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28 \title Developer documentation for Simon Tatham's puzzle collection
30 This is a guide to the internal structure of Simon Tatham's Portable
31 Puzzle Collection (henceforth referred to simply as \q{Puzzles}),
32 for use by anyone attempting to implement a new puzzle or port to a
35 This guide is believed correct as of r6190. Hopefully it will be
36 updated along with the code in future, but if not, I've at least
37 left this version number in here so you can figure out what's
38 changed by tracking commit comments from there onwards.
40 \C{intro} Introduction
42 The Puzzles code base is divided into four parts: a set of
43 interchangeable front ends, a set of interchangeable back ends, a
44 universal \q{middle end} which acts as a buffer between the two, and
45 a bunch of miscellaneous utility functions. In the following
46 sections I give some general discussion of each of these parts.
48 \H{intro-frontend} Front end
50 The front end is the non-portable part of the code: it's the bit
51 that you replace completely when you port to a different platform.
52 So it's responsible for all system calls, all GUI interaction, and
53 anything else platform-specific.
55 The current front ends in the main code base are for Windows, GTK
56 and MacOS X; I also know of a third-party front end for PalmOS.
58 The front end contains \cw{main()} or the local platform's
59 equivalent. Top-level control over the application's execution flow
60 belongs to the front end (it isn't, for example, a set of functions
61 called by a universal \cw{main()} somewhere else).
63 The front end has complete freedom to design the GUI for any given
64 port of Puzzles. There is no centralised mechanism for maintaining
65 the menu layout, for example. This has a cost in consistency (when I
66 \e{do} want the same menu layout on more than one platform, I have
67 to edit two pieces of code in parallel every time I make a change),
68 but the advantage is that local GUI conventions can be conformed to
69 and local constraints adapted to. For example, MacOS X has strict
70 human interface guidelines which specify a different menu layout
71 from the one I've used on Windows and GTK; there's nothing stopping
72 the OS X front end from providing a menu layout consistent with
75 Although the front end is mostly caller rather than the callee in
76 its interactions with other parts of the code, it is required to
77 implement a small API for other modules to call, mostly of drawing
78 functions for games to use when drawing their graphics. The drawing
79 API is documented in \k{drawing}; the other miscellaneous front end
80 API functions are documented in \k{frontend-api}.
82 \H{intro-backend} Back end
84 A \q{back end}, in this collection, is synonymous with a \q{puzzle}.
85 Each back end implements a different game.
87 At the top level, a back end is simply a data structure, containing
88 a few constants (flag words, preferred pixel size) and a large
89 number of function pointers. Back ends are almost invariably callee
90 rather than caller, which means there's a limitation on what a back
91 end can do on its own initiative.
93 The persistent state in a back end is divided into a number of data
94 structures, which are used for different purposes and therefore
95 likely to be switched around, changed without notice, and otherwise
96 updated by the rest of the code. It is important when designing a
97 back end to put the right pieces of data into the right structures,
98 or standard midend-provided features (such as Undo) may fail to
101 The functions and variables provided in the back end data structure
102 are documented in \k{backend}.
104 \H{intro-midend} Middle end
106 Puzzles has a single and universal \q{middle end}. This code is
107 common to all platforms and all games; it sits in between the front
108 end and the back end and provides standard functionality everywhere.
110 People adding new back ends or new front ends should generally not
111 need to edit the middle end. On rare occasions there might be a
112 change that can be made to the middle end to permit a new game to do
113 something not currently anticipated by the middle end's present
114 design; however, this is terribly easy to get wrong and should
115 probably not be undertaken without consulting the primary maintainer
116 (me). Patch submissions containing unannounced mid-end changes will
117 be treated on their merits like any other patch; this is just a
118 friendly warning that mid-end changes will need quite a lot of
119 merits to make them acceptable.
121 Functionality provided by the mid-end includes:
123 \b Maintaining a list of game state structures and moving back and
124 forth along that list to provide Undo and Redo.
126 \b Handling timers (for move animations, flashes on completion, and
127 in some cases actually timing the game).
129 \b Handling the container format of game IDs: receiving them,
130 picking them apart into parameters, description and/or random seed,
131 and so on. The game back end need only handle the individual parts
132 of a game ID (encoded parameters and encoded game description);
133 everything else is handled centrally by the mid-end.
135 \b Handling standard keystrokes and menu commands, such as \q{New
136 Game}, \q{Restart Game} and \q{Quit}.
138 \b Pre-processing mouse events so that the game back ends can rely
139 on them arriving in a sensible order (no missing button-release
140 events, no sudden changes of which button is currently pressed,
143 \b Handling the dialog boxes which ask the user for a game ID.
145 \b Handling serialisation of entire games (for loading and saving a
146 half-finished game to a disk file, or for handling application
147 shutdown and restart on platforms such as PalmOS where state is
148 expected to be saved).
150 Thus, there's a lot of work done once by the mid-end so that
151 individual back ends don't have to worry about it. All the back end
152 has to do is cooperate in ensuring the mid-end can do its work
155 The API of functions provided by the mid-end to be called by the
156 front end is documented in \k{midend}.
158 \H{intro-utils} Miscellaneous utilities
160 In addition to these three major structural components, the Puzzles
161 code also contains a variety of utility modules usable by all of the
162 above components. There is a set of functions to provide
163 platform-independent random number generation; functions to make
164 memory allocation easier; functions which implement a balanced tree
165 structure to be used as necessary in complex algorithms; and a few
166 other miscellaneous functions. All of these are documented in
169 \H{intro-structure} Structure of this guide
171 There are a number of function call interfaces within Puzzles, and
172 this guide will discuss each one in a chapter of its own. After
173 that, \k{writing} discusses how to design new games, with some
174 general design thoughts and tips.
176 \C{backend} Interface to the back end
178 This chapter gives a detailed discussion of the interface that each
179 back end must implement.
181 At the top level, each back end source file exports a single global
182 symbol, which is a \c{const struct game} containing a large number
183 of function pointers and a small amount of constant data. This
184 structure is called by different names depending on what kind of
185 platform the puzzle set is being compiled on:
187 \b On platforms such as Windows and GTK, which build a separate
188 binary for each puzzle, the game structure in every back end has the
189 same name, \cq{thegame}; the front end refers directly to this name,
190 so that compiling the same front end module against a different back
191 end module builds a different puzzle.
193 \b On platforms such as MacOS X and PalmOS, which build all the
194 puzzles into a single monolithic binary, the game structure in each
195 back end must have a different name, and there's a helper module
196 \c{list.c} (constructed automatically by the same Perl script that
197 builds the \cw{Makefile}s) which contains a complete list of those
200 On the latter type of platform, source files may assume that the
201 preprocessor symbol \c{COMBINED} has been defined. Thus, the usual
202 code to declare the game structure looks something like this:
205 \c #define thegame net /* or whatever this game is called */
206 \e iii iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii
209 \c const struct game thegame = {
210 \c /* lots of structure initialisation in here */
211 \e iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii
214 Game back ends must also internally define a number of data
215 structures, for storing their various persistent state. This chapter
216 will first discuss the nature and use of those structures, and then
217 go on to give details of every element of the game structure.
219 \H{backend-structs} Data structures
221 Each game is required to define four separate data structures. This
222 section discusses each one and suggests what sorts of things need to
225 \S{backend-game-params} \c{game_params}
227 The \c{game_params} structure contains anything which affects the
228 automatic generation of new puzzles. So if puzzle generation is
229 parametrised in any way, those parameters need to be stored in
232 Most puzzles currently in this collection are played on a grid of
233 squares, meaning that the most obvious parameter is the grid size.
234 Many puzzles have additional parameters; for example, Mines allows
235 you to control the number of mines in the grid independently of its
236 size, Net can be wrapping or non-wrapping, Solo has difficulty
237 levels and symmetry settings, and so on.
239 A simple rule for deciding whether a data item needs to go in
240 \c{game_params} is: would the user expect to be able to control this
241 data item from either the preset-game-types menu or the \q{Custom}
242 game type configuration? If so, it's part of \c{game_params}.
244 \c{game_params} structures are permitted to contain pointers to
245 subsidiary data if they need to. The back end is required to provide
246 functions to create and destroy \c{game_params}, and those functions
247 can allocate and free additional memory if necessary. (It has not
248 yet been necessary to do this in any puzzle so far, but the
249 capability is there just in case.)
251 \c{game_params} is also the only structure which the game's
252 \cw{compute_size()} function may refer to; this means that any
253 aspect of the game which affects the size of the window it needs to
254 be drawn in must be stored in \c{game_params}. In particular, this
255 imposes the fundamental limitation that random game generation may
256 not have a random effect on the window size: game generation
257 algorithms are constrained to work by starting from the grid size
258 rather than generating it as an emergent phenomenon. (Although this
259 is a restriction in theory, it has not yet seemed to be a problem.)
261 \S{backend-game-state} \c{game_state}
263 While the user is actually playing a puzzle, the \c{game_state}
264 structure stores all the data corresponding to the current state of
267 The mid-end keeps \c{game_state}s in a list, and adds to the list
268 every time the player makes a move; the Undo and Redo functions step
269 back and forth through that list.
271 Therefore, a good means of deciding whether a data item needs to go
272 in \c{game_state} is: would a player expect that data item to be
273 restored on undo? If so, put it in \c{game_state}, and this will
274 automatically happen without you having to lift a finger. If not
275 \dash for example, the deaths counter in Mines is precisely
276 something that does \e{not} want to be reset to its previous state
277 on an undo \dash then you might have found a data item that needs to
278 go in \c{game_ui} instead.
280 During play, \c{game_state}s are often passed around without an
281 accompanying \c{game_params} structure. Therefore, any information
282 in \c{game_params} which is important during play (such as the grid
283 size) must be duplicated within the \c{game_state}. One simple
284 method of doing this is to have the \c{game_state} structure
285 \e{contain} a \c{game_params} structure as one of its members,
286 although this isn't obligatory if you prefer to do it another way.
288 \S{backend-game-drawstate} \c{game_drawstate}
290 \c{game_drawstate} carries persistent state relating to the current
291 graphical contents of the puzzle window. The same \c{game_drawstate}
292 is passed to every call to the game redraw function, so that it can
293 remember what it has already drawn and what needs redrawing.
295 A typical use for a \c{game_drawstate} is to have an array mirroring
296 the array of grid squares in the \c{game_state}; then every time the
297 redraw function was passed a \c{game_state}, it would loop over all
298 the squares, and physically redraw any whose description in the
299 \c{game_state} (i.e. what the square needs to look like when the
300 redraw is completed) did not match its description in the
301 \c{game_drawstate} (i.e. what the square currently looks like).
303 \c{game_drawstate} is occasionally completely torn down and
304 reconstructed by the mid-end, if the user somehow forces a full
305 redraw. Therefore, no data should be stored in \c{game_drawstate}
306 which is \e{not} related to the state of the puzzle window, because
307 it might be unexpectedly destroyed.
309 The back end provides functions to create and destroy
310 \c{game_drawstate}, which means it can contain pointers to
311 subsidiary allocated data if it needs to. A common thing to want to
312 allocate in a \c{game_drawstate} is a \c{blitter}; see
313 \k{drawing-blitter} for more on this subject.
315 \S{backend-game-ui} \c{game_ui}
317 \c{game_ui} contains whatever doesn't fit into the above three
320 A new \c{game_ui} is created when the user begins playing a new
321 instance of a puzzle (i.e. during \q{New Game} or after entering a
322 game ID etc). It persists until the user finishes playing that game
323 and begins another one (or closes the window); in particular,
324 \q{Restart Game} does \e{not} destroy the \c{game_ui}.
326 \c{game_ui} is useful for implementing user-interface state which is
327 not part of \c{game_state}. Common examples are keyboard control
328 (you wouldn't want to have to separately Undo through every cursor
329 motion) and mouse dragging. See \k{writing-keyboard-cursor} and
330 \k{writing-howto-dragging}, respectively, for more details.
332 Another use for \c{game_ui} is to store highly persistent data such
333 as the Mines death counter. This is conceptually rather different:
334 where the Net cursor position was \e{not important enough} to
335 preserve for the player to restore by Undo, the Mines death counter
336 is \e{too important} to permit the player to revert by Undo!
338 A final use for \c{game_ui} is to pass information to the redraw
339 function about recent changes to the game state. This is used in
340 Mines, for example, to indicate whether a requested \q{flash} should
341 be a white flash for victory or a red flash for defeat; see
342 \k{writing-flash-types}.
344 \H{backend-simple} Simple data in the back end
346 In this section I begin to discuss each individual element in the
347 back end structure. To begin with, here are some simple
348 self-contained data elements.
350 \S{backend-name} \c{name}
354 This is a simple ASCII string giving the name of the puzzle. This
355 name will be used in window titles, in game selection menus on
356 monolithic platforms, and anywhere else that the front end needs to
357 know the name of a game.
359 \S{backend-winhelp} \c{winhelp_topic}
361 \c const char *winhelp_topic;
363 This member is used on Windows only, to provide online help.
364 Although the Windows front end provides a separate binary for each
365 puzzle, it has a single monolithic help file; so when a user selects
366 \q{Help} from the menu, the program needs to open the help file and
367 jump to the chapter describing that particular puzzle.
369 Therefore, each chapter in \c{puzzles.but} is labelled with a
370 \e{help topic} name, similar to this:
372 \c \cfg{winhelp-topic}{games.net}
374 And then the corresponding game back end encodes the topic string
375 (here \cq{games.net}) in the \c{winhelp_topic} element of the game
378 \H{backend-params} Handling game parameter sets
380 In this section I present the various functions which handle the
381 \c{game_params} structure.
383 \S{backend-default-params} \cw{default_params()}
385 \c game_params *(*default_params)(void);
387 This function allocates a new \c{game_params} structure, fills it
388 with the default values, and returns a pointer to it.
390 \S{backend-fetch-preset} \cw{fetch_preset()}
392 \c int (*fetch_preset)(int i, char **name, game_params **params);
394 This function is used to populate the \q{Type} menu, which provides
395 a list of conveniently accessible preset parameters for most games.
397 The function is called with \c{i} equal to the index of the preset
398 required (numbering from zero). It returns \cw{FALSE} if that preset
399 does not exist (if \c{i} is less than zero or greater than the
400 largest preset index). Otherwise, it sets \c{*params} to point at a
401 newly allocated \c{game_params} structure containing the preset
402 information, sets \c{*name} to point at a newly allocated C string
403 containing the preset title (to go on the \q{Type} menu), and
406 If the game does not wish to support any presets at all, this
407 function is permitted to return \cw{FALSE} always.
409 \S{backend-encode-params} \cw{encode_params()}
411 \c char *(*encode_params)(game_params *params, int full);
413 The job of this function is to take a \c{game_params}, and encode it
414 in a string form for use in game IDs. The return value must be a
415 newly allocated C string, and \e{must} not contain a colon or a hash
416 (since those characters are used to mark the end of the parameter
417 section in a game ID).
419 Ideally, it should also not contain any other potentially
420 controversial punctuation; bear in mind when designing a string
421 parameter format that it will probably be used on both Windows and
422 Unix command lines under a variety of exciting shell quoting and
423 metacharacter rules. Sticking entirely to alphanumerics is the
424 safest thing; if you really need punctuation, you can probably get
425 away with commas, periods or underscores without causing anybody any
426 major inconvenience. If you venture far beyond that, you're likely
427 to irritate \e{somebody}.
429 (At the time of writing this, all existing games have purely
430 alphanumeric string parameter formats. Usually these involve a
431 letter denoting a parameter, followed optionally by a number giving
432 the value of that parameter, with a few mandatory parts at the
433 beginning such as numeric width and height separated by \cq{x}.)
435 If the \c{full} parameter is \cw{TRUE}, this function should encode
436 absolutely everything in the \c{game_params}, such that a subsequent
437 call to \cw{decode_params()} (\k{backend-decode-params}) will yield
438 an identical structure. If \c{full} is \cw{FALSE}, however, you
439 should leave out anything which is not necessary to describe a
440 \e{specific puzzle instance}, i.e. anything which only takes effect
441 when a new puzzle is \e{generated}. For example, the Solo
442 \c{game_params} includes a difficulty rating used when constructing
443 new puzzles; but a Solo game ID need not explicitly include the
444 difficulty, since to describe a puzzle once generated it's
445 sufficient to give the grid dimensions and the location and contents
446 of the clue squares. (Indeed, one might very easily type in a puzzle
447 out of a newspaper without \e{knowing} what its difficulty level is
448 in Solo's terminology.) Therefore, Solo's \cw{encode_params()} only
449 encodes the difficulty level if \c{full} is set.
451 \S{backend-decode-params} \cw{decode_params()}
453 \c void (*decode_params)(game_params *params, char const *string);
455 This function is the inverse of \cw{encode_params()}
456 (\k{backend-encode-params}). It parses the supplied string and fills
457 in the supplied \c{game_params} structure. Note that the structure
458 will \e{already} have been allocated: this function is not expected
459 to create a \e{new} \c{game_params}, but to modify an existing one.
461 This function can receive a string which only encodes a subset of
462 the parameters. The most obvious way in which this can happen is if
463 the string was constructed by \cw{encode_params()} with its \c{full}
464 parameter set to \cw{FALSE}; however, it could also happen if the
465 user typed in a parameter set manually and missed something out. Be
466 prepared to deal with a wide range of possibilities.
468 When dealing with a parameter which is not specified in the input
469 string, what to do requires a judgment call on the part of the
470 programmer. Sometimes it makes sense to adjust other parameters to
471 bring them into line with the new ones. In Mines, for example, you
472 would probably not want to keep the same mine count if the user
473 dropped the grid size and didn't specify one, since you might easily
474 end up with more mines than would actually fit in the grid! On the
475 other hand, sometimes it makes sense to leave the parameter alone: a
476 Solo player might reasonably expect to be able to configure size and
477 difficulty independently of one another.
479 This function currently has no direct means of returning an error if
480 the string cannot be parsed at all. However, the returned
481 \c{game_params} is almost always subsequently passed to
482 \cw{validate_params()} (\k{backend-validate-params}), so if you
483 really want to signal parse errors, you could always have a \c{char
484 *} in your parameters structure which stored an error message, and
485 have \cw{validate_params()} return it if it is non-\cw{NULL}.
487 \S{backend-free-params} \cw{free_params()}
489 \c void (*free_params)(game_params *params);
491 This function frees a \c{game_params} structure, and any subsidiary
492 allocations contained within it.
494 \S{backend-dup-params} \cw{dup_params()}
496 \c game_params *(*dup_params)(game_params *params);
498 This function allocates a new \c{game_params} structure and
499 initialises it with an exact copy of the information in the one
500 provided as input. It returns a pointer to the new duplicate.
502 \S{backend-can-configure} \c{can_configure}
504 \c int can_configure;
506 This boolean data element is set to \cw{TRUE} if the back end
507 supports custom parameter configuration via a dialog box. If it is
508 \cw{TRUE}, then the functions \cw{configure()} and
509 \cw{custom_params()} are expected to work. See \k{backend-configure}
510 and \k{backend-custom-params} for more details.
512 \S{backend-configure} \cw{configure()}
514 \c config_item *(*configure)(game_params *params);
516 This function is called when the user requests a dialog box for
517 custom parameter configuration. It returns a newly allocated array
518 of \cw{config_item} structures, describing the GUI elements required
519 in the dialog box. The array should have one more element than the
520 number of controls, since it is terminated with a \cw{C_END} marker
521 (see below). Each array element describes the control together with
522 its initial value; the front end will modify the value fields and
523 return the updated array to \cw{custom_params()} (see
524 \k{backend-custom-params}).
526 The \cw{config_item} structure contains the following elements:
533 \c{name} is an ASCII string giving the textual label for a GUI
534 control. It is \e{not} expected to be dynamically allocated.
536 \c{type} contains one of a small number of \c{enum} values defining
537 what type of control is being described. The meaning of the \c{sval}
538 and \c{ival} fields depends on the value in \c{type}. The valid
543 \dd Describes a text input box. (This is also used for numeric
544 input. The back end does not bother informing the front end that the
545 box is numeric rather than textual; some front ends do have the
546 capacity to take this into account, but I decided it wasn't worth
547 the extra complexity in the interface.) For this type, \c{ival} is
548 unused, and \c{sval} contains a dynamically allocated string
549 representing the contents of the input box.
553 \dd Describes a simple checkbox. For this type, \c{sval} is unused,
554 and \c{ival} is \cw{TRUE} or \cw{FALSE}.
558 \dd Describes a drop-down list presenting one of a small number of
559 fixed choices. For this type, \c{sval} contains a list of strings
560 describing the choices; the very first character of \c{sval} is used
561 as a delimiter when processing the rest (so that the strings
562 \cq{:zero:one:two}, \cq{!zero!one!two} and \cq{xzeroxonextwo} all
563 define a three-element list containing \cq{zero}, \cq{one} and
564 \cq{two}). \c{ival} contains the index of the currently selected
565 element, numbering from zero (so that in the above example, 0 would
566 mean \cq{zero} and 2 would mean \cq{two}).
570 Note that for this control type, \c{sval} is \e{not} dynamically
571 allocated, whereas it was for \c{C_STRING}.
577 \dd Marks the end of the array of \c{config_item}s. All other fields
580 The array returned from this function is expected to have filled in
581 the initial values of all the controls according to the input
582 \c{game_params} structure.
584 If the game's \c{can_configure} flag is set to \cw{FALSE}, this
585 function is never called and need not do anything at all.
587 \S{backend-custom-params} \cw{custom_params()}
589 \c game_params *(*custom_params)(config_item *cfg);
591 This function is the counterpart to \cw{configure()}
592 (\k{backend-configure}). It receives as input an array of
593 \c{config_item}s which was originally created by \cw{configure()},
594 but in which the control values have since been changed in
595 accordance with user input. Its function is to read the new values
596 out of the controls and return a newly allocated \c{game_params}
597 structure representing the user's chosen parameter set.
599 (The front end will have modified the controls' \e{values}, but
600 there will still always be the same set of controls, in the same
601 order, as provided by \cw{configure()}. It is not necessary to check
602 the \c{name} and \c{type} fields, although you could use
603 \cw{assert()} if you were feeling energetic.)
605 This function is not expected to (and indeed \e{must not}) free the
606 input \c{config_item} array. (If the parameters fail to validate,
607 the dialog box will stay open.)
609 If the game's \c{can_configure} flag is set to \cw{FALSE}, this
610 function is never called and need not do anything at all.
612 \S{backend-validate-params} \cw{validate_params()}
614 \c char *(*validate_params)(game_params *params, int full);
616 This function takes a \c{game_params} structure as input, and checks
617 that the parameters described in it fall within sensible limits. (At
618 the very least, grid dimensions should almost certainly be strictly
619 positive, for example.)
621 Return value is \cw{NULL} if no problems were found, or
622 alternatively a (non-dynamically-allocated) ASCII string describing
623 the error in human-readable form.
625 If the \c{full} parameter is set, full validation should be
626 performed: any set of parameters which would not permit generation
627 of a sensible puzzle should be faulted. If \c{full} is \e{not} set,
628 the implication is that these parameters are not going to be used
629 for \e{generating} a puzzle; so parameters which can't even sensibly
630 \e{describe} a valid puzzle should still be faulted, but parameters
631 which only affect puzzle generation should not be.
633 (The \c{full} option makes a difference when parameter combinations
634 are non-orthogonal. For example, Net has a boolean option
635 controlling whether it enforces a unique solution; it turns out that
636 it's impossible to generate a uniquely soluble puzzle with wrapping
637 walls and width 2, so \cw{validate_params()} will complain if you
638 ask for one. However, if the user had just been playing a unique
639 wrapping puzzle of a more sensible width, and then pastes in a game
640 ID acquired from somebody else which happens to describe a
641 \e{non}-unique wrapping width-2 puzzle, then \cw{validate_params()}
642 will be passed a \c{game_params} containing the width and wrapping
643 settings from the new game ID and the uniqueness setting from the
644 old one. This would be faulted, if it weren't for the fact that
645 \c{full} is not set during this call, so Net ignores the
646 inconsistency. The resulting \c{game_params} is never subsequently
647 used to generate a puzzle; this is a promise made by the mid-end
648 when it asks for a non-full validation.)
650 \H{backend-descs} Handling game descriptions
652 In this section I present the functions that deal with a textual
653 description of a puzzle, i.e. the part that comes after the colon in
654 a descriptive-format game ID.
656 \S{backend-new-desc} \cw{new_desc()}
658 \c char *(*new_desc)(game_params *params, random_state *rs,
659 \c char **aux, int interactive);
661 This function is where all the really hard work gets done. This is
662 the function whose job is to randomly generate a new puzzle,
663 ensuring solubility and uniqueness as appropriate.
665 As input it is given a \c{game_params} structure and a random state
666 (see \k{utils-random} for the random number API). It must invent a
667 puzzle instance, encode it in string form, and return a dynamically
668 allocated C string containing that encoding.
670 Additionally, it may return a second dynamically allocated string in
671 \c{*aux}. (If it doesn't want to, then it can leave that parameter
672 completely alone; it isn't required to set it to \cw{NULL}, although
673 doing so is harmless.) That string, if present, will be passed to
674 \cw{solve()} (\k{backend-solve}) later on; so if the puzzle is
675 generated in such a way that a solution is known, then information
676 about that solution can be saved in \c{*aux} for \cw{solve()} to
679 The \c{interactive} parameter should be ignored by almost all
680 puzzles. Its purpose is to distinguish between generating a puzzle
681 within a GUI context for immediate play, and generating a puzzle in
682 a command-line context for saving to be played later. The only
683 puzzle that currently uses this distinction (and, I fervently hope,
684 the only one which will \e{ever} need to use it) is Mines, which
685 chooses a random first-click location when generating puzzles
686 non-interactively, but which waits for the user to place the first
687 click when interactive. If you think you have come up with another
688 puzzle which needs to make use of this parameter, please think for
689 at least ten minutes about whether there is \e{any} alternative!
691 Note that game description strings are not required to contain an
692 encoding of parameters such as grid size; a game description is
693 never separated from the \c{game_params} it was generated with, so
694 any information contained in that structure need not be encoded
695 again in the game description.
697 \S{backend-validate-desc} \cw{validate_desc()}
699 \c char *(*validate_desc)(game_params *params, char *desc);
701 This function is given a game description, and its job is to
702 validate that it describes a puzzle which makes sense.
704 To some extent it's up to the user exactly how far they take the
705 phrase \q{makes sense}; there are no particularly strict rules about
706 how hard the user is permitted to shoot themself in the foot when
707 typing in a bogus game description by hand. (For example, Rectangles
708 will not verify that the sum of all the numbers in the grid equals
709 the grid's area. So a user could enter a puzzle which was provably
710 not soluble, and the program wouldn't complain; there just wouldn't
711 happen to be any sequence of moves which solved it.)
713 The one non-negotiable criterion is that any game description which
714 makes it through \cw{validate_desc()} \e{must not} subsequently
715 cause a crash or an assertion failure when fed to \cw{new_game()}
716 and thence to the rest of the back end.
718 The return value is \cw{NULL} on success, or a
719 non-dynamically-allocated C string containing an error message.
721 \S{backend-new-game} \cw{new_game()}
723 \c game_state *(*new_game)(midend *me, game_params *params,
726 This function takes a game description as input, together with its
727 accompanying \c{game_params}, and constructs a \c{game_state}
728 describing the initial state of the puzzle. It returns a newly
729 allocated \c{game_state} structure.
731 Almost all puzzles should ignore the \c{me} parameter. It is
732 required by Mines, which needs it for later passing to
733 \cw{midend_supersede_game_desc()} (see \k{backend-supersede}) once
734 the user has placed the first click. I fervently hope that no other
735 puzzle will be awkward enough to require it, so everybody else
736 should ignore it. As with the \c{interactive} parameter in
737 \cw{new_desc()} (\k{backend-new-desc}), if you think you have a
738 reason to need this parameter, please try very hard to think of an
739 alternative approach!
741 \H{backend-states} Handling game states
743 This section describes the functions which create and destroy
744 \c{game_state} structures.
746 (Well, except \cw{new_game()}, which is in \k{backend-new-game}
747 instead of under here; but it deals with game descriptions \e{and}
748 game states and it had to go in one section or the other.)
750 \S{backend-dup-game} \cw{dup_game()}
752 \c game_state *(*dup_game)(game_state *state);
754 This function allocates a new \c{game_state} structure and
755 initialises it with an exact copy of the information in the one
756 provided as input. It returns a pointer to the new duplicate.
758 \S{backend-free-game} \cw{free_game()}
760 \c void (*free_game)(game_state *state);
762 This function frees a \c{game_state} structure, and any subsidiary
763 allocations contained within it.
765 \H{backend-ui} Handling \c{game_ui}
767 \S{backend-new-ui} \cw{new_ui()}
769 \c game_ui *(*new_ui)(game_state *state);
771 This function allocates and returns a new \c{game_ui} structure for
772 playing a particular puzzle. It is passed a pointer to the initial
773 \c{game_state}, in case it needs to refer to that when setting up
774 the initial values for the new game.
776 \S{backend-free-ui} \cw{free_ui()}
778 \c void (*free_ui)(game_ui *ui);
780 This function frees a \c{game_ui} structure, and any subsidiary
781 allocations contained within it.
783 \S{backend-encode-ui} \cw{encode_ui()}
785 \c char *(*encode_ui)(game_ui *ui);
787 This function encodes any \e{important} data in a \c{game_ui}
788 structure in string form. It is only called when saving a
789 half-finished game to a file.
791 It should be used sparingly. Almost all data in a \c{game_ui} is not
792 important enough to save. The location of the keyboard-controlled
793 cursor, for example, can be reset to a default position on reloading
794 the game without impacting the user experience. If the user should
795 somehow manage to save a game while a mouse drag was in progress,
796 then discarding that mouse drag would be an outright \e{feature}.
798 A typical thing that \e{would} be worth encoding in this function is
799 the Mines death counter: it's in the \c{game_ui} rather than the
800 \c{game_state} because it's too important to allow the user to
801 revert it by using Undo, and therefore it's also too important to
802 allow the user to revert it by saving and reloading. (Of course, the
803 user could edit the save file by hand... But if the user is \e{that}
804 determined to cheat, they could just as easily modify the game's
807 \S{backend-decode-ui} \cw{decode_ui()}
809 \c void (*decode_ui)(game_ui *ui, char *encoding);
811 This function parses a string previously output by \cw{encode_ui()},
812 and writes the decoded data back into the provided \c{game_ui}
815 \S{backend-changed-state} \cw{changed_state()}
817 \c void (*changed_state)(game_ui *ui, game_state *oldstate,
818 \c game_state *newstate);
820 This function is called by the mid-end whenever the current game
821 state changes, for any reason. Those reasons include:
823 \b a fresh move being made by \cw{interpret_move()} and
826 \b a solve operation being performed by \cw{solve()} and
829 \b the user moving back and forth along the undo list by means of
830 the Undo and Redo operations
832 \b the user selecting Restart to go back to the initial game state.
834 The job of \cw{changed_state()} is to update the \c{game_ui} for
835 consistency with the new game state, if any update is necessary. For
836 example, Same Game stores data about the currently selected tile
837 group in its \c{game_ui}, and this data is intrinsically related to
838 the game state it was derived from. So it's very likely to become
839 invalid when the game state changes; thus, Same Game's
840 \cw{changed_state()} function clears the current selection whenever
843 When \cw{anim_length()} or \cw{flash_length()} are called, you can
844 be sure that there has been a previous call to \cw{changed_state()}.
845 So \cw{changed_state()} can set up data in the \c{game_ui} which will
846 be read by \cw{anim_length()} and \cw{flash_length()}, and those
847 functions will not have to worry about being called without the data
848 having been initialised.
850 \H{backend-moves} Making moves
852 This section describes the functions which actually make moves in
853 the game: that is, the functions which process user input and end up
854 producing new \c{game_state}s.
856 \S{backend-interpret-move} \cw{interpret_move()}
858 \c char *(*interpret_move)(game_state *state, game_ui *ui,
859 \c const game_drawstate *ds,
860 \c int x, int y, int button);
862 This function receives user input and processes it. Its input
863 parameters are the current \c{game_state}, the current \c{game_ui}
864 and the current \c{game_drawstate}, plus details of the input event.
865 \c{button} is either an ASCII value or a special code (listed below)
866 indicating an arrow or function key or a mouse event; when
867 \c{button} is a mouse event, \c{x} and \c{y} contain the pixel
868 coordinates of the mouse pointer relative to the top left of the
869 puzzle's drawing area.
871 (The pointer to the \c{game_drawstate} is marked \c{const}, because
872 \c{interpret_move} should not write to it. The normal use of that
873 pointer will be to read the game's tile size parameter in order to
874 divide mouse coordinates by it.)
876 \cw{interpret_move()} may return in three different ways:
878 \b Returning \cw{NULL} indicates that no action whatsoever occurred
879 in response to the input event; the puzzle was not interested in it
882 \b Returning the empty string (\cw{""}) indicates that the input
883 event has resulted in a change being made to the \c{game_ui} which
884 will require a redraw of the game window, but that no actual
885 \e{move} was made (i.e. no new \c{game_state} needs to be created).
887 \b Returning anything else indicates that a move was made and that a
888 new \c{game_state} must be created. However, instead of actually
889 constructing a new \c{game_state} itself, this function is required
890 to return a string description of the details of the move. This
891 string will be passed to \cw{execute_move()}
892 (\k{backend-execute-move}) to actually create the new
893 \c{game_state}. (Encoding moves as strings in this way means that
894 the mid-end can keep the strings as well as the game states, and the
895 strings can be written to disk when saving the game and fed to
896 \cw{execute_move()} again on reloading.)
898 The return value from \cw{interpret_move()} is expected to be
899 dynamically allocated if and only if it is not either \cw{NULL}
900 \e{or} the empty string.
902 After this function is called, the back end is permitted to rely on
903 some subsequent operations happening in sequence:
905 \b \cw{execute_move()} will be called to convert this move
906 description into a new \c{game_state}
908 \b \cw{changed_state()} will be called with the new \c{game_state}.
910 This means that if \cw{interpret_move()} needs to do updates to the
911 \c{game_ui} which are easier to perform by referring to the new
912 \c{game_state}, it can safely leave them to be done in
913 \cw{changed_state()} and not worry about them failing to happen.
915 (Note, however, that \cw{execute_move()} may \e{also} be called in
916 other circumstances. It is only \cw{interpret_move()} which can rely
917 on a subsequent call to \cw{changed_state()}.)
919 The special key codes supported by this function are:
921 \dt \cw{LEFT_BUTTON}, \cw{MIDDLE_BUTTON}, \cw{RIGHT_BUTTON}
923 \dd Indicate that one of the mouse buttons was pressed down.
925 \dt \cw{LEFT_DRAG}, \cw{MIDDLE_DRAG}, \cw{RIGHT_DRAG}
927 \dd Indicate that the mouse was moved while one of the mouse buttons
928 was still down. The mid-end guarantees that when one of these events
929 is received, it will always have been preceded by a button-down
930 event (and possibly other drag events) for the same mouse button,
931 and no event involving another mouse button will have appeared in
934 \dt \cw{LEFT_RELEASE}, \cw{MIDDLE_RELEASE}, \cw{RIGHT_RELEASE}
936 \dd Indicate that a mouse button was released. The mid-end
937 guarantees that when one of these events is received, it will always
938 have been preceded by a button-down event (and possibly some drag
939 events) for the same mouse button, and no event involving another
940 mouse button will have appeared in between.
942 \dt \cw{CURSOR_UP}, \cw{CURSOR_DOWN}, \cw{CURSOR_LEFT},
945 \dd Indicate that an arrow key was pressed.
947 \dt \cw{CURSOR_SELECT}
949 \dd On platforms which have a prominent \q{select} button alongside
950 their cursor keys, indicates that that button was pressed.
952 In addition, there are some modifiers which can be bitwise-ORed into
953 the \c{button} parameter:
955 \dt \cw{MOD_CTRL}, \cw{MOD_SHFT}
957 \dd These indicate that the Control or Shift key was pressed
958 alongside the key. They only apply to the cursor keys, not to mouse
959 buttons or anything else.
961 \dt \cw{MOD_NUM_KEYPAD}
963 \dd This applies to some ASCII values, and indicates that the key
964 code was input via the numeric keypad rather than the main keyboard.
965 Some puzzles may wish to treat this differently (for example, a
966 puzzle might want to use the numeric keypad as an eight-way
967 directional pad), whereas others might not (a game involving numeric
968 input probably just wants to treat the numeric keypad as numbers).
972 \dd This mask is the bitwise OR of all the available modifiers; you
973 can bitwise-AND with \cw{~MOD_MASK} to strip all the modifiers off
976 \S{backend-execute-move} \cw{execute_move()}
978 \c game_state *(*execute_move)(game_state *state, char *move);
980 This function takes an input \c{game_state} and a move string as
981 output from \cw{interpret_move()}. It returns a newly allocated
982 \c{game_state} which contains the result of applying the specified
983 move to the input game state.
985 This function may return \cw{NULL} if it cannot parse the move
986 string (and this is definitely preferable to crashing or failing an
987 assertion, since one way this can happen is if loading a corrupt
988 save file). However, it must not return \cw{NULL} for any move
989 string that really was output from \cw{interpret_move()}: this is
990 punishable by assertion failure in the mid-end.
992 \S{backend-can-solve} \c{can_solve}
996 This boolean field is set to \cw{TRUE} if the game's \cw{solve()}
997 function does something. If it's set to \cw{FALSE}, the game will
998 not even offer the \q{Solve} menu option.
1000 \S{backend-solve} \cw{solve()}
1002 \c char *(*solve)(game_state *orig, game_state *curr,
1003 \c char *aux, char **error);
1005 This function is called when the user selects the \q{Solve} option
1008 It is passed two input game states: \c{orig} is the game state from
1009 the very start of the puzzle, and \c{curr} is the current one.
1010 (Different games find one or other or both of these convenient.) It
1011 is also passed the \c{aux} string saved by \cw{new_desc()}
1012 (\k{backend-new-desc}), in case that encodes important information
1013 needed to provide the solution.
1015 If this function is unable to produce a solution (perhaps, for
1016 example, the game has no in-built solver so it can only solve
1017 puzzles it invented internally and has an \c{aux} string for) then
1018 it may return \cw{NULL}. If it does this, it must also set
1019 \c{*error} to an error message to be presented to the user (such as
1020 \q{Solution not known for this puzzle}); that error message is not
1021 expected to be dynamically allocated.
1023 If this function \e{does} produce a solution, it returns a move
1024 string suitable for feeding to \cw{execute_move()}
1025 (\k{backend-execute-move}).
1027 \H{backend-drawing} Drawing the game graphics
1029 This section discusses the back end functions that deal with
1032 \S{backend-new-drawstate} \cw{new_drawstate()}
1034 \c game_drawstate *(*new_drawstate)(drawing *dr, game_state *state);
1036 This function allocates and returns a new \c{game_drawstate}
1037 structure for drawing a particular puzzle. It is passed a pointer to
1038 a \c{game_state}, in case it needs to refer to that when setting up
1041 This function may not rely on the puzzle having been newly started;
1042 a new draw state can be constructed at any time if the front end
1043 requests a forced redraw. For games like Pattern, in which initial
1044 game states are much simpler than general ones, this might be
1045 important to keep in mind.
1047 The parameter \c{dr} is a drawing object (see \k{drawing}) which the
1048 function might need to use to allocate blitters. (However, this
1049 isn't recommended; it's usually more sensible to wait to allocate a
1050 blitter until \cw{set_size()} is called, because that way you can
1051 tailor it to the scale at which the puzzle is being drawn.)
1053 \S{backend-free-drawstate} \cw{free_drawstate()}
1055 \c void (*free_drawstate)(drawing *dr, game_drawstate *ds);
1057 This function frees a \c{game_drawstate} structure, and any
1058 subsidiary allocations contained within it.
1060 The parameter \c{dr} is a drawing object (see \k{drawing}), which
1061 might be required if you are freeing a blitter.
1063 \S{backend-preferred-tilesize} \c{preferred_tilesize}
1065 \c int preferred_tilesize;
1067 Each game is required to define a single integer parameter which
1068 expresses, in some sense, the scale at which it is drawn. This is
1069 described in the APIs as \cq{tilesize}, since most puzzles are on a
1070 square (or possibly triangular or hexagonal) grid and hence a
1071 sensible interpretation of this parameter is to define it as the
1072 size of one grid tile in pixels; however, there's no actual
1073 requirement that the \q{tile size} be proportional to the game
1074 window size. Window size is required to increase monotonically with
1075 \q{tile size}, however.
1077 The data element \c{preferred_tilesize} indicates the tile size
1078 which should be used in the absence of a good reason to do otherwise
1079 (such as the screen being too small, or the user explicitly
1080 requesting a resize if that ever gets implemented).
1082 \S{backend-compute-size} \cw{compute_size()}
1084 \c void (*compute_size)(game_params *params, int tilesize,
1087 This function is passed a \c{game_params} structure and a tile size.
1088 It returns, in \c{*x} and \c{*y}, the size in pixels of the drawing
1089 area that would be required to render a puzzle with those parameters
1092 \S{backend-set-size} \cw{set_size()}
1094 \c void (*set_size)(drawing *dr, game_drawstate *ds,
1095 \c game_params *params, int tilesize);
1097 This function is responsible for setting up a \c{game_drawstate} to
1098 draw at a given tile size. Typically this will simply involve
1099 copying the supplied \c{tilesize} parameter into a \c{tilesize}
1100 field inside the draw state; for some more complex games it might
1101 also involve setting up other dimension fields, or possibly
1102 allocating a blitter (see \k{drawing-blitter}).
1104 The parameter \c{dr} is a drawing object (see \k{drawing}), which is
1105 required if a blitter needs to be allocated.
1107 Back ends may assume (and may enforce by assertion) that this
1108 function will be called at most once for any \c{game_drawstate}. If
1109 a puzzle needs to be redrawn at a different size, the mid-end will
1110 create a fresh drawstate.
1112 \S{backend-colours} \cw{colours()}
1114 \c float *(*colours)(frontend *fe, int *ncolours);
1116 This function is responsible for telling the front end what colours
1117 the puzzle will need to draw itself.
1119 It returns the number of colours required in \c{*ncolours}, and the
1120 return value from the function itself is a dynamically allocated
1121 array of three times that many \c{float}s, containing the red, green
1122 and blue components of each colour respectively as numbers in the
1125 The second parameter passed to this function is a front end handle.
1126 The only things it is permitted to do with this handle are to call
1127 the front-end function called \cw{frontend_default_colour()} (see
1128 \k{frontend-default-colour}) or the utility function called
1129 \cw{game_mkhighlight()} (see \k{utils-game-mkhighlight}). (The
1130 latter is a wrapper on the former, so front end implementors only
1131 need to provide \cw{frontend_default_colour()}.) This allows
1132 \cw{colours()} to take local configuration into account when
1133 deciding on its own colour allocations. Most games use the front
1134 end's default colour as their background, apart from a few which
1135 depend on drawing relief highlights so they adjust the background
1136 colour if it's too light for highlights to show up against it.
1138 Note that the colours returned from this function are for
1139 \e{drawing}, not for printing. Printing has an entirely different
1140 colour allocation policy.
1142 \S{backend-anim-length} \cw{anim_length()}
1144 \c float (*anim_length)(game_state *oldstate, game_state *newstate,
1145 \c int dir, game_ui *ui);
1147 This function is called when a move is made, undone or redone. It is
1148 given the old and the new \c{game_state}, and its job is to decide
1149 whether the transition between the two needs to be animated or can
1152 \c{oldstate} is the state that was current until this call;
1153 \c{newstate} is the state that will be current after it. \c{dir}
1154 specifies the chronological order of those states: if it is
1155 positive, then the transition is the result of a move or a redo (and
1156 so \c{newstate} is the later of the two moves), whereas if it is
1157 negative then the transition is the result of an undo (so that
1158 \c{newstate} is the \e{earlier} move).
1160 If this function decides the transition should be animated, it
1161 returns the desired length of the animation in seconds. If not, it
1164 State changes as a result of a Restart operation are never animated;
1165 the mid-end will handle them internally and never consult this
1166 function at all. State changes as a result of Solve operations are
1167 also not animated by default, although you can change this for a
1168 particular game by setting a flag in \c{flags} (\k{backend-flags}).
1170 The function is also passed a pointer to the local \c{game_ui}. It
1171 may refer to information in here to help with its decision (see
1172 \k{writing-conditional-anim} for an example of this), and/or it may
1173 \e{write} information about the nature of the animation which will
1174 be read later by \cw{redraw()}.
1176 When this function is called, it may rely on \cw{changed_state()}
1177 having been called previously, so if \cw{anim_length()} needs to
1178 refer to information in the \c{game_ui}, then \cw{changed_state()}
1179 is a reliable place to have set that information up.
1181 Move animations do not inhibit further input events. If the user
1182 continues playing before a move animation is complete, the animation
1183 will be abandoned and the display will jump straight to the final
1186 \S{backend-flash-length} \cw{flash_length()}
1188 \c float (*flash_length)(game_state *oldstate, game_state *newstate,
1189 \c int dir, game_ui *ui);
1191 This function is called when a move is completed. (\q{Completed}
1192 means that not only has the move been made, but any animation which
1193 accompanied it has finished.) It decides whether the transition from
1194 \c{oldstate} to \c{newstate} merits a \q{flash}.
1196 A flash is much like a move animation, but it is \e{not} interrupted
1197 by further user interface activity; it runs to completion in
1198 parallel with whatever else might be going on on the display. The
1199 only thing which will rush a flash to completion is another flash.
1201 The purpose of flashes is to indicate that the game has been
1202 completed. They were introduced as a separate concept from move
1203 animations because of Net: the habit of most Net players (and
1204 certainly me) is to rotate a tile into place and immediately lock
1205 it, then move on to another tile. When you make your last move, at
1206 the instant the final tile is rotated into place the screen starts
1207 to flash to indicate victory \dash but if you then press the lock
1208 button out of habit, then the move animation is cancelled, and the
1209 victory flash does not complete. (And if you \e{don't} press the
1210 lock button, the completed grid will look untidy because there will
1211 be one unlocked square.) Therefore, I introduced a specific concept
1212 of a \q{flash} which is separate from a move animation and can
1213 proceed in parallel with move animations and any other display
1214 activity, so that the victory flash in Net is not cancelled by that
1217 The input parameters to \cw{flash_length()} are exactly the same as
1218 the ones to \cw{anim_length()}.
1220 Just like \cw{anim_length()}, when this function is called, it may
1221 rely on \cw{changed_state()} having been called previously, so if it
1222 needs to refer to information in the \c{game_ui} then
1223 \cw{changed_state()} is a reliable place to have set that
1226 (Some games use flashes to indicate defeat as well as victory;
1227 Mines, for example, flashes in a different colour when you tread on
1228 a mine from the colour it uses when you complete the game. In order
1229 to achieve this, its \cw{flash_length()} function has to store a
1230 flag in the \c{game_ui} to indicate which flash type is required.)
1232 \S{backend-status} \cw{status()}
1234 \c int (*status)(game_state *state);
1236 This function returns a status value indicating whether the current
1237 game is still in play, or has been won, or has been conclusively lost.
1238 The mid-end uses this to implement \cw{midend_status()}
1239 (\k{midend-status}).
1241 The return value should be +1 if the game has been successfully
1242 solved. If the game has been lost in a situation where further play is
1243 unlikely, the return value should be -1. If neither is true (so play
1244 is still ongoing), return zero.
1246 Front ends may wish to use a non-zero status as a cue to proactively
1247 offer the option of starting a new game. Therefore, back ends should
1248 not return -1 if the game has been \e{technically} lost but undoing
1249 and continuing is still a realistic possibility.
1251 (For instance, games with hidden information such as Guess or Mines
1252 might well return a non-zero status whenever they reveal the solution,
1253 whether or not the player guessed it correctly, on the grounds that a
1254 player would be unlikely to hide the solution and continue playing
1255 after the answer was spoiled. On the other hand, games where you can
1256 merely get into a dead end such as Same Game or Inertia might choose
1257 to return 0 in that situation, on the grounds that the player would
1258 quite likely press Undo and carry on playing.)
1260 \S{backend-redraw} \cw{redraw()}
1262 \c void (*redraw)(drawing *dr, game_drawstate *ds,
1263 \c game_state *oldstate, game_state *newstate, int dir,
1264 \c game_ui *ui, float anim_time, float flash_time);
1266 This function is responsible for actually drawing the contents of
1267 the game window, and for redrawing every time the game state or the
1268 \c{game_ui} changes.
1270 The parameter \c{dr} is a drawing object which may be passed to the
1271 drawing API functions (see \k{drawing} for documentation of the
1272 drawing API). This function may not save \c{dr} and use it
1273 elsewhere; it must only use it for calling back to the drawing API
1274 functions within its own lifetime.
1276 \c{ds} is the local \c{game_drawstate}, of course, and \c{ui} is the
1279 \c{newstate} is the semantically-current game state, and is always
1280 non-\cw{NULL}. If \c{oldstate} is also non-\cw{NULL}, it means that
1281 a move has recently been made and the game is still in the process
1282 of displaying an animation linking the old and new states; in this
1283 situation, \c{anim_time} will give the length of time (in seconds)
1284 that the animation has already been running. If \c{oldstate} is
1285 \cw{NULL}, then \c{anim_time} is unused (and will hopefully be set
1286 to zero to avoid confusion).
1288 \c{flash_time}, if it is is non-zero, denotes that the game is in
1289 the middle of a flash, and gives the time since the start of the
1290 flash. See \k{backend-flash-length} for general discussion of
1293 The very first time this function is called for a new
1294 \c{game_drawstate}, it is expected to redraw the \e{entire} drawing
1295 area. Since this often involves drawing visual furniture which is
1296 never subsequently altered, it is often simplest to arrange this by
1297 having a special \q{first time} flag in the draw state, and
1298 resetting it after the first redraw.
1300 When this function (or any subfunction) calls the drawing API, it is
1301 expected to pass colour indices which were previously defined by the
1302 \cw{colours()} function.
1304 \H{backend-printing} Printing functions
1306 This section discusses the back end functions that deal with
1307 printing puzzles out on paper.
1309 \S{backend-can-print} \c{can_print}
1313 This flag is set to \cw{TRUE} if the puzzle is capable of printing
1314 itself on paper. (This makes sense for some puzzles, such as Solo,
1315 which can be filled in with a pencil. Other puzzles, such as
1316 Twiddle, inherently involve moving things around and so would not
1317 make sense to print.)
1319 If this flag is \cw{FALSE}, then the functions \cw{print_size()}
1320 and \cw{print()} will never be called.
1322 \S{backend-can-print-in-colour} \c{can_print_in_colour}
1324 \c int can_print_in_colour;
1326 This flag is set to \cw{TRUE} if the puzzle is capable of printing
1327 itself differently when colour is available. For example, Map can
1328 actually print coloured regions in different \e{colours} rather than
1329 resorting to cross-hatching.
1331 If the \c{can_print} flag is \cw{FALSE}, then this flag will be
1334 \S{backend-print-size} \cw{print_size()}
1336 \c void (*print_size)(game_params *params, float *x, float *y);
1338 This function is passed a \c{game_params} structure and a tile size.
1339 It returns, in \c{*x} and \c{*y}, the preferred size in
1340 \e{millimetres} of that puzzle if it were to be printed out on paper.
1342 If the \c{can_print} flag is \cw{FALSE}, this function will never be
1345 \S{backend-print} \cw{print()}
1347 \c void (*print)(drawing *dr, game_state *state, int tilesize);
1349 This function is called when a puzzle is to be printed out on paper.
1350 It should use the drawing API functions (see \k{drawing}) to print
1353 This function is separate from \cw{redraw()} because it is often
1356 \b The printing function may not depend on pixel accuracy, since
1357 printer resolution is variable. Draw as if your canvas had infinite
1360 \b The printing function sometimes needs to display things in a
1361 completely different style. Net, for example, is very different as
1362 an on-screen puzzle and as a printed one.
1364 \b The printing function is often much simpler since it has no need
1365 to deal with repeated partial redraws.
1367 However, there's no reason the printing and redraw functions can't
1368 share some code if they want to.
1370 When this function (or any subfunction) calls the drawing API, the
1371 colour indices it passes should be colours which have been allocated
1372 by the \cw{print_*_colour()} functions within this execution of
1373 \cw{print()}. This is very different from the fixed small number of
1374 colours used in \cw{redraw()}, because printers do not have a
1375 limitation on the total number of colours that may be used. Some
1376 puzzles' printing functions might wish to allocate only one \q{ink}
1377 colour and use it for all drawing; others might wish to allocate
1378 \e{more} colours than are used on screen.
1380 One possible colour policy worth mentioning specifically is that a
1381 puzzle's printing function might want to allocate the \e{same}
1382 colour indices as are used by the redraw function, so that code
1383 shared between drawing and printing does not have to keep switching
1384 its colour indices. In order to do this, the simplest thing is to
1385 make use of the fact that colour indices returned from
1386 \cw{print_*_colour()} are guaranteed to be in increasing order from
1387 zero. So if you have declared an \c{enum} defining three colours
1388 \cw{COL_BACKGROUND}, \cw{COL_THIS} and \cw{COL_THAT}, you might then
1392 \c c = print_mono_colour(dr, 1); assert(c == COL_BACKGROUND);
1393 \c c = print_mono_colour(dr, 0); assert(c == COL_THIS);
1394 \c c = print_mono_colour(dr, 0); assert(c == COL_THAT);
1396 If the \c{can_print} flag is \cw{FALSE}, this function will never be
1399 \H{backend-misc} Miscellaneous
1401 \S{backend-can-format-as-text-ever} \c{can_format_as_text_ever}
1403 \c int can_format_as_text_ever;
1405 This boolean field is \cw{TRUE} if the game supports formatting a
1406 game state as ASCII text (typically ASCII art) for copying to the
1407 clipboard and pasting into other applications. If it is \cw{FALSE},
1408 front ends will not offer the \q{Copy} command at all.
1410 If this field is \cw{TRUE}, the game does not necessarily have to
1411 support text formatting for \e{all} games: e.g. a game which can be
1412 played on a square grid or a triangular one might only support copy
1413 and paste for the former, because triangular grids in ASCII art are
1416 If this field is \cw{FALSE}, the functions
1417 \cw{can_format_as_text_now()} (\k{backend-can-format-as-text-now})
1418 and \cw{text_format()} (\k{backend-text-format}) are never called.
1420 \S{backend-can-format-as-text-now} \c{can_format_as_text_now()}
1422 \c int (*can_format_as_text_now)(game_params *params);
1424 This function is passed a \c{game_params} and returns a boolean,
1425 which is \cw{TRUE} if the game can support ASCII text output for
1426 this particular game type. If it returns \cw{FALSE}, front ends will
1427 grey out or otherwise disable the \q{Copy} command.
1429 Games may enable and disable the copy-and-paste function for
1430 different game \e{parameters}, but are currently constrained to
1431 return the same answer from this function for all game \e{states}
1432 sharing the same parameters. In other words, the \q{Copy} function
1433 may enable or disable itself when the player changes game preset,
1434 but will never change during play of a single game or when another
1435 game of exactly the same type is generated.
1437 This function should not take into account aspects of the game
1438 parameters which are not encoded by \cw{encode_params()}
1439 (\k{backend-encode-params}) when the \c{full} parameter is set to
1440 \cw{FALSE}. Such parameters will not necessarily match up between a
1441 call to this function and a subsequent call to \cw{text_format()}
1442 itself. (For instance, game \e{difficulty} should not affect whether
1443 the game can be copied to the clipboard. Only the actual visible
1444 \e{shape} of the game can affect that.)
1446 \S{backend-text-format} \cw{text_format()}
1448 \c char *(*text_format)(game_state *state);
1450 This function is passed a \c{game_state}, and returns a newly
1451 allocated C string containing an ASCII representation of that game
1452 state. It is used to implement the \q{Copy} operation in many front
1455 This function will only ever be called if the back end field
1456 \c{can_format_as_text_ever} (\k{backend-can-format-as-text-ever}) is
1457 \cw{TRUE} \e{and} the function \cw{can_format_as_text_now()}
1458 (\k{backend-can-format-as-text-now}) has returned \cw{TRUE} for the
1459 currently selected game parameters.
1461 The returned string may contain line endings (and will probably want
1462 to), using the normal C internal \cq{\\n} convention. For
1463 consistency between puzzles, all multi-line textual puzzle
1464 representations should \e{end} with a newline as well as containing
1465 them internally. (There are currently no puzzles which have a
1466 one-line ASCII representation, so there's no precedent yet for
1467 whether that should come with a newline or not.)
1469 \S{backend-wants-statusbar} \cw{wants_statusbar}
1471 \c int wants_statusbar;
1473 This boolean field is set to \cw{TRUE} if the puzzle has a use for a
1474 textual status line (to display score, completion status, currently
1477 \S{backend-is-timed} \c{is_timed}
1481 This boolean field is \cw{TRUE} if the puzzle is time-critical. If
1482 so, the mid-end will maintain a game timer while the user plays.
1484 If this field is \cw{FALSE}, then \cw{timing_state()} will never be
1485 called and need not do anything.
1487 \S{backend-timing-state} \cw{timing_state()}
1489 \c int (*timing_state)(game_state *state, game_ui *ui);
1491 This function is passed the current \c{game_state} and the local
1492 \c{game_ui}; it returns \cw{TRUE} if the game timer should currently
1495 A typical use for the \c{game_ui} in this function is to note when
1496 the game was first completed (by setting a flag in
1497 \cw{changed_state()} \dash see \k{backend-changed-state}), and
1498 freeze the timer thereafter so that the user can undo back through
1499 their solution process without altering their time.
1501 \S{backend-flags} \c{flags}
1505 This field contains miscellaneous per-backend flags. It consists of
1506 the bitwise OR of some combination of the following:
1508 \dt \cw{BUTTON_BEATS(x,y)}
1510 \dd Given any \cw{x} and \cw{y} from the set \{\cw{LEFT_BUTTON},
1511 \cw{MIDDLE_BUTTON}, \cw{RIGHT_BUTTON}\}, this macro evaluates to a
1512 bit flag which indicates that when buttons \cw{x} and \cw{y} are
1513 both pressed simultaneously, the mid-end should consider \cw{x} to
1514 have priority. (In the absence of any such flags, the mid-end will
1515 always consider the most recently pressed button to have priority.)
1517 \dt \cw{SOLVE_ANIMATES}
1519 \dd This flag indicates that moves generated by \cw{solve()}
1520 (\k{backend-solve}) are candidates for animation just like any other
1521 move. For most games, solve moves should not be animated, so the
1522 mid-end doesn't even bother calling \cw{anim_length()}
1523 (\k{backend-anim-length}), thus saving some special-case code in
1524 each game. On the rare occasion that animated solve moves are
1525 actually required, you can set this flag.
1527 \dt \cw{REQUIRE_RBUTTON}
1529 \dd This flag indicates that the puzzle cannot be usefully played
1530 without the use of mouse buttons other than the left one. On some
1531 PDA platforms, this flag is used by the front end to enable
1532 right-button emulation through an appropriate gesture. Note that a
1533 puzzle is not required to set this just because it \e{uses} the
1534 right button, but only if its use of the right button is critical to
1535 playing the game. (Slant, for example, uses the right button to
1536 cycle through the three square states in the opposite order from the
1537 left button, and hence can manage fine without it.)
1539 \dt \cw{REQUIRE_NUMPAD}
1541 \dd This flag indicates that the puzzle cannot be usefully played
1542 without the use of number-key input. On some PDA platforms it causes
1543 an emulated number pad to appear on the screen. Similarly to
1544 \cw{REQUIRE_RBUTTON}, a puzzle need not specify this simply if its
1545 use of the number keys is not critical.
1547 \H{backend-initiative} Things a back end may do on its own initiative
1549 This section describes a couple of things that a back end may choose
1550 to do by calling functions elsewhere in the program, which would not
1551 otherwise be obvious.
1553 \S{backend-newrs} Create a random state
1555 If a back end needs random numbers at some point during normal play,
1556 it can create a fresh \c{random_state} by first calling
1557 \c{get_random_seed} (\k{frontend-get-random-seed}) and then passing
1558 the returned seed data to \cw{random_new()}.
1560 This is likely not to be what you want. If a puzzle needs randomness
1561 in the middle of play, it's likely to be more sensible to store some
1562 sort of random state within the \c{game_state}, so that the random
1563 numbers are tied to the particular game state and hence the player
1564 can't simply keep undoing their move until they get numbers they
1567 This facility is currently used only in Net, to implement the
1568 \q{jumble} command, which sets every unlocked tile to a new random
1569 orientation. This randomness \e{is} a reasonable use of the feature,
1570 because it's non-adversarial \dash there's no advantage to the user
1571 in getting different random numbers.
1573 \S{backend-supersede} Supersede its own game description
1575 In response to a move, a back end is (reluctantly) permitted to call
1576 \cw{midend_supersede_game_desc()}:
1578 \c void midend_supersede_game_desc(midend *me,
1579 \c char *desc, char *privdesc);
1581 When the user selects \q{New Game}, the mid-end calls
1582 \cw{new_desc()} (\k{backend-new-desc}) to get a new game
1583 description, and (as well as using that to generate an initial game
1584 state) stores it for the save file and for telling to the user. The
1585 function above overwrites that game description, and also splits it
1586 in two. \c{desc} becomes the new game description which is provided
1587 to the user on request, and is also the one used to construct a new
1588 initial game state if the user selects \q{Restart}. \c{privdesc} is
1589 a \q{private} game description, used to reconstruct the game's
1590 initial state when reloading.
1592 The distinction between the two, as well as the need for this
1593 function at all, comes from Mines. Mines begins with a blank grid
1594 and no idea of where the mines actually are; \cw{new_desc()} does
1595 almost no work in interactive mode, and simply returns a string
1596 encoding the \c{random_state}. When the user first clicks to open a
1597 tile, \e{then} Mines generates the mine positions, in such a way
1598 that the game is soluble from that starting point. Then it uses this
1599 function to supersede the random-state game description with a
1600 proper one. But it needs two: one containing the initial click
1601 location (because that's what you want to happen if you restart the
1602 game, and also what you want to send to a friend so that they play
1603 \e{the same game} as you), and one without the initial click
1604 location (because when you save and reload the game, you expect to
1605 see the same blank initial state as you had before saving).
1607 I should stress again that this function is a horrid hack. Nobody
1608 should use it if they're not Mines; if you think you need to use it,
1609 think again repeatedly in the hope of finding a better way to do
1610 whatever it was you needed to do.
1612 \C{drawing} The drawing API
1614 The back end function \cw{redraw()} (\k{backend-redraw}) is required
1615 to draw the puzzle's graphics on the window's drawing area, or on
1616 paper if the puzzle is printable. To do this portably, it is
1617 provided with a drawing API allowing it to talk directly to the
1618 front end. In this chapter I document that API, both for the benefit
1619 of back end authors trying to use it and for front end authors
1620 trying to implement it.
1622 The drawing API as seen by the back end is a collection of global
1623 functions, each of which takes a pointer to a \c{drawing} structure
1624 (a \q{drawing object}). These objects are supplied as parameters to
1625 the back end's \cw{redraw()} and \cw{print()} functions.
1627 In fact these global functions are not implemented directly by the
1628 front end; instead, they are implemented centrally in \c{drawing.c}
1629 and form a small piece of middleware. The drawing API as supplied by
1630 the front end is a structure containing a set of function pointers,
1631 plus a \cq{void *} handle which is passed to each of those
1632 functions. This enables a single front end to switch between
1633 multiple implementations of the drawing API if necessary. For
1634 example, the Windows API supplies a printing mechanism integrated
1635 into the same GDI which deals with drawing in windows, and therefore
1636 the same API implementation can handle both drawing and printing;
1637 but on Unix, the most common way for applications to print is by
1638 producing PostScript output directly, and although it would be
1639 \e{possible} to write a single (say) \cw{draw_rect()} function which
1640 checked a global flag to decide whether to do GTK drawing operations
1641 or output PostScript to a file, it's much nicer to have two separate
1642 functions and switch between them as appropriate.
1644 When drawing, the puzzle window is indexed by pixel coordinates,
1645 with the top left pixel defined as \cw{(0,0)} and the bottom right
1646 pixel \cw{(w-1,h-1)}, where \c{w} and \c{h} are the width and height
1647 values returned by the back end function \cw{compute_size()}
1648 (\k{backend-compute-size}).
1650 When printing, the puzzle's print area is indexed in exactly the
1651 same way (with an arbitrary tile size provided by the printing
1652 module \c{printing.c}), to facilitate sharing of code between the
1653 drawing and printing routines. However, when printing, puzzles may
1654 no longer assume that the coordinate unit has any relationship to a
1655 pixel; the printer's actual resolution might very well not even be
1656 known at print time, so the coordinate unit might be smaller or
1657 larger than a pixel. Puzzles' print functions should restrict
1658 themselves to drawing geometric shapes rather than fiddly pixel
1661 \e{Puzzles' redraw functions may assume that the surface they draw
1662 on is persistent}. It is the responsibility of every front end to
1663 preserve the puzzle's window contents in the face of GUI window
1664 expose issues and similar. It is not permissible to request that the
1665 back end redraw any part of a window that it has already drawn,
1666 unless something has actually changed as a result of making moves in
1669 Most front ends accomplish this by having the drawing routines draw
1670 on a stored bitmap rather than directly on the window, and copying
1671 the bitmap to the window every time a part of the window needs to be
1672 redrawn. Therefore, it is vitally important that whenever the back
1673 end does any drawing it informs the front end of which parts of the
1674 window it has accessed, and hence which parts need repainting. This
1675 is done by calling \cw{draw_update()} (\k{drawing-draw-update}).
1677 Persistence of old drawing is convenient. However, a puzzle should
1678 be very careful about how it updates its drawing area. The problem
1679 is that some front ends do anti-aliased drawing: rather than simply
1680 choosing between leaving each pixel untouched or painting it a
1681 specified colour, an antialiased drawing function will \e{blend} the
1682 original and new colours in pixels at a figure's boundary according
1683 to the proportion of the pixel occupied by the figure (probably
1684 modified by some heuristic fudge factors). All of this produces a
1685 smoother appearance for curves and diagonal lines.
1687 An unfortunate effect of drawing an anti-aliased figure repeatedly
1688 is that the pixels around the figure's boundary come steadily more
1689 saturated with \q{ink} and the boundary appears to \q{spread out}.
1690 Worse, redrawing a figure in a different colour won't fully paint
1691 over the old boundary pixels, so the end result is a rather ugly
1694 A good strategy to avoid unpleasant anti-aliasing artifacts is to
1695 identify a number of rectangular areas which need to be redrawn,
1696 clear them to the background colour, and then redraw their contents
1697 from scratch, being careful all the while not to stray beyond the
1698 boundaries of the original rectangles. The \cw{clip()} function
1699 (\k{drawing-clip}) comes in very handy here. Games based on a square
1700 grid can often do this fairly easily. Other games may need to be
1701 somewhat more careful. For example, Loopy's redraw function first
1702 identifies portions of the display which need to be updated. Then,
1703 if the changes are fairly well localised, it clears and redraws a
1704 rectangle containing each changed area. Otherwise, it gives up and
1705 redraws the entire grid from scratch.
1707 It is possible to avoid clearing to background and redrawing from
1708 scratch if one is very careful about which drawing functions one
1709 uses: if a function is documented as not anti-aliasing under some
1710 circumstances, you can rely on each pixel in a drawing either being
1711 left entirely alone or being set to the requested colour, with no
1712 blending being performed.
1714 In the following sections I first discuss the drawing API as seen by
1715 the back end, and then the \e{almost} identical function-pointer
1716 form seen by the front end.
1718 \H{drawing-backend} Drawing API as seen by the back end
1720 This section documents the back-end drawing API, in the form of
1721 functions which take a \c{drawing} object as an argument.
1723 \S{drawing-draw-rect} \cw{draw_rect()}
1725 \c void draw_rect(drawing *dr, int x, int y, int w, int h,
1728 Draws a filled rectangle in the puzzle window.
1730 \c{x} and \c{y} give the coordinates of the top left pixel of the
1731 rectangle. \c{w} and \c{h} give its width and height. Thus, the
1732 horizontal extent of the rectangle runs from \c{x} to \c{x+w-1}
1733 inclusive, and the vertical extent from \c{y} to \c{y+h-1}
1736 \c{colour} is an integer index into the colours array returned by
1737 the back end function \cw{colours()} (\k{backend-colours}).
1739 There is no separate pixel-plotting function. If you want to plot a
1740 single pixel, the approved method is to use \cw{draw_rect()} with
1741 width and height set to 1.
1743 Unlike many of the other drawing functions, this function is
1744 guaranteed to be pixel-perfect: the rectangle will be sharply
1745 defined and not anti-aliased or anything like that.
1747 This function may be used for both drawing and printing.
1749 \S{drawing-draw-rect-outline} \cw{draw_rect_outline()}
1751 \c void draw_rect_outline(drawing *dr, int x, int y, int w, int h,
1754 Draws an outline rectangle in the puzzle window.
1756 \c{x} and \c{y} give the coordinates of the top left pixel of the
1757 rectangle. \c{w} and \c{h} give its width and height. Thus, the
1758 horizontal extent of the rectangle runs from \c{x} to \c{x+w-1}
1759 inclusive, and the vertical extent from \c{y} to \c{y+h-1}
1762 \c{colour} is an integer index into the colours array returned by
1763 the back end function \cw{colours()} (\k{backend-colours}).
1765 From a back end perspective, this function may be considered to be
1766 part of the drawing API. However, front ends are not required to
1767 implement it, since it is actually implemented centrally (in
1768 \cw{misc.c}) as a wrapper on \cw{draw_polygon()}.
1770 This function may be used for both drawing and printing.
1772 \S{drawing-draw-line} \cw{draw_line()}
1774 \c void draw_line(drawing *dr, int x1, int y1, int x2, int y2,
1777 Draws a straight line in the puzzle window.
1779 \c{x1} and \c{y1} give the coordinates of one end of the line.
1780 \c{x2} and \c{y2} give the coordinates of the other end. The line
1781 drawn includes both those points.
1783 \c{colour} is an integer index into the colours array returned by
1784 the back end function \cw{colours()} (\k{backend-colours}).
1786 Some platforms may perform anti-aliasing on this function.
1787 Therefore, do not assume that you can erase a line by drawing the
1788 same line over it in the background colour; anti-aliasing might lead
1789 to perceptible ghost artefacts around the vanished line. Horizontal
1790 and vertical lines, however, are pixel-perfect and not anti-aliased.
1792 This function may be used for both drawing and printing.
1794 \S{drawing-draw-polygon} \cw{draw_polygon()}
1796 \c void draw_polygon(drawing *dr, int *coords, int npoints,
1797 \c int fillcolour, int outlinecolour);
1799 Draws an outlined or filled polygon in the puzzle window.
1801 \c{coords} is an array of \cw{(2*npoints)} integers, containing the
1802 \c{x} and \c{y} coordinates of \c{npoints} vertices.
1804 \c{fillcolour} and \c{outlinecolour} are integer indices into the
1805 colours array returned by the back end function \cw{colours()}
1806 (\k{backend-colours}). \c{fillcolour} may also be \cw{-1} to
1807 indicate that the polygon should be outlined only.
1809 The polygon defined by the specified list of vertices is first
1810 filled in \c{fillcolour}, if specified, and then outlined in
1813 \c{outlinecolour} may \e{not} be \cw{-1}; it must be a valid colour
1814 (and front ends are permitted to enforce this by assertion). This is
1815 because different platforms disagree on whether a filled polygon
1816 should include its boundary line or not, so drawing \e{only} a
1817 filled polygon would have non-portable effects. If you want your
1818 filled polygon not to have a visible outline, you must set
1819 \c{outlinecolour} to the same as \c{fillcolour}.
1821 Some platforms may perform anti-aliasing on this function.
1822 Therefore, do not assume that you can erase a polygon by drawing the
1823 same polygon over it in the background colour. Also, be prepared for
1824 the polygon to extend a pixel beyond its obvious bounding box as a
1825 result of this; if you really need it not to do this to avoid
1826 interfering with other delicate graphics, you should probably use
1827 \cw{clip()} (\k{drawing-clip}). You can rely on horizontal and
1828 vertical lines not being anti-aliased.
1830 This function may be used for both drawing and printing.
1832 \S{drawing-draw-circle} \cw{draw_circle()}
1834 \c void draw_circle(drawing *dr, int cx, int cy, int radius,
1835 \c int fillcolour, int outlinecolour);
1837 Draws an outlined or filled circle in the puzzle window.
1839 \c{cx} and \c{cy} give the coordinates of the centre of the circle.
1840 \c{radius} gives its radius. The total horizontal pixel extent of
1841 the circle is from \c{cx-radius+1} to \c{cx+radius-1} inclusive, and
1842 the vertical extent similarly around \c{cy}.
1844 \c{fillcolour} and \c{outlinecolour} are integer indices into the
1845 colours array returned by the back end function \cw{colours()}
1846 (\k{backend-colours}). \c{fillcolour} may also be \cw{-1} to
1847 indicate that the circle should be outlined only.
1849 The circle is first filled in \c{fillcolour}, if specified, and then
1850 outlined in \c{outlinecolour}.
1852 \c{outlinecolour} may \e{not} be \cw{-1}; it must be a valid colour
1853 (and front ends are permitted to enforce this by assertion). This is
1854 because different platforms disagree on whether a filled circle
1855 should include its boundary line or not, so drawing \e{only} a
1856 filled circle would have non-portable effects. If you want your
1857 filled circle not to have a visible outline, you must set
1858 \c{outlinecolour} to the same as \c{fillcolour}.
1860 Some platforms may perform anti-aliasing on this function.
1861 Therefore, do not assume that you can erase a circle by drawing the
1862 same circle over it in the background colour. Also, be prepared for
1863 the circle to extend a pixel beyond its obvious bounding box as a
1864 result of this; if you really need it not to do this to avoid
1865 interfering with other delicate graphics, you should probably use
1866 \cw{clip()} (\k{drawing-clip}).
1868 This function may be used for both drawing and printing.
1870 \S{drawing-draw-thick-line} \cw{draw_thick_line()}
1872 \c void draw_thick_line(drawing *dr, float thickness,
1873 \c float x1, float y1, float x2, float y2,
1876 Draws a line in the puzzle window, giving control over the line's
1879 \c{x1} and \c{y1} give the coordinates of one end of the line.
1880 \c{x2} and \c{y2} give the coordinates of the other end.
1881 \c{thickness} gives the thickness of the line, in pixels.
1883 Note that the coordinates and thickness are floating-point: the
1884 continuous coordinate system is in effect here. It's important to
1885 be able to address points with better-than-pixel precision in this
1886 case, because one can't otherwise properly express the endpoints of
1887 lines with both odd and even thicknesses.
1889 Some platforms may perform anti-aliasing on this function. The
1890 precise pixels affected by a thick-line drawing operation may vary
1891 between platforms, and no particular guarantees are provided.
1892 Indeed, even horizontal or vertical lines may be anti-aliased.
1894 This function may be used for both drawing and printing.
1896 \S{drawing-draw-text} \cw{draw_text()}
1898 \c void draw_text(drawing *dr, int x, int y, int fonttype,
1899 \c int fontsize, int align, int colour, char *text);
1901 Draws text in the puzzle window.
1903 \c{x} and \c{y} give the coordinates of a point. The relation of
1904 this point to the location of the text is specified by \c{align},
1905 which is a bitwise OR of horizontal and vertical alignment flags:
1907 \dt \cw{ALIGN_VNORMAL}
1909 \dd Indicates that \c{y} is aligned with the baseline of the text.
1911 \dt \cw{ALIGN_VCENTRE}
1913 \dd Indicates that \c{y} is aligned with the vertical centre of the
1914 text. (In fact, it's aligned with the vertical centre of normal
1915 \e{capitalised} text: displaying two pieces of text with
1916 \cw{ALIGN_VCENTRE} at the same \cw{y}-coordinate will cause their
1917 baselines to be aligned with one another, even if one is an ascender
1918 and the other a descender.)
1920 \dt \cw{ALIGN_HLEFT}
1922 \dd Indicates that \c{x} is aligned with the left-hand end of the
1925 \dt \cw{ALIGN_HCENTRE}
1927 \dd Indicates that \c{x} is aligned with the horizontal centre of
1930 \dt \cw{ALIGN_HRIGHT}
1932 \dd Indicates that \c{x} is aligned with the right-hand end of the
1935 \c{fonttype} is either \cw{FONT_FIXED} or \cw{FONT_VARIABLE}, for a
1936 monospaced or proportional font respectively. (No more detail than
1937 that may be specified; it would only lead to portability issues
1938 between different platforms.)
1940 \c{fontsize} is the desired size, in pixels, of the text. This size
1941 corresponds to the overall point size of the text, not to any
1942 internal dimension such as the cap-height.
1944 \c{colour} is an integer index into the colours array returned by
1945 the back end function \cw{colours()} (\k{backend-colours}).
1947 This function may be used for both drawing and printing.
1949 The character set used to encode the text passed to this function is
1950 specified \e{by the drawing object}, although it must be a superset
1951 of ASCII. If a puzzle wants to display text that is not contained in
1952 ASCII, it should use the \cw{text_fallback()} function
1953 (\k{drawing-text-fallback}) to query the drawing object for an
1954 appropriate representation of the characters it wants.
1956 \S{drawing-text-fallback} \cw{text_fallback()}
1958 \c char *text_fallback(drawing *dr, const char *const *strings,
1961 This function is used to request a translation of UTF-8 text into
1962 whatever character encoding is expected by the drawing object's
1963 implementation of \cw{draw_text()}.
1965 The input is a list of strings encoded in UTF-8: \cw{nstrings} gives
1966 the number of strings in the list, and \cw{strings[0]},
1967 \cw{strings[1]}, ..., \cw{strings[nstrings-1]} are the strings
1970 The returned string (which is dynamically allocated and must be
1971 freed when finished with) is derived from the first string in the
1972 list that the drawing object expects to be able to display reliably;
1973 it will consist of that string translated into the character set
1974 expected by \cw{draw_text()}.
1976 Drawing implementations are not required to handle anything outside
1977 ASCII, but are permitted to assume that \e{some} string will be
1978 successfully translated. So every call to this function must include
1979 a string somewhere in the list (presumably the last element) which
1980 consists of nothing but ASCII, to be used by any front end which
1981 cannot handle anything else.
1983 For example, if a puzzle wished to display a string including a
1984 multiplication sign (U+00D7 in Unicode, represented by the bytes C3
1985 97 in UTF-8), it might do something like this:
1987 \c static const char *const times_signs[] = { "\xC3\x97", "x" };
1988 \c char *times_sign = text_fallback(dr, times_signs, 2);
1989 \c sprintf(buffer, "%d%s%d", width, times_sign, height);
1990 \c draw_text(dr, x, y, font, size, align, colour, buffer);
1993 which would draw a string with a times sign in the middle on
1994 platforms that support it, and fall back to a simple ASCII \cq{x}
1995 where there was no alternative.
1997 \S{drawing-clip} \cw{clip()}
1999 \c void clip(drawing *dr, int x, int y, int w, int h);
2001 Establishes a clipping rectangle in the puzzle window.
2003 \c{x} and \c{y} give the coordinates of the top left pixel of the
2004 clipping rectangle. \c{w} and \c{h} give its width and height. Thus,
2005 the horizontal extent of the rectangle runs from \c{x} to \c{x+w-1}
2006 inclusive, and the vertical extent from \c{y} to \c{y+h-1}
2007 inclusive. (These are exactly the same semantics as
2010 After this call, no drawing operation will affect anything outside
2011 the specified rectangle. The effect can be reversed by calling
2012 \cw{unclip()} (\k{drawing-unclip}). The clipping rectangle is
2013 pixel-perfect: pixels within the rectangle are affected as usual by
2014 drawing functions; pixels outside are completely untouched.
2016 Back ends should not assume that a clipping rectangle will be
2017 automatically cleared up by the front end if it's left lying around;
2018 that might work on current front ends, but shouldn't be relied upon.
2019 Always explicitly call \cw{unclip()}.
2021 This function may be used for both drawing and printing.
2023 \S{drawing-unclip} \cw{unclip()}
2025 \c void unclip(drawing *dr);
2027 Reverts the effect of a previous call to \cw{clip()}. After this
2028 call, all drawing operations will be able to affect the entire
2029 puzzle window again.
2031 This function may be used for both drawing and printing.
2033 \S{drawing-draw-update} \cw{draw_update()}
2035 \c void draw_update(drawing *dr, int x, int y, int w, int h);
2037 Informs the front end that a rectangular portion of the puzzle
2038 window has been drawn on and needs to be updated.
2040 \c{x} and \c{y} give the coordinates of the top left pixel of the
2041 update rectangle. \c{w} and \c{h} give its width and height. Thus,
2042 the horizontal extent of the rectangle runs from \c{x} to \c{x+w-1}
2043 inclusive, and the vertical extent from \c{y} to \c{y+h-1}
2044 inclusive. (These are exactly the same semantics as
2047 The back end redraw function \e{must} call this function to report
2048 any changes it has made to the window. Otherwise, those changes may
2049 not become immediately visible, and may then appear at an
2050 unpredictable subsequent time such as the next time the window is
2051 covered and re-exposed.
2053 This function is only important when drawing. It may be called when
2054 printing as well, but doing so is not compulsory, and has no effect.
2055 (So if you have a shared piece of code between the drawing and
2056 printing routines, that code may safely call \cw{draw_update()}.)
2058 \S{drawing-status-bar} \cw{status_bar()}
2060 \c void status_bar(drawing *dr, char *text);
2062 Sets the text in the game's status bar to \c{text}. The text is copied
2063 from the supplied buffer, so the caller is free to deallocate or
2064 modify the buffer after use.
2066 (This function is not exactly a \e{drawing} function, but it shares
2067 with the drawing API the property that it may only be called from
2068 within the back end redraw function, so this is as good a place as
2069 any to document it.)
2071 The supplied text is filtered through the mid-end for optional
2072 rewriting before being passed on to the front end; the mid-end will
2073 prepend the current game time if the game is timed (and may in
2074 future perform other rewriting if it seems like a good idea).
2076 This function is for drawing only; it must never be called during
2079 \S{drawing-blitter} Blitter functions
2081 This section describes a group of related functions which save and
2082 restore a section of the puzzle window. This is most commonly used
2083 to implement user interfaces involving dragging a puzzle element
2084 around the window: at the end of each call to \cw{redraw()}, if an
2085 object is currently being dragged, the back end saves the window
2086 contents under that location and then draws the dragged object, and
2087 at the start of the next \cw{redraw()} the first thing it does is to
2088 restore the background.
2090 The front end defines an opaque type called a \c{blitter}, which is
2091 capable of storing a rectangular area of a specified size.
2093 Blitter functions are for drawing only; they must never be called
2096 \S2{drawing-blitter-new} \cw{blitter_new()}
2098 \c blitter *blitter_new(drawing *dr, int w, int h);
2100 Creates a new blitter object which stores a rectangle of size \c{w}
2101 by \c{h} pixels. Returns a pointer to the blitter object.
2103 Blitter objects are best stored in the \c{game_drawstate}. A good
2104 time to create them is in the \cw{set_size()} function
2105 (\k{backend-set-size}), since it is at this point that you first
2106 know how big a rectangle they will need to save.
2108 \S2{drawing-blitter-free} \cw{blitter_free()}
2110 \c void blitter_free(drawing *dr, blitter *bl);
2112 Disposes of a blitter object. Best called in \cw{free_drawstate()}.
2113 (However, check that the blitter object is not \cw{NULL} before
2114 attempting to free it; it is possible that a draw state might be
2115 created and freed without ever having \cw{set_size()} called on it
2118 \S2{drawing-blitter-save} \cw{blitter_save()}
2120 \c void blitter_save(drawing *dr, blitter *bl, int x, int y);
2122 This is a true drawing API function, in that it may only be called
2123 from within the game redraw routine. It saves a rectangular portion
2124 of the puzzle window into the specified blitter object.
2126 \c{x} and \c{y} give the coordinates of the top left corner of the
2127 saved rectangle. The rectangle's width and height are the ones
2128 specified when the blitter object was created.
2130 This function is required to cope and do the right thing if \c{x}
2131 and \c{y} are out of range. (The right thing probably means saving
2132 whatever part of the blitter rectangle overlaps with the visible
2133 area of the puzzle window.)
2135 \S2{drawing-blitter-load} \cw{blitter_load()}
2137 \c void blitter_load(drawing *dr, blitter *bl, int x, int y);
2139 This is a true drawing API function, in that it may only be called
2140 from within the game redraw routine. It restores a rectangular
2141 portion of the puzzle window from the specified blitter object.
2143 \c{x} and \c{y} give the coordinates of the top left corner of the
2144 rectangle to be restored. The rectangle's width and height are the
2145 ones specified when the blitter object was created.
2147 Alternatively, you can specify both \c{x} and \c{y} as the special
2148 value \cw{BLITTER_FROMSAVED}, in which case the rectangle will be
2149 restored to exactly where it was saved from. (This is probably what
2150 you want to do almost all the time, if you're using blitters to
2151 implement draggable puzzle elements.)
2153 This function is required to cope and do the right thing if \c{x}
2154 and \c{y} (or the equivalent ones saved in the blitter) are out of
2155 range. (The right thing probably means restoring whatever part of
2156 the blitter rectangle overlaps with the visible area of the puzzle
2159 If this function is called on a blitter which had previously been
2160 saved from a partially out-of-range rectangle, then the parts of the
2161 saved bitmap which were not visible at save time are undefined. If
2162 the blitter is restored to a different position so as to make those
2163 parts visible, the effect on the drawing area is undefined.
2165 \S{print-mono-colour} \cw{print_mono_colour()}
2167 \c int print_mono_colour(drawing *dr, int grey);
2169 This function allocates a colour index for a simple monochrome
2170 colour during printing.
2172 \c{grey} must be 0 or 1. If \c{grey} is 0, the colour returned is
2173 black; if \c{grey} is 1, the colour is white.
2175 \S{print-grey-colour} \cw{print_grey_colour()}
2177 \c int print_grey_colour(drawing *dr, float grey);
2179 This function allocates a colour index for a grey-scale colour
2182 \c{grey} may be any number between 0 (black) and 1 (white); for
2183 example, 0.5 indicates a medium grey.
2185 The chosen colour will be rendered to the limits of the printer's
2186 halftoning capability.
2188 \S{print-hatched-colour} \cw{print_hatched_colour()}
2190 \c int print_hatched_colour(drawing *dr, int hatch);
2192 This function allocates a colour index which does not represent a
2193 literal \e{colour}. Instead, regions shaded in this colour will be
2194 hatched with parallel lines. The \c{hatch} parameter defines what
2195 type of hatching should be used in place of this colour:
2197 \dt \cw{HATCH_SLASH}
2199 \dd This colour will be hatched by lines slanting to the right at 45
2202 \dt \cw{HATCH_BACKSLASH}
2204 \dd This colour will be hatched by lines slanting to the left at 45
2207 \dt \cw{HATCH_HORIZ}
2209 \dd This colour will be hatched by horizontal lines.
2213 \dd This colour will be hatched by vertical lines.
2217 \dd This colour will be hatched by criss-crossing horizontal and
2222 \dd This colour will be hatched by criss-crossing diagonal lines.
2224 Colours defined to use hatching may not be used for drawing lines or
2225 text; they may only be used for filling areas. That is, they may be
2226 used as the \c{fillcolour} parameter to \cw{draw_circle()} and
2227 \cw{draw_polygon()}, and as the colour parameter to
2228 \cw{draw_rect()}, but may not be used as the \c{outlinecolour}
2229 parameter to \cw{draw_circle()} or \cw{draw_polygon()}, or with
2230 \cw{draw_line()} or \cw{draw_text()}.
2232 \S{print-rgb-mono-colour} \cw{print_rgb_mono_colour()}
2234 \c int print_rgb_mono_colour(drawing *dr, float r, float g,
2235 \c float b, float grey);
2237 This function allocates a colour index for a fully specified RGB
2238 colour during printing.
2240 \c{r}, \c{g} and \c{b} may each be anywhere in the range from 0 to 1.
2242 If printing in black and white only, these values will be ignored,
2243 and either pure black or pure white will be used instead, according
2244 to the \q{grey} parameter. (The fallback colour is the same as the
2245 one which would be allocated by \cw{print_mono_colour(grey)}.)
2247 \S{print-rgb-grey-colour} \cw{print_rgb_grey_colour()}
2249 \c int print_rgb_grey_colour(drawing *dr, float r, float g,
2250 \c float b, float grey);
2252 This function allocates a colour index for a fully specified RGB
2253 colour during printing.
2255 \c{r}, \c{g} and \c{b} may each be anywhere in the range from 0 to 1.
2257 If printing in black and white only, these values will be ignored,
2258 and a shade of grey given by the \c{grey} parameter will be used
2259 instead. (The fallback colour is the same as the one which would be
2260 allocated by \cw{print_grey_colour(grey)}.)
2262 \S{print-rgb-hatched-colour} \cw{print_rgb_hatched_colour()}
2264 \c int print_rgb_hatched_colour(drawing *dr, float r, float g,
2265 \c float b, float hatched);
2267 This function allocates a colour index for a fully specified RGB
2268 colour during printing.
2270 \c{r}, \c{g} and \c{b} may each be anywhere in the range from 0 to 1.
2272 If printing in black and white only, these values will be ignored,
2273 and a form of cross-hatching given by the \c{hatch} parameter will
2274 be used instead; see \k{print-hatched-colour} for the possible
2275 values of this parameter. (The fallback colour is the same as the
2276 one which would be allocated by \cw{print_hatched_colour(hatch)}.)
2278 \S{print-line-width} \cw{print_line_width()}
2280 \c void print_line_width(drawing *dr, int width);
2282 This function is called to set the thickness of lines drawn during
2283 printing. It is meaningless in drawing: all lines drawn by
2284 \cw{draw_line()}, \cw{draw_circle} and \cw{draw_polygon()} are one
2285 pixel in thickness. However, in printing there is no clear
2286 definition of a pixel and so line widths must be explicitly
2289 The line width is specified in the usual coordinate system. Note,
2290 however, that it is a hint only: the central printing system may
2291 choose to vary line thicknesses at user request or due to printer
2294 \S{print-line-dotted} \cw{print_line_dotted()}
2296 \c void print_line_dotted(drawing *dr, int dotted);
2298 This function is called to toggle the drawing of dotted lines during
2299 printing. It is not supported during drawing.
2301 The parameter \cq{dotted} is a boolean; \cw{TRUE} means that future
2302 lines drawn by \cw{draw_line()}, \cw{draw_circle} and
2303 \cw{draw_polygon()} will be dotted, and \cw{FALSE} means that they
2306 Some front ends may impose restrictions on the width of dotted
2307 lines. Asking for a dotted line via this front end will override any
2308 line width request if the front end requires it.
2310 \H{drawing-frontend} The drawing API as implemented by the front end
2312 This section describes the drawing API in the function-pointer form
2313 in which it is implemented by a front end.
2315 (It isn't only platform-specific front ends which implement this
2316 API; the platform-independent module \c{ps.c} also provides an
2317 implementation of it which outputs PostScript. Thus, any platform
2318 which wants to do PS printing can do so with minimum fuss.)
2320 The following entries all describe function pointer fields in a
2321 structure called \c{drawing_api}. Each of the functions takes a
2322 \cq{void *} context pointer, which it should internally cast back to
2323 a more useful type. Thus, a drawing \e{object} (\c{drawing *)}
2324 suitable for passing to the back end redraw or printing functions
2325 is constructed by passing a \c{drawing_api} and a \cq{void *} to the
2326 function \cw{drawing_new()} (see \k{drawing-new}).
2328 \S{drawingapi-draw-text} \cw{draw_text()}
2330 \c void (*draw_text)(void *handle, int x, int y, int fonttype,
2331 \c int fontsize, int align, int colour, char *text);
2333 This function behaves exactly like the back end \cw{draw_text()}
2334 function; see \k{drawing-draw-text}.
2336 \S{drawingapi-draw-rect} \cw{draw_rect()}
2338 \c void (*draw_rect)(void *handle, int x, int y, int w, int h,
2341 This function behaves exactly like the back end \cw{draw_rect()}
2342 function; see \k{drawing-draw-rect}.
2344 \S{drawingapi-draw-line} \cw{draw_line()}
2346 \c void (*draw_line)(void *handle, int x1, int y1, int x2, int y2,
2349 This function behaves exactly like the back end \cw{draw_line()}
2350 function; see \k{drawing-draw-line}.
2352 \S{drawingapi-draw-polygon} \cw{draw_polygon()}
2354 \c void (*draw_polygon)(void *handle, int *coords, int npoints,
2355 \c int fillcolour, int outlinecolour);
2357 This function behaves exactly like the back end \cw{draw_polygon()}
2358 function; see \k{drawing-draw-polygon}.
2360 \S{drawingapi-draw-circle} \cw{draw_circle()}
2362 \c void (*draw_circle)(void *handle, int cx, int cy, int radius,
2363 \c int fillcolour, int outlinecolour);
2365 This function behaves exactly like the back end \cw{draw_circle()}
2366 function; see \k{drawing-draw-circle}.
2368 \S{drawingapi-draw-thick-line} \cw{draw_thick_line()}
2370 \c void draw_thick_line(drawing *dr, float thickness,
2371 \c float x1, float y1, float x2, float y2,
2374 This function behaves exactly like the back end
2375 \cw{draw_thick_line()} function; see \k{drawing-draw-thick-line}.
2377 An implementation of this API which doesn't provide high-quality
2378 rendering of thick lines is permitted to define this function
2379 pointer to be \cw{NULL}. The middleware in \cw{drawing.c} will notice
2380 and provide a low-quality alternative using \cw{draw_polygon()}.
2382 \S{drawingapi-draw-update} \cw{draw_update()}
2384 \c void (*draw_update)(void *handle, int x, int y, int w, int h);
2386 This function behaves exactly like the back end \cw{draw_update()}
2387 function; see \k{drawing-draw-update}.
2389 An implementation of this API which only supports printing is
2390 permitted to define this function pointer to be \cw{NULL} rather
2391 than bothering to define an empty function. The middleware in
2392 \cw{drawing.c} will notice and avoid calling it.
2394 \S{drawingapi-clip} \cw{clip()}
2396 \c void (*clip)(void *handle, int x, int y, int w, int h);
2398 This function behaves exactly like the back end \cw{clip()}
2399 function; see \k{drawing-clip}.
2401 \S{drawingapi-unclip} \cw{unclip()}
2403 \c void (*unclip)(void *handle);
2405 This function behaves exactly like the back end \cw{unclip()}
2406 function; see \k{drawing-unclip}.
2408 \S{drawingapi-start-draw} \cw{start_draw()}
2410 \c void (*start_draw)(void *handle);
2412 This function is called at the start of drawing. It allows the front
2413 end to initialise any temporary data required to draw with, such as
2416 Implementations of this API which do not provide drawing services
2417 may define this function pointer to be \cw{NULL}; it will never be
2418 called unless drawing is attempted.
2420 \S{drawingapi-end-draw} \cw{end_draw()}
2422 \c void (*end_draw)(void *handle);
2424 This function is called at the end of drawing. It allows the front
2425 end to do cleanup tasks such as deallocating device contexts and
2426 scheduling appropriate GUI redraw events.
2428 Implementations of this API which do not provide drawing services
2429 may define this function pointer to be \cw{NULL}; it will never be
2430 called unless drawing is attempted.
2432 \S{drawingapi-status-bar} \cw{status_bar()}
2434 \c void (*status_bar)(void *handle, char *text);
2436 This function behaves exactly like the back end \cw{status_bar()}
2437 function; see \k{drawing-status-bar}.
2439 Front ends implementing this function need not worry about it being
2440 called repeatedly with the same text; the middleware code in
2441 \cw{status_bar()} will take care of this.
2443 Implementations of this API which do not provide drawing services
2444 may define this function pointer to be \cw{NULL}; it will never be
2445 called unless drawing is attempted.
2447 \S{drawingapi-blitter-new} \cw{blitter_new()}
2449 \c blitter *(*blitter_new)(void *handle, int w, int h);
2451 This function behaves exactly like the back end \cw{blitter_new()}
2452 function; see \k{drawing-blitter-new}.
2454 Implementations of this API which do not provide drawing services
2455 may define this function pointer to be \cw{NULL}; it will never be
2456 called unless drawing is attempted.
2458 \S{drawingapi-blitter-free} \cw{blitter_free()}
2460 \c void (*blitter_free)(void *handle, blitter *bl);
2462 This function behaves exactly like the back end \cw{blitter_free()}
2463 function; see \k{drawing-blitter-free}.
2465 Implementations of this API which do not provide drawing services
2466 may define this function pointer to be \cw{NULL}; it will never be
2467 called unless drawing is attempted.
2469 \S{drawingapi-blitter-save} \cw{blitter_save()}
2471 \c void (*blitter_save)(void *handle, blitter *bl, int x, int y);
2473 This function behaves exactly like the back end \cw{blitter_save()}
2474 function; see \k{drawing-blitter-save}.
2476 Implementations of this API which do not provide drawing services
2477 may define this function pointer to be \cw{NULL}; it will never be
2478 called unless drawing is attempted.
2480 \S{drawingapi-blitter-load} \cw{blitter_load()}
2482 \c void (*blitter_load)(void *handle, blitter *bl, int x, int y);
2484 This function behaves exactly like the back end \cw{blitter_load()}
2485 function; see \k{drawing-blitter-load}.
2487 Implementations of this API which do not provide drawing services
2488 may define this function pointer to be \cw{NULL}; it will never be
2489 called unless drawing is attempted.
2491 \S{drawingapi-begin-doc} \cw{begin_doc()}
2493 \c void (*begin_doc)(void *handle, int pages);
2495 This function is called at the beginning of a printing run. It gives
2496 the front end an opportunity to initialise any required printing
2497 subsystem. It also provides the number of pages in advance.
2499 Implementations of this API which do not provide printing services
2500 may define this function pointer to be \cw{NULL}; it will never be
2501 called unless printing is attempted.
2503 \S{drawingapi-begin-page} \cw{begin_page()}
2505 \c void (*begin_page)(void *handle, int number);
2507 This function is called during printing, at the beginning of each
2508 page. It gives the page number (numbered from 1 rather than 0, so
2509 suitable for use in user-visible contexts).
2511 Implementations of this API which do not provide printing services
2512 may define this function pointer to be \cw{NULL}; it will never be
2513 called unless printing is attempted.
2515 \S{drawingapi-begin-puzzle} \cw{begin_puzzle()}
2517 \c void (*begin_puzzle)(void *handle, float xm, float xc,
2518 \c float ym, float yc, int pw, int ph, float wmm);
2520 This function is called during printing, just before printing a
2521 single puzzle on a page. It specifies the size and location of the
2524 \c{xm} and \c{xc} specify the horizontal position of the puzzle on
2525 the page, as a linear function of the page width. The front end is
2526 expected to multiply the page width by \c{xm}, add \c{xc} (measured
2527 in millimetres), and use the resulting x-coordinate as the left edge
2530 Similarly, \c{ym} and \c{yc} specify the vertical position of the
2531 puzzle as a function of the page height: the page height times
2532 \c{ym}, plus \c{yc} millimetres, equals the desired distance from
2533 the top of the page to the top of the puzzle.
2535 (This unwieldy mechanism is required because not all printing
2536 systems can communicate the page size back to the software. The
2537 PostScript back end, for example, writes out PS which determines the
2538 page size at print time by means of calling \cq{clippath}, and
2539 centres the puzzles within that. Thus, exactly the same PS file
2540 works on A4 or on US Letter paper without needing local
2541 configuration, which simplifies matters.)
2543 \cw{pw} and \cw{ph} give the size of the puzzle in drawing API
2544 coordinates. The printing system will subsequently call the puzzle's
2545 own print function, which will in turn call drawing API functions in
2546 the expectation that an area \cw{pw} by \cw{ph} units is available
2547 to draw the puzzle on.
2549 Finally, \cw{wmm} gives the desired width of the puzzle in
2550 millimetres. (The aspect ratio is expected to be preserved, so if
2551 the desired puzzle height is also needed then it can be computed as
2554 Implementations of this API which do not provide printing services
2555 may define this function pointer to be \cw{NULL}; it will never be
2556 called unless printing is attempted.
2558 \S{drawingapi-end-puzzle} \cw{end_puzzle()}
2560 \c void (*end_puzzle)(void *handle);
2562 This function is called after the printing of a specific puzzle is
2565 Implementations of this API which do not provide printing services
2566 may define this function pointer to be \cw{NULL}; it will never be
2567 called unless printing is attempted.
2569 \S{drawingapi-end-page} \cw{end_page()}
2571 \c void (*end_page)(void *handle, int number);
2573 This function is called after the printing of a page is finished.
2575 Implementations of this API which do not provide printing services
2576 may define this function pointer to be \cw{NULL}; it will never be
2577 called unless printing is attempted.
2579 \S{drawingapi-end-doc} \cw{end_doc()}
2581 \c void (*end_doc)(void *handle);
2583 This function is called after the printing of the entire document is
2584 finished. This is the moment to close files, send things to the
2585 print spooler, or whatever the local convention is.
2587 Implementations of this API which do not provide printing services
2588 may define this function pointer to be \cw{NULL}; it will never be
2589 called unless printing is attempted.
2591 \S{drawingapi-line-width} \cw{line_width()}
2593 \c void (*line_width)(void *handle, float width);
2595 This function is called to set the line thickness, during printing
2596 only. Note that the width is a \cw{float} here, where it was an
2597 \cw{int} as seen by the back end. This is because \cw{drawing.c} may
2598 have scaled it on the way past.
2600 However, the width is still specified in the same coordinate system
2601 as the rest of the drawing.
2603 Implementations of this API which do not provide printing services
2604 may define this function pointer to be \cw{NULL}; it will never be
2605 called unless printing is attempted.
2607 \S{drawingapi-text-fallback} \cw{text_fallback()}
2609 \c char *(*text_fallback)(void *handle, const char *const *strings,
2612 This function behaves exactly like the back end \cw{text_fallback()}
2613 function; see \k{drawing-text-fallback}.
2615 Implementations of this API which do not support any characters
2616 outside ASCII may define this function pointer to be \cw{NULL}, in
2617 which case the central code in \cw{drawing.c} will provide a default
2620 \H{drawingapi-frontend} The drawing API as called by the front end
2622 There are a small number of functions provided in \cw{drawing.c}
2623 which the front end needs to \e{call}, rather than helping to
2624 implement. They are described in this section.
2626 \S{drawing-new} \cw{drawing_new()}
2628 \c drawing *drawing_new(const drawing_api *api, midend *me,
2631 This function creates a drawing object. It is passed a
2632 \c{drawing_api}, which is a structure containing nothing but
2633 function pointers; and also a \cq{void *} handle. The handle is
2634 passed back to each function pointer when it is called.
2636 The \c{midend} parameter is used for rewriting the status bar
2637 contents: \cw{status_bar()} (see \k{drawing-status-bar}) has to call
2638 a function in the mid-end which might rewrite the status bar text.
2639 If the drawing object is to be used only for printing, or if the
2640 game is known not to call \cw{status_bar()}, this parameter may be
2643 \S{drawing-free} \cw{drawing_free()}
2645 \c void drawing_free(drawing *dr);
2647 This function frees a drawing object. Note that the \cq{void *}
2648 handle is not freed; if that needs cleaning up it must be done by
2651 \S{drawing-print-get-colour} \cw{print_get_colour()}
2653 \c void print_get_colour(drawing *dr, int colour, int printincolour,
2654 \c int *hatch, float *r, float *g, float *b)
2656 This function is called by the implementations of the drawing API
2657 functions when they are called in a printing context. It takes a
2658 colour index as input, and returns the description of the colour as
2659 requested by the back end.
2661 \c{printincolour} is \cw{TRUE} iff the implementation is printing in
2662 colour. This will alter the results returned if the colour in
2663 question was specified with a black-and-white fallback value.
2665 If the colour should be rendered by hatching, \c{*hatch} is filled
2666 with the type of hatching desired. See \k{print-grey-colour} for
2667 details of the values this integer can take.
2669 If the colour should be rendered as solid colour, \c{*hatch} is
2670 given a negative value, and \c{*r}, \c{*g} and \c{*b} are filled
2671 with the RGB values of the desired colour (if printing in colour),
2672 or all filled with the grey-scale value (if printing in black and
2675 \C{midend} The API provided by the mid-end
2677 This chapter documents the API provided by the mid-end to be called
2678 by the front end. You probably only need to read this if you are a
2679 front end implementor, i.e. you are porting Puzzles to a new
2680 platform. If you're only interested in writing new puzzles, you can
2681 safely skip this chapter.
2683 All the persistent state in the mid-end is encapsulated within a
2684 \c{midend} structure, to facilitate having multiple mid-ends in any
2685 port which supports multiple puzzle windows open simultaneously.
2686 Each \c{midend} is intended to handle the contents of a single
2689 \H{midend-new} \cw{midend_new()}
2691 \c midend *midend_new(frontend *fe, const game *ourgame,
2692 \c const drawing_api *drapi, void *drhandle)
2694 Allocates and returns a new mid-end structure.
2696 The \c{fe} argument is stored in the mid-end. It will be used when
2697 calling back to functions such as \cw{activate_timer()}
2698 (\k{frontend-activate-timer}), and will be passed on to the back end
2699 function \cw{colours()} (\k{backend-colours}).
2701 The parameters \c{drapi} and \c{drhandle} are passed to
2702 \cw{drawing_new()} (\k{drawing-new}) to construct a drawing object
2703 which will be passed to the back end function \cw{redraw()}
2704 (\k{backend-redraw}). Hence, all drawing-related function pointers
2705 defined in \c{drapi} can expect to be called with \c{drhandle} as
2706 their first argument.
2708 The \c{ourgame} argument points to a container structure describing
2709 a game back end. The mid-end thus created will only be capable of
2710 handling that one game. (So even in a monolithic front end
2711 containing all the games, this imposes the constraint that any
2712 individual puzzle window is tied to a single game. Unless, of
2713 course, you feel brave enough to change the mid-end for the window
2714 without closing the window...)
2716 \H{midend-free} \cw{midend_free()}
2718 \c void midend_free(midend *me);
2720 Frees a mid-end structure and all its associated data.
2722 \H{midend-tilesize} \cw{midend_tilesize()}
2724 \c int midend_tilesize(midend *me);
2726 Returns the \cq{tilesize} parameter being used to display the
2727 current puzzle (\k{backend-preferred-tilesize}).
2729 \H{midend-set-params} \cw{midend_set_params()}
2731 \c void midend_set_params(midend *me, game_params *params);
2733 Sets the current game parameters for a mid-end. Subsequent games
2734 generated by \cw{midend_new_game()} (\k{midend-new-game}) will use
2735 these parameters until further notice.
2737 The usual way in which the front end will have an actual
2738 \c{game_params} structure to pass to this function is if it had
2739 previously got it from \cw{midend_fetch_preset()}
2740 (\k{midend-fetch-preset}). Thus, this function is usually called in
2741 response to the user making a selection from the presets menu.
2743 \H{midend-get-params} \cw{midend_get_params()}
2745 \c game_params *midend_get_params(midend *me);
2747 Returns the current game parameters stored in this mid-end.
2749 The returned value is dynamically allocated, and should be freed
2750 when finished with by passing it to the game's own
2751 \cw{free_params()} function (see \k{backend-free-params}).
2753 \H{midend-size} \cw{midend_size()}
2755 \c void midend_size(midend *me, int *x, int *y, int user_size);
2757 Tells the mid-end to figure out its window size.
2759 On input, \c{*x} and \c{*y} should contain the maximum or requested
2760 size for the window. (Typically this will be the size of the screen
2761 that the window has to fit on, or similar.) The mid-end will
2762 repeatedly call the back end function \cw{compute_size()}
2763 (\k{backend-compute-size}), searching for a tile size that best
2764 satisfies the requirements. On exit, \c{*x} and \c{*y} will contain
2765 the size needed for the puzzle window's drawing area. (It is of
2766 course up to the front end to adjust this for any additional window
2767 furniture such as menu bars and window borders, if necessary. The
2768 status bar is also not included in this size.)
2770 Use \c{user_size} to indicate whether \c{*x} and \c{*y} are a
2771 requested size, or just a maximum size.
2773 If \c{user_size} is set to \cw{TRUE}, the mid-end will treat the
2774 input size as a request, and will pick a tile size which
2775 approximates it \e{as closely as possible}, going over the game's
2776 preferred tile size if necessary to achieve this. The mid-end will
2777 also use the resulting tile size as its preferred one until further
2778 notice, on the assumption that this size was explicitly requested
2779 by the user. Use this option if you want your front end to support
2780 dynamic resizing of the puzzle window with automatic scaling of the
2783 If \c{user_size} is set to \cw{FALSE}, then the game's tile size
2784 will never go over its preferred one, although it may go under in
2785 order to fit within the maximum bounds specified by \c{*x} and
2786 \c{*y}. This is the recommended approach when opening a new window
2787 at default size: the game will use its preferred size unless it has
2788 to use a smaller one to fit on the screen. If the tile size is
2789 shrunk for this reason, the change will not persist; if a smaller
2790 grid is subsequently chosen, the tile size will recover.
2792 The mid-end will try as hard as it can to return a size which is
2793 less than or equal to the input size, in both dimensions. In extreme
2794 circumstances it may fail (if even the lowest possible tile size
2795 gives window dimensions greater than the input), in which case it
2796 will return a size greater than the input size. Front ends should be
2797 prepared for this to happen (i.e. don't crash or fail an assertion),
2798 but may handle it in any way they see fit: by rejecting the game
2799 parameters which caused the problem, by opening a window larger than
2800 the screen regardless of inconvenience, by introducing scroll bars
2801 on the window, by drawing on a large bitmap and scaling it into a
2802 smaller window, or by any other means you can think of. It is likely
2803 that when the tile size is that small the game will be unplayable
2804 anyway, so don't put \e{too} much effort into handling it
2807 If your platform has no limit on window size (or if you're planning
2808 to use scroll bars for large puzzles), you can pass dimensions of
2809 \cw{INT_MAX} as input to this function. You should probably not do
2810 that \e{and} set the \c{user_size} flag, though!
2812 The midend relies on the frontend calling \cw{midend_new_game()}
2813 (\k{midend-new-game}) before calling \cw{midend_size()}.
2815 \H{midend-new-game} \cw{midend_new_game()}
2817 \c void midend_new_game(midend *me);
2819 Causes the mid-end to begin a new game. Normally the game will be a
2820 new randomly generated puzzle. However, if you have previously
2821 called \cw{midend_game_id()} or \cw{midend_set_config()}, the game
2822 generated might be dictated by the results of those functions. (In
2823 particular, you \e{must} call \cw{midend_new_game()} after calling
2824 either of those functions, or else no immediate effect will be
2827 You will probably need to call \cw{midend_size()} after calling this
2828 function, because if the game parameters have been changed since the
2829 last new game then the window size might need to change. (If you
2830 know the parameters \e{haven't} changed, you don't need to do this.)
2832 This function will create a new \c{game_drawstate}, but does not
2833 actually perform a redraw (since you often need to call
2834 \cw{midend_size()} before the redraw can be done). So after calling
2835 this function and after calling \cw{midend_size()}, you should then
2836 call \cw{midend_redraw()}. (It is not necessary to call
2837 \cw{midend_force_redraw()}; that will discard the draw state and
2838 create a fresh one, which is unnecessary in this case since there's
2839 a fresh one already. It would work, but it's usually excessive.)
2841 \H{midend-restart-game} \cw{midend_restart_game()}
2843 \c void midend_restart_game(midend *me);
2845 This function causes the current game to be restarted. This is done
2846 by placing a new copy of the original game state on the end of the
2847 undo list (so that an accidental restart can be undone).
2849 This function automatically causes a redraw, i.e. the front end can
2850 expect its drawing API to be called from \e{within} a call to this
2851 function. Some back ends require that \cw{midend_size()}
2852 (\k{midend-size}) is called before \cw{midend_restart_game()}.
2854 \H{midend-force-redraw} \cw{midend_force_redraw()}
2856 \c void midend_force_redraw(midend *me);
2858 Forces a complete redraw of the puzzle window, by means of
2859 discarding the current \c{game_drawstate} and creating a new one
2860 from scratch before calling the game's \cw{redraw()} function.
2862 The front end can expect its drawing API to be called from within a
2863 call to this function. Some back ends require that \cw{midend_size()}
2864 (\k{midend-size}) is called before \cw{midend_force_redraw()}.
2866 \H{midend-redraw} \cw{midend_redraw()}
2868 \c void midend_redraw(midend *me);
2870 Causes a partial redraw of the puzzle window, by means of simply
2871 calling the game's \cw{redraw()} function. (That is, the only things
2872 redrawn will be things that have changed since the last redraw.)
2874 The front end can expect its drawing API to be called from within a
2875 call to this function. Some back ends require that \cw{midend_size()}
2876 (\k{midend-size}) is called before \cw{midend_redraw()}.
2878 \H{midend-process-key} \cw{midend_process_key()}
2880 \c int midend_process_key(midend *me, int x, int y, int button);
2882 The front end calls this function to report a mouse or keyboard
2883 event. The parameters \c{x}, \c{y} and \c{button} are almost
2884 identical to the ones passed to the back end function
2885 \cw{interpret_move()} (\k{backend-interpret-move}), except that the
2886 front end is \e{not} required to provide the guarantees about mouse
2887 event ordering. The mid-end will sort out multiple simultaneous
2888 button presses and changes of button; the front end's responsibility
2889 is simply to pass on the mouse events it receives as accurately as
2892 (Some platforms may need to emulate absent mouse buttons by means of
2893 using a modifier key such as Shift with another mouse button. This
2894 tends to mean that if Shift is pressed or released in the middle of
2895 a mouse drag, the mid-end will suddenly stop receiving, say,
2896 \cw{LEFT_DRAG} events and start receiving \cw{RIGHT_DRAG}s, with no
2897 intervening button release or press events. This too is something
2898 which the mid-end will sort out for you; the front end has no
2899 obligation to maintain sanity in this area.)
2901 The front end \e{should}, however, always eventually send some kind
2902 of button release. On some platforms this requires special effort:
2903 Windows, for example, requires a call to the system API function
2904 \cw{SetCapture()} in order to ensure that your window receives a
2905 mouse-up event even if the pointer has left the window by the time
2906 the mouse button is released. On any platform that requires this
2907 sort of thing, the front end \e{is} responsible for doing it.
2909 Calling this function is very likely to result in calls back to the
2910 front end's drawing API and/or \cw{activate_timer()}
2911 (\k{frontend-activate-timer}).
2913 The return value from \cw{midend_process_key()} is non-zero, unless
2914 the effect of the keypress was to request termination of the
2915 program. A front end should shut down the puzzle in response to a
2918 \H{midend-colours} \cw{midend_colours()}
2920 \c float *midend_colours(midend *me, int *ncolours);
2922 Returns an array of the colours required by the game, in exactly the
2923 same format as that returned by the back end function \cw{colours()}
2924 (\k{backend-colours}). Front ends should call this function rather
2925 than calling the back end's version directly, since the mid-end adds
2926 standard customisation facilities. (At the time of writing, those
2927 customisation facilities are implemented hackily by means of
2928 environment variables, but it's not impossible that they may become
2929 more full and formal in future.)
2931 \H{midend-timer} \cw{midend_timer()}
2933 \c void midend_timer(midend *me, float tplus);
2935 If the mid-end has called \cw{activate_timer()}
2936 (\k{frontend-activate-timer}) to request regular callbacks for
2937 purposes of animation or timing, this is the function the front end
2938 should call on a regular basis. The argument \c{tplus} gives the
2939 time, in seconds, since the last time either this function was
2940 called or \cw{activate_timer()} was invoked.
2942 One of the major purposes of timing in the mid-end is to perform
2943 move animation. Therefore, calling this function is very likely to
2944 result in calls back to the front end's drawing API.
2946 \H{midend-num-presets} \cw{midend_num_presets()}
2948 \c int midend_num_presets(midend *me);
2950 Returns the number of game parameter presets supplied by this game.
2951 Front ends should use this function and \cw{midend_fetch_preset()}
2952 to configure their presets menu rather than calling the back end
2953 directly, since the mid-end adds standard customisation facilities.
2954 (At the time of writing, those customisation facilities are
2955 implemented hackily by means of environment variables, but it's not
2956 impossible that they may become more full and formal in future.)
2958 \H{midend-fetch-preset} \cw{midend_fetch_preset()}
2960 \c void midend_fetch_preset(midend *me, int n,
2961 \c char **name, game_params **params);
2963 Returns one of the preset game parameter structures for the game. On
2964 input \c{n} must be a non-negative integer and less than the value
2965 returned from \cw{midend_num_presets()}. On output, \c{*name} is set
2966 to an ASCII string suitable for entering in the game's presets menu,
2967 and \c{*params} is set to the corresponding \c{game_params}
2970 Both of the two output values are dynamically allocated, but they
2971 are owned by the mid-end structure: the front end should not ever
2972 free them directly, because they will be freed automatically during
2975 \H{midend-which-preset} \cw{midend_which_preset()}
2977 \c int midend_which_preset(midend *me);
2979 Returns the numeric index of the preset game parameter structure
2980 which matches the current game parameters, or a negative number if
2981 no preset matches. Front ends could use this to maintain a tick
2982 beside one of the items in the menu (or tick the \q{Custom} option
2983 if the return value is less than zero).
2985 \H{midend-wants-statusbar} \cw{midend_wants_statusbar()}
2987 \c int midend_wants_statusbar(midend *me);
2989 This function returns \cw{TRUE} if the puzzle has a use for a
2990 textual status line (to display score, completion status, currently
2991 active tiles, time, or anything else).
2993 Front ends should call this function rather than talking directly to
2996 \H{midend-get-config} \cw{midend_get_config()}
2998 \c config_item *midend_get_config(midend *me, int which,
2999 \c char **wintitle);
3001 Returns a dialog box description for user configuration.
3003 On input, \cw{which} should be set to one of three values, which
3004 select which of the various dialog box descriptions is returned:
3006 \dt \cw{CFG_SETTINGS}
3008 \dd Requests the GUI parameter configuration box generated by the
3009 puzzle itself. This should be used when the user selects \q{Custom}
3010 from the game types menu (or equivalent). The mid-end passes this
3011 request on to the back end function \cw{configure()}
3012 (\k{backend-configure}).
3016 \dd Requests a box suitable for entering a descriptive game ID (and
3017 viewing the existing one). The mid-end generates this dialog box
3018 description itself. This should be used when the user selects
3019 \q{Specific} from the game menu (or equivalent).
3023 \dd Requests a box suitable for entering a random-seed game ID (and
3024 viewing the existing one). The mid-end generates this dialog box
3025 description itself. This should be used when the user selects
3026 \q{Random Seed} from the game menu (or equivalent).
3028 The returned value is an array of \cw{config_item}s, exactly as
3029 described in \k{backend-configure}. Another returned value is an
3030 ASCII string giving a suitable title for the configuration window,
3033 Both returned values are dynamically allocated and will need to be
3034 freed. The window title can be freed in the obvious way; the
3035 \cw{config_item} array is a slightly complex structure, so a utility
3036 function \cw{free_cfg()} is provided to free it for you. See
3039 (Of course, you will probably not want to free the \cw{config_item}
3040 array until the dialog box is dismissed, because before then you
3041 will probably need to pass it to \cw{midend_set_config}.)
3043 \H{midend-set-config} \cw{midend_set_config()}
3045 \c char *midend_set_config(midend *me, int which,
3046 \c config_item *cfg);
3048 Passes the mid-end the results of a configuration dialog box.
3049 \c{which} should have the same value which it had when
3050 \cw{midend_get_config()} was called; \c{cfg} should be the array of
3051 \c{config_item}s returned from \cw{midend_get_config()}, modified to
3052 contain the results of the user's editing operations.
3054 This function returns \cw{NULL} on success, or otherwise (if the
3055 configuration data was in some way invalid) an ASCII string
3056 containing an error message suitable for showing to the user.
3058 If the function succeeds, it is likely that the game parameters will
3059 have been changed and it is certain that a new game will be
3060 requested. The front end should therefore call
3061 \cw{midend_new_game()}, and probably also re-think the window size
3062 using \cw{midend_size()} and eventually perform a refresh using
3063 \cw{midend_redraw()}.
3065 \H{midend-game-id} \cw{midend_game_id()}
3067 \c char *midend_game_id(midend *me, char *id);
3069 Passes the mid-end a string game ID (of any of the valid forms
3070 \cq{params}, \cq{params:description} or \cq{params#seed}) which the
3071 mid-end will process and use for the next generated game.
3073 This function returns \cw{NULL} on success, or otherwise (if the
3074 configuration data was in some way invalid) an ASCII string
3075 containing an error message (not dynamically allocated) suitable for
3076 showing to the user. In the event of an error, the mid-end's
3077 internal state will be left exactly as it was before the call.
3079 If the function succeeds, it is likely that the game parameters will
3080 have been changed and it is certain that a new game will be
3081 requested. The front end should therefore call
3082 \cw{midend_new_game()}, and probably also re-think the window size
3083 using \cw{midend_size()} and eventually case a refresh using
3084 \cw{midend_redraw()}.
3086 \H{midend-get-game-id} \cw{midend_get_game_id()}
3088 \c char *midend_get_game_id(midend *me)
3090 Returns a descriptive game ID (i.e. one in the form
3091 \cq{params:description}) describing the game currently active in the
3092 mid-end. The returned string is dynamically allocated.
3094 \H{midend-get-random-seed} \cw{midend_get_random_seed()}
3096 \c char *midend_get_random_seed(midend *me)
3098 Returns a random game ID (i.e. one in the form \cq{params#seedstring})
3099 describing the game currently active in the mid-end, if there is one.
3100 If the game was created by entering a description, no random seed will
3101 currently exist and this function will return \cw{NULL}.
3103 The returned string, if it is non-\cw{NULL}, is dynamically allocated.
3105 \H{midend-can-format-as-text-now} \cw{midend_can_format_as_text_now()}
3107 \c int midend_can_format_as_text_now(midend *me);
3109 Returns \cw{TRUE} if the game code is capable of formatting puzzles
3110 of the currently selected game type as ASCII.
3112 If this returns \cw{FALSE}, then \cw{midend_text_format()}
3113 (\k{midend-text-format}) will return \cw{NULL}.
3115 \H{midend-text-format} \cw{midend_text_format()}
3117 \c char *midend_text_format(midend *me);
3119 Formats the current game's current state as ASCII text suitable for
3120 copying to the clipboard. The returned string is dynamically
3123 If the game's \c{can_format_as_text_ever} flag is \cw{FALSE}, or if
3124 its \cw{can_format_as_text_now()} function returns \cw{FALSE}, then
3125 this function will return \cw{NULL}.
3127 If the returned string contains multiple lines (which is likely), it
3128 will use the normal C line ending convention (\cw{\\n} only). On
3129 platforms which use a different line ending convention for data in
3130 the clipboard, it is the front end's responsibility to perform the
3133 \H{midend-solve} \cw{midend_solve()}
3135 \c char *midend_solve(midend *me);
3137 Requests the mid-end to perform a Solve operation.
3139 On success, \cw{NULL} is returned. On failure, an error message (not
3140 dynamically allocated) is returned, suitable for showing to the
3143 The front end can expect its drawing API and/or
3144 \cw{activate_timer()} to be called from within a call to this
3145 function. Some back ends require that \cw{midend_size()}
3146 (\k{midend-size}) is called before \cw{midend_solve()}.
3148 \H{midend-status} \cw{midend_status()}
3150 \c int midend_status(midend *me);
3152 This function returns +1 if the midend is currently displaying a game
3153 in a solved state, -1 if the game is in a permanently lost state, or 0
3154 otherwise. This function just calls the back end's \cw{status()}
3155 function. Front ends may wish to use this as a cue to proactively
3156 offer the option of starting a new game.
3158 (See \k{backend-status} for more detail about the back end's
3159 \cw{status()} function and discussion of what should count as which
3162 \H{midend-can-undo} \cw{midend_can_undo()}
3164 \c int midend_can_undo(midend *me);
3166 Returns \cw{TRUE} if the midend is currently in a state where the undo
3167 operation is meaningful (i.e. at least one position exists on the undo
3168 chain before the present one). Front ends may wish to use this to
3169 visually activate and deactivate an undo button.
3171 \H{midend-can-redo} \cw{midend_can_redo()}
3173 \c int midend_can_redo(midend *me);
3175 Returns \cw{TRUE} if the midend is currently in a state where the redo
3176 operation is meaningful (i.e. at least one position exists on the redo
3177 chain after the present one). Front ends may wish to use this to
3178 visually activate and deactivate a redo button.
3180 \H{midend-serialise} \cw{midend_serialise()}
3182 \c void midend_serialise(midend *me,
3183 \c void (*write)(void *ctx, void *buf, int len),
3186 Calling this function causes the mid-end to convert its entire
3187 internal state into a long ASCII text string, and to pass that
3188 string (piece by piece) to the supplied \c{write} function.
3190 Desktop implementations can use this function to save a game in any
3191 state (including half-finished) to a disk file, by supplying a
3192 \c{write} function which is a wrapper on \cw{fwrite()} (or local
3193 equivalent). Other implementations may find other uses for it, such
3194 as compressing the large and sprawling mid-end state into a
3195 manageable amount of memory when a palmtop application is suspended
3196 so that another one can run; in this case \cw{write} might want to
3197 write to a memory buffer rather than a file. There may be other uses
3200 This function will call back to the supplied \c{write} function a
3201 number of times, with the first parameter (\c{ctx}) equal to
3202 \c{wctx}, and the other two parameters pointing at a piece of the
3205 \H{midend-deserialise} \cw{midend_deserialise()}
3207 \c char *midend_deserialise(midend *me,
3208 \c int (*read)(void *ctx, void *buf, int len),
3211 This function is the counterpart to \cw{midend_serialise()}. It
3212 calls the supplied \cw{read} function repeatedly to read a quantity
3213 of data, and attempts to interpret that data as a serialised mid-end
3214 as output by \cw{midend_serialise()}.
3216 The \cw{read} function is called with the first parameter (\c{ctx})
3217 equal to \c{rctx}, and should attempt to read \c{len} bytes of data
3218 into the buffer pointed to by \c{buf}. It should return \cw{FALSE}
3219 on failure or \cw{TRUE} on success. It should not report success
3220 unless it has filled the entire buffer; on platforms which might be
3221 reading from a pipe or other blocking data source, \c{read} is
3222 responsible for looping until the whole buffer has been filled.
3224 If the de-serialisation operation is successful, the mid-end's
3225 internal data structures will be replaced by the results of the
3226 load, and \cw{NULL} will be returned. Otherwise, the mid-end's state
3227 will be completely unchanged and an error message (typically some
3228 variation on \q{save file is corrupt}) will be returned. As usual,
3229 the error message string is not dynamically allocated.
3231 If this function succeeds, it is likely that the game parameters
3232 will have been changed. The front end should therefore probably
3233 re-think the window size using \cw{midend_size()}, and probably
3234 cause a refresh using \cw{midend_redraw()}.
3236 Because each mid-end is tied to a specific game back end, this
3237 function will fail if you attempt to read in a save file generated by
3238 a different game from the one configured in this mid-end, even if your
3239 application is a monolithic one containing all the puzzles. See
3240 \k{identify-game} for a helper function which will allow you to
3241 identify a save file before you instantiate your mid-end in the first
3244 \H{identify-game} \cw{identify_game()}
3246 \c char *identify_game(char **name,
3247 \c int (*read)(void *ctx, void *buf, int len),
3250 This function examines a serialised midend stream, of the same kind
3251 used by \cw{midend_serialise()} and \cw{midendd_deserialise()}, and
3252 returns the \cw{name} field of the game back end from which it was
3255 You might want this if your front end was a monolithic one containing
3256 all the puzzles, and you wanted to be able to load an arbitrary save
3257 file and automatically switch to the right game. Probably your next
3258 step would be to iterate through \cw{gamelist} (\k{frontend-backend})
3259 looking for a game structure whose \cw{name} field matched the
3260 returned string, and give an error if you didn't find one.
3262 On success, the return value of this function is \cw{NULL}, and the
3263 game name string is written into \cw{*name}. The caller should free
3264 that string after using it.
3266 On failure, \cw{*name} is \cw{NULL}, and the return value is an error
3267 message (which does not need freeing at all).
3269 (This isn't strictly speaking a midend function, since it doesn't
3270 accept or return a pointer to a midend. You'd probably call it just
3271 \e{before} deciding what kind of midend you wanted to instantiate.)
3273 \H{frontend-backend} Direct reference to the back end structure by
3276 Although \e{most} things the front end needs done should be done by
3277 calling the mid-end, there are a few situations in which the front
3278 end needs to refer directly to the game back end structure.
3280 The most obvious of these is
3282 \b passing the game back end as a parameter to \cw{midend_new()}.
3284 There are a few other back end features which are not wrapped by the
3285 mid-end because there didn't seem much point in doing so:
3287 \b fetching the \c{name} field to use in window titles and similar
3289 \b reading the \c{can_configure}, \c{can_solve} and
3290 \c{can_format_as_text_ever} fields to decide whether to add those
3291 items to the menu bar or equivalent
3293 \b reading the \c{winhelp_topic} field (Windows only)
3295 \b the GTK front end provides a \cq{--generate} command-line option
3296 which directly calls the back end to do most of its work. This is
3297 not really part of the main front end code, though, and I'm not sure
3300 In order to find the game back end structure, the front end does one
3303 \b If the particular front end is compiling a separate binary per
3304 game, then the back end structure is a global variable with the
3305 standard name \cq{thegame}:
3309 \c extern const game thegame;
3313 \b If the front end is compiled as a monolithic application
3314 containing all the puzzles together (in which case the preprocessor
3315 symbol \cw{COMBINED} must be defined when compiling most of the code
3316 base), then there will be two global variables defined:
3320 \c extern const game *gamelist[];
3321 \c extern const int gamecount;
3323 \c{gamelist} will be an array of \c{gamecount} game structures,
3324 declared in the automatically constructed source module \c{list.c}.
3325 The application should search that array for the game it wants,
3326 probably by reaching into each game structure and looking at its
3331 \H{frontend-api} Mid-end to front-end calls
3333 This section describes the small number of functions which a front
3334 end must provide to be called by the mid-end or other standard
3337 \H{frontend-get-random-seed} \cw{get_random_seed()}
3339 \c void get_random_seed(void **randseed, int *randseedsize);
3341 This function is called by a new mid-end, and also occasionally by
3342 game back ends. Its job is to return a piece of data suitable for
3343 using as a seed for initialisation of a new \c{random_state}.
3345 On exit, \c{*randseed} should be set to point at a newly allocated
3346 piece of memory containing some seed data, and \c{*randseedsize}
3347 should be set to the length of that data.
3349 A simple and entirely adequate implementation is to return a piece
3350 of data containing the current system time at the highest
3351 conveniently available resolution.
3353 \H{frontend-activate-timer} \cw{activate_timer()}
3355 \c void activate_timer(frontend *fe);
3357 This is called by the mid-end to request that the front end begin
3358 calling it back at regular intervals.
3360 The timeout interval is left up to the front end; the finer it is,
3361 the smoother move animations will be, but the more CPU time will be
3362 used. Current front ends use values around 20ms (i.e. 50Hz).
3364 After this function is called, the mid-end will expect to receive
3365 calls to \cw{midend_timer()} on a regular basis.
3367 \H{frontend-deactivate-timer} \cw{deactivate_timer()}
3369 \c void deactivate_timer(frontend *fe);
3371 This is called by the mid-end to request that the front end stop
3372 calling \cw{midend_timer()}.
3374 \H{frontend-fatal} \cw{fatal()}
3376 \c void fatal(char *fmt, ...);
3378 This is called by some utility functions if they encounter a
3379 genuinely fatal error such as running out of memory. It is a
3380 variadic function in the style of \cw{printf()}, and is expected to
3381 show the formatted error message to the user any way it can and then
3382 terminate the application. It must not return.
3384 \H{frontend-default-colour} \cw{frontend_default_colour()}
3386 \c void frontend_default_colour(frontend *fe, float *output);
3388 This function expects to be passed a pointer to an array of three
3389 \cw{float}s. It returns the platform's local preferred background
3390 colour in those three floats, as red, green and blue values (in that
3391 order) ranging from \cw{0.0} to \cw{1.0}.
3393 This function should only ever be called by the back end function
3394 \cw{colours()} (\k{backend-colours}). (Thus, it isn't a
3395 \e{midend}-to-frontend function as such, but there didn't seem to be
3396 anywhere else particularly good to put it. Sorry.)
3398 \C{utils} Utility APIs
3400 This chapter documents a variety of utility APIs provided for the
3401 general use of the rest of the Puzzles code.
3403 \H{utils-random} Random number generation
3405 Platforms' local random number generators vary widely in quality and
3406 seed size. Puzzles therefore supplies its own high-quality random
3407 number generator, with the additional advantage of giving the same
3408 results if fed the same seed data on different platforms. This
3409 allows game random seeds to be exchanged between different ports of
3410 Puzzles and still generate the same games.
3412 Unlike the ANSI C \cw{rand()} function, the Puzzles random number
3413 generator has an \e{explicit} state object called a
3414 \c{random_state}. One of these is managed by each mid-end, for
3415 example, and passed to the back end to generate a game with.
3417 \S{utils-random-init} \cw{random_new()}
3419 \c random_state *random_new(char *seed, int len);
3421 Allocates, initialises and returns a new \c{random_state}. The input
3422 data is used as the seed for the random number stream (i.e. using
3423 the same seed at a later time will generate the same stream).
3425 The seed data can be any data at all; there is no requirement to use
3426 printable ASCII, or NUL-terminated strings, or anything like that.
3428 \S{utils-random-copy} \cw{random_copy()}
3430 \c random_state *random_copy(random_state *tocopy);
3432 Allocates a new \c{random_state}, copies the contents of another
3433 \c{random_state} into it, and returns the new state. If exactly the
3434 same sequence of functions is subseqently called on both the copy and
3435 the original, the results will be identical. This may be useful for
3436 speculatively performing some operation using a given random state,
3437 and later replaying that operation precisely.
3439 \S{utils-random-free} \cw{random_free()}
3441 \c void random_free(random_state *state);
3443 Frees a \c{random_state}.
3445 \S{utils-random-bits} \cw{random_bits()}
3447 \c unsigned long random_bits(random_state *state, int bits);
3449 Returns a random number from 0 to \cw{2^bits-1} inclusive. \c{bits}
3450 should be between 1 and 32 inclusive.
3452 \S{utils-random-upto} \cw{random_upto()}
3454 \c unsigned long random_upto(random_state *state, unsigned long limit);
3456 Returns a random number from 0 to \cw{limit-1} inclusive.
3458 \S{utils-random-state-encode} \cw{random_state_encode()}
3460 \c char *random_state_encode(random_state *state);
3462 Encodes the entire contents of a \c{random_state} in printable
3463 ASCII. Returns a dynamically allocated string containing that
3464 encoding. This can subsequently be passed to
3465 \cw{random_state_decode()} to reconstruct the same \c{random_state}.
3467 \S{utils-random-state-decode} \cw{random_state_decode()}
3469 \c random_state *random_state_decode(char *input);
3471 Decodes a string generated by \cw{random_state_encode()} and
3472 reconstructs an equivalent \c{random_state} to the one encoded, i.e.
3473 it should produce the same stream of random numbers.
3475 This function has no error reporting; if you pass it an invalid
3476 string it will simply generate an arbitrary random state, which may
3477 turn out to be noticeably non-random.
3479 \S{utils-shuffle} \cw{shuffle()}
3481 \c void shuffle(void *array, int nelts, int eltsize, random_state *rs);
3483 Shuffles an array into a random order. The interface is much like
3484 ANSI C \cw{qsort()}, except that there's no need for a compare
3487 \c{array} is a pointer to the first element of the array. \c{nelts}
3488 is the number of elements in the array; \c{eltsize} is the size of a
3489 single element (typically measured using \c{sizeof}). \c{rs} is a
3490 \c{random_state} used to generate all the random numbers for the
3493 \H{utils-alloc} Memory allocation
3495 Puzzles has some central wrappers on the standard memory allocation
3496 functions, which provide compile-time type checking, and run-time
3497 error checking by means of quitting the application if it runs out
3498 of memory. This doesn't provide the best possible recovery from
3499 memory shortage, but on the other hand it greatly simplifies the
3500 rest of the code, because nothing else anywhere needs to worry about
3501 \cw{NULL} returns from allocation.
3503 \S{utils-snew} \cw{snew()}
3505 \c var = snew(type);
3508 This macro takes a single argument which is a \e{type name}. It
3509 allocates space for one object of that type. If allocation fails it
3510 will call \cw{fatal()} and not return; so if it does return, you can
3511 be confident that its return value is non-\cw{NULL}.
3513 The return value is cast to the specified type, so that the compiler
3514 will type-check it against the variable you assign it into. Thus,
3515 this ensures you don't accidentally allocate memory the size of the
3516 wrong type and assign it into a variable of the right one (or vice
3519 \S{utils-snewn} \cw{snewn()}
3521 \c var = snewn(n, type);
3524 This macro is the array form of \cw{snew()}. It takes two arguments;
3525 the first is a number, and the second is a type name. It allocates
3526 space for that many objects of that type, and returns a type-checked
3527 non-\cw{NULL} pointer just as \cw{snew()} does.
3529 \S{utils-sresize} \cw{sresize()}
3531 \c var = sresize(var, n, type);
3534 This macro is a type-checked form of \cw{realloc()}. It takes three
3535 arguments: an input memory block, a new size in elements, and a
3536 type. It re-sizes the input memory block to a size sufficient to
3537 contain that many elements of that type. It returns a type-checked
3538 non-\cw{NULL} pointer, like \cw{snew()} and \cw{snewn()}.
3540 The input memory block can be \cw{NULL}, in which case this function
3541 will behave exactly like \cw{snewn()}. (In principle any
3542 ANSI-compliant \cw{realloc()} implementation ought to cope with
3543 this, but I've never quite trusted it to work everywhere.)
3545 \S{utils-sfree} \cw{sfree()}
3547 \c void sfree(void *p);
3549 This function is pretty much equivalent to \cw{free()}. It is
3550 provided with a dynamically allocated block, and frees it.
3552 The input memory block can be \cw{NULL}, in which case this function
3553 will do nothing. (In principle any ANSI-compliant \cw{free()}
3554 implementation ought to cope with this, but I've never quite trusted
3555 it to work everywhere.)
3557 \S{utils-dupstr} \cw{dupstr()}
3559 \c char *dupstr(const char *s);
3561 This function dynamically allocates a duplicate of a C string. Like
3562 the \cw{snew()} functions, it guarantees to return non-\cw{NULL} or
3565 (Many platforms provide the function \cw{strdup()}. As well as
3566 guaranteeing never to return \cw{NULL}, my version has the advantage
3567 of being defined \e{everywhere}, rather than inconveniently not
3570 \S{utils-free-cfg} \cw{free_cfg()}
3572 \c void free_cfg(config_item *cfg);
3574 This function correctly frees an array of \c{config_item}s,
3575 including walking the array until it gets to the end and freeing
3576 precisely those \c{sval} fields which are expected to be dynamically
3579 (See \k{backend-configure} for details of the \c{config_item}
3582 \H{utils-tree234} Sorted and counted tree functions
3584 Many games require complex algorithms for generating random puzzles,
3585 and some require moderately complex algorithms even during play. A
3586 common requirement during these algorithms is for a means of
3587 maintaining sorted or unsorted lists of items, such that items can
3588 be removed and added conveniently.
3590 For general use, Puzzles provides the following set of functions
3591 which maintain 2-3-4 trees in memory. (A 2-3-4 tree is a balanced
3592 tree structure, with the property that all lookups, insertions,
3593 deletions, splits and joins can be done in \cw{O(log N)} time.)
3595 All these functions expect you to be storing a tree of \c{void *}
3596 pointers. You can put anything you like in those pointers.
3598 By the use of per-node element counts, these tree structures have
3599 the slightly unusual ability to look elements up by their numeric
3600 index within the list represented by the tree. This means that they
3601 can be used to store an unsorted list (in which case, every time you
3602 insert a new element, you must explicitly specify the position where
3603 you wish to insert it). They can also do numeric lookups in a sorted
3604 tree, which might be useful for (for example) tracking the median of
3605 a changing data set.
3607 As well as storing sorted lists, these functions can be used for
3608 storing \q{maps} (associative arrays), by defining each element of a
3609 tree to be a (key, value) pair.
3611 \S{utils-newtree234} \cw{newtree234()}
3613 \c tree234 *newtree234(cmpfn234 cmp);
3615 Creates a new empty tree, and returns a pointer to it.
3617 The parameter \c{cmp} determines the sorting criterion on the tree.
3620 \c typedef int (*cmpfn234)(void *, void *);
3622 If you want a sorted tree, you should provide a function matching
3623 this prototype, which returns like \cw{strcmp()} does (negative if
3624 the first argument is smaller than the second, positive if it is
3625 bigger, zero if they compare equal). In this case, the function
3626 \cw{addpos234()} will not be usable on your tree (because all
3627 insertions must respect the sorting order).
3629 If you want an unsorted tree, pass \cw{NULL}. In this case you will
3630 not be able to use either \cw{add234()} or \cw{del234()}, or any
3631 other function such as \cw{find234()} which depends on a sorting
3632 order. Your tree will become something more like an array, except
3633 that it will efficiently support insertion and deletion as well as
3634 lookups by numeric index.
3636 \S{utils-freetree234} \cw{freetree234()}
3638 \c void freetree234(tree234 *t);
3640 Frees a tree. This function will not free the \e{elements} of the
3641 tree (because they might not be dynamically allocated, or you might
3642 be storing the same set of elements in more than one tree); it will
3643 just free the tree structure itself. If you want to free all the
3644 elements of a tree, you should empty it before passing it to
3645 \cw{freetree234()}, by means of code along the lines of
3647 \c while ((element = delpos234(tree, 0)) != NULL)
3648 \c sfree(element); /* or some more complicated free function */
3649 \e iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii
3651 \S{utils-add234} \cw{add234()}
3653 \c void *add234(tree234 *t, void *e);
3655 Inserts a new element \c{e} into the tree \c{t}. This function
3656 expects the tree to be sorted; the new element is inserted according
3659 If an element comparing equal to \c{e} is already in the tree, then
3660 the insertion will fail, and the return value will be the existing
3661 element. Otherwise, the insertion succeeds, and \c{e} is returned.
3663 \S{utils-addpos234} \cw{addpos234()}
3665 \c void *addpos234(tree234 *t, void *e, int index);
3667 Inserts a new element into an unsorted tree. Since there is no
3668 sorting order to dictate where the new element goes, you must
3669 specify where you want it to go. Setting \c{index} to zero puts the
3670 new element right at the start of the list; setting \c{index} to the
3671 current number of elements in the tree puts the new element at the
3674 Return value is \c{e}, in line with \cw{add234()} (although this
3675 function cannot fail except by running out of memory, in which case
3676 it will bomb out and die rather than returning an error indication).
3678 \S{utils-index234} \cw{index234()}
3680 \c void *index234(tree234 *t, int index);
3682 Returns a pointer to the \c{index}th element of the tree, or
3683 \cw{NULL} if \c{index} is out of range. Elements of the tree are
3686 \S{utils-find234} \cw{find234()}
3688 \c void *find234(tree234 *t, void *e, cmpfn234 cmp);
3690 Searches for an element comparing equal to \c{e} in a sorted tree.
3692 If \c{cmp} is \cw{NULL}, the tree's ordinary comparison function
3693 will be used to perform the search. However, sometimes you don't
3694 want that; suppose, for example, each of your elements is a big
3695 structure containing a \c{char *} name field, and you want to find
3696 the element with a given name. You \e{could} achieve this by
3697 constructing a fake element structure, setting its name field
3698 appropriately, and passing it to \cw{find234()}, but you might find
3699 it more convenient to pass \e{just} a name string to \cw{find234()},
3700 supplying an alternative comparison function which expects one of
3701 its arguments to be a bare name and the other to be a large
3702 structure containing a name field.
3704 Therefore, if \c{cmp} is not \cw{NULL}, then it will be used to
3705 compare \c{e} to elements of the tree. The first argument passed to
3706 \c{cmp} will always be \c{e}; the second will be an element of the
3709 (See \k{utils-newtree234} for the definition of the \c{cmpfn234}
3710 function pointer type.)
3712 The returned value is the element found, or \cw{NULL} if the search
3715 \S{utils-findrel234} \cw{findrel234()}
3717 \c void *findrel234(tree234 *t, void *e, cmpfn234 cmp, int relation);
3719 This function is like \cw{find234()}, but has the additional ability
3720 to do a \e{relative} search. The additional parameter \c{relation}
3721 can be one of the following values:
3725 \dd Find only an element that compares equal to \c{e}. This is
3726 exactly the behaviour of \cw{find234()}.
3730 \dd Find the greatest element that compares strictly less than
3731 \c{e}. \c{e} may be \cw{NULL}, in which case it finds the greatest
3732 element in the whole tree (which could also be done by
3733 \cw{index234(t, count234(t)-1)}).
3737 \dd Find the greatest element that compares less than or equal to
3738 \c{e}. (That is, find an element that compares equal to \c{e} if
3739 possible, but failing that settle for something just less than it.)
3743 \dd Find the smallest element that compares strictly greater than
3744 \c{e}. \c{e} may be \cw{NULL}, in which case it finds the smallest
3745 element in the whole tree (which could also be done by
3746 \cw{index234(t, 0)}).
3750 \dd Find the smallest element that compares greater than or equal to
3751 \c{e}. (That is, find an element that compares equal to \c{e} if
3752 possible, but failing that settle for something just bigger than
3755 Return value, as before, is the element found or \cw{NULL} if no
3756 element satisfied the search criterion.
3758 \S{utils-findpos234} \cw{findpos234()}
3760 \c void *findpos234(tree234 *t, void *e, cmpfn234 cmp, int *index);
3762 This function is like \cw{find234()}, but has the additional feature
3763 of returning the index of the element found in the tree; that index
3764 is written to \c{*index} in the event of a successful search (a
3765 non-\cw{NULL} return value).
3767 \c{index} may be \cw{NULL}, in which case this function behaves
3768 exactly like \cw{find234()}.
3770 \S{utils-findrelpos234} \cw{findrelpos234()}
3772 \c void *findrelpos234(tree234 *t, void *e, cmpfn234 cmp, int relation,
3775 This function combines all the features of \cw{findrel234()} and
3778 \S{utils-del234} \cw{del234()}
3780 \c void *del234(tree234 *t, void *e);
3782 Finds an element comparing equal to \c{e} in the tree, deletes it,
3785 The input tree must be sorted.
3787 The element found might be \c{e} itself, or might merely compare
3790 Return value is \cw{NULL} if no such element is found.
3792 \S{utils-delpos234} \cw{delpos234()}
3794 \c void *delpos234(tree234 *t, int index);
3796 Deletes the element at position \c{index} in the tree, and returns
3799 Return value is \cw{NULL} if the index is out of range.
3801 \S{utils-count234} \cw{count234()}
3803 \c int count234(tree234 *t);
3805 Returns the number of elements currently in the tree.
3807 \S{utils-splitpos234} \cw{splitpos234()}
3809 \c tree234 *splitpos234(tree234 *t, int index, int before);
3811 Splits the input tree into two pieces at a given position, and
3812 creates a new tree containing all the elements on one side of that
3815 If \c{before} is \cw{TRUE}, then all the items at or after position
3816 \c{index} are left in the input tree, and the items before that
3817 point are returned in the new tree. Otherwise, the reverse happens:
3818 all the items at or after \c{index} are moved into the new tree, and
3819 those before that point are left in the old one.
3821 If \c{index} is equal to 0 or to the number of elements in the input
3822 tree, then one of the two trees will end up empty (and this is not
3823 an error condition). If \c{index} is further out of range in either
3824 direction, the operation will fail completely and return \cw{NULL}.
3826 This operation completes in \cw{O(log N)} time, no matter how large
3827 the tree or how balanced or unbalanced the split.
3829 \S{utils-split234} \cw{split234()}
3831 \c tree234 *split234(tree234 *t, void *e, cmpfn234 cmp, int rel);
3833 Splits a sorted tree according to its sort order.
3835 \c{rel} can be any of the relation constants described in
3836 \k{utils-findrel234}, \e{except} for \cw{REL234_EQ}. All the
3837 elements having that relation to \c{e} will be transferred into the
3838 new tree; the rest will be left in the old one.
3840 The parameter \c{cmp} has the same semantics as it does in
3841 \cw{find234()}: if it is not \cw{NULL}, it will be used in place of
3842 the tree's own comparison function when comparing elements to \c{e},
3843 in such a way that \c{e} itself is always the first of its two
3846 Again, this operation completes in \cw{O(log N)} time, no matter how
3847 large the tree or how balanced or unbalanced the split.
3849 \S{utils-join234} \cw{join234()}
3851 \c tree234 *join234(tree234 *t1, tree234 *t2);
3853 Joins two trees together by concatenating the lists they represent.
3854 All the elements of \c{t2} are moved into \c{t1}, in such a way that
3855 they appear \e{after} the elements of \c{t1}. The tree \c{t2} is
3856 freed; the return value is \c{t1}.
3858 If you apply this function to a sorted tree and it violates the sort
3859 order (i.e. the smallest element in \c{t2} is smaller than or equal
3860 to the largest element in \c{t1}), the operation will fail and
3863 This operation completes in \cw{O(log N)} time, no matter how large
3864 the trees being joined together.
3866 \S{utils-join234r} \cw{join234r()}
3868 \c tree234 *join234r(tree234 *t1, tree234 *t2);
3870 Joins two trees together in exactly the same way as \cw{join234()},
3871 but this time the combined tree is returned in \c{t2}, and \c{t1} is
3872 destroyed. The elements in \c{t1} still appear before those in
3875 Again, this operation completes in \cw{O(log N)} time, no matter how
3876 large the trees being joined together.
3878 \S{utils-copytree234} \cw{copytree234()}
3880 \c tree234 *copytree234(tree234 *t, copyfn234 copyfn,
3881 \c void *copyfnstate);
3883 Makes a copy of an entire tree.
3885 If \c{copyfn} is \cw{NULL}, the tree will be copied but the elements
3886 will not be; i.e. the new tree will contain pointers to exactly the
3887 same physical elements as the old one.
3889 If you want to copy each actual element during the operation, you
3890 can instead pass a function in \c{copyfn} which makes a copy of each
3891 element. That function has the prototype
3893 \c typedef void *(*copyfn234)(void *state, void *element);
3895 and every time it is called, the \c{state} parameter will be set to
3896 the value you passed in as \c{copyfnstate}.
3898 \H{utils-misc} Miscellaneous utility functions and macros
3900 This section contains all the utility functions which didn't
3901 sensibly fit anywhere else.
3903 \S{utils-truefalse} \cw{TRUE} and \cw{FALSE}
3905 The main Puzzles header file defines the macros \cw{TRUE} and
3906 \cw{FALSE}, which are used throughout the code in place of 1 and 0
3907 (respectively) to indicate that the values are in a boolean context.
3908 For code base consistency, I'd prefer it if submissions of new code
3909 followed this convention as well.
3911 \S{utils-maxmin} \cw{max()} and \cw{min()}
3913 The main Puzzles header file defines the pretty standard macros
3914 \cw{max()} and \cw{min()}, each of which is given two arguments and
3915 returns the one which compares greater or less respectively.
3917 These macros may evaluate their arguments multiple times. Avoid side
3920 \S{utils-pi} \cw{PI}
3922 The main Puzzles header file defines a macro \cw{PI} which expands
3923 to a floating-point constant representing pi.
3925 (I've never understood why ANSI's \cw{<math.h>} doesn't define this.
3928 \S{utils-obfuscate-bitmap} \cw{obfuscate_bitmap()}
3930 \c void obfuscate_bitmap(unsigned char *bmp, int bits, int decode);
3932 This function obscures the contents of a piece of data, by
3933 cryptographic methods. It is useful for games of hidden information
3934 (such as Mines, Guess or Black Box), in which the game ID
3935 theoretically reveals all the information the player is supposed to
3936 be trying to guess. So in order that players should be able to send
3937 game IDs to one another without accidentally spoiling the resulting
3938 game by looking at them, these games obfuscate their game IDs using
3941 Although the obfuscation function is cryptographic, it cannot
3942 properly be called encryption because it has no key. Therefore,
3943 anybody motivated enough can re-implement it, or hack it out of the
3944 Puzzles source, and strip the obfuscation off one of these game IDs
3945 to see what lies beneath. (Indeed, they could usually do it much
3946 more easily than that, by entering the game ID into their own copy
3947 of the puzzle and hitting Solve.) The aim is not to protect against
3948 a determined attacker; the aim is simply to protect people who
3949 wanted to play the game honestly from \e{accidentally} spoiling
3952 The input argument \c{bmp} points at a piece of memory to be
3953 obfuscated. \c{bits} gives the length of the data. Note that that
3954 length is in \e{bits} rather than bytes: if you ask for obfuscation
3955 of a partial number of bytes, then you will get it. Bytes are
3956 considered to be used from the top down: thus, for example, setting
3957 \c{bits} to 10 will cover the whole of \cw{bmp[0]} and the \e{top
3958 two} bits of \cw{bmp[1]}. The remainder of a partially used byte is
3959 undefined (i.e. it may be corrupted by the function).
3961 The parameter \c{decode} is \cw{FALSE} for an encoding operation,
3962 and \cw{TRUE} for a decoding operation. Each is the inverse of the
3963 other. (There's no particular reason you shouldn't obfuscate by
3964 decoding and restore cleartext by encoding, if you really wanted to;
3965 it should still work.)
3967 The input bitmap is processed in place.
3969 \S{utils-bin2hex} \cw{bin2hex()}
3971 \c char *bin2hex(const unsigned char *in, int inlen);
3973 This function takes an input byte array and converts it into an
3974 ASCII string encoding those bytes in (lower-case) hex. It returns a
3975 dynamically allocated string containing that encoding.
3977 This function is useful for encoding the result of
3978 \cw{obfuscate_bitmap()} in printable ASCII for use in game IDs.
3980 \S{utils-hex2bin} \cw{hex2bin()}
3982 \c unsigned char *hex2bin(const char *in, int outlen);
3984 This function takes an ASCII string containing hex digits, and
3985 converts it back into a byte array of length \c{outlen}. If there
3986 aren't enough hex digits in the string, the contents of the
3987 resulting array will be undefined.
3989 This function is the inverse of \cw{bin2hex()}.
3991 \S{utils-game-mkhighlight} \cw{game_mkhighlight()}
3993 \c void game_mkhighlight(frontend *fe, float *ret,
3994 \c int background, int highlight, int lowlight);
3996 It's reasonably common for a puzzle game's graphics to use
3997 highlights and lowlights to indicate \q{raised} or \q{lowered}
3998 sections. Fifteen, Sixteen and Twiddle are good examples of this.
4000 Puzzles using this graphical style are running a risk if they just
4001 use whatever background colour is supplied to them by the front end,
4002 because that background colour might be too light to see any
4003 highlights on at all. (In particular, it's not unheard of for the
4004 front end to specify a default background colour of white.)
4006 Therefore, such puzzles can call this utility function from their
4007 \cw{colours()} routine (\k{backend-colours}). You pass it your front
4008 end handle, a pointer to the start of your return array, and three
4009 colour indices. It will:
4011 \b call \cw{frontend_default_colour()} (\k{frontend-default-colour})
4012 to fetch the front end's default background colour
4014 \b alter the brightness of that colour if it's unsuitable
4016 \b define brighter and darker variants of the colour to be used as
4017 highlights and lowlights
4019 \b write those results into the relevant positions in the \c{ret}
4022 Thus, \cw{ret[background*3]} to \cw{ret[background*3+2]} will be set
4023 to RGB values defining a sensible background colour, and similary
4024 \c{highlight} and \c{lowlight} will be set to sensible colours.
4026 \C{writing} How to write a new puzzle
4028 This chapter gives a guide to how to actually write a new puzzle:
4029 where to start, what to do first, how to solve common problems.
4031 The previous chapters have been largely composed of facts. This one
4034 \H{writing-editorial} Choosing a puzzle
4036 Before you start writing a puzzle, you have to choose one. Your
4037 taste in puzzle games is up to you, of course; and, in fact, you're
4038 probably reading this guide because you've \e{already} thought of a
4039 game you want to write. But if you want to get it accepted into the
4040 official Puzzles distribution, then there's a criterion it has to
4043 The current Puzzles editorial policy is that all games should be
4044 \e{fair}. A fair game is one which a player can only fail to
4045 complete through demonstrable lack of skill \dash that is, such that
4046 a better player in the same situation would have \e{known} to do
4047 something different.
4049 For a start, that means every game presented to the user must have
4050 \e{at least one solution}. Giving the unsuspecting user a puzzle
4051 which is actually impossible is not acceptable. (There is an
4052 exception: if the user has selected some non-default option which is
4053 clearly labelled as potentially unfair, \e{then} you're allowed to
4054 generate possibly insoluble puzzles, because the user isn't
4055 unsuspecting any more. Same Game and Mines both have options of this
4058 Also, this actually \e{rules out} games such as Klondike, or the
4059 normal form of Mahjong Solitaire. Those games have the property that
4060 even if there is a solution (i.e. some sequence of moves which will
4061 get from the start state to the solved state), the player doesn't
4062 necessarily have enough information to \e{find} that solution. In
4063 both games, it is possible to reach a dead end because you had an
4064 arbitrary choice to make and made it the wrong way. This violates
4065 the fairness criterion, because a better player couldn't have known
4066 they needed to make the other choice.
4068 (GNOME has a variant on Mahjong Solitaire which makes it fair: there
4069 is a Shuffle operation which randomly permutes all the remaining
4070 tiles without changing their positions, which allows you to get out
4071 of a sticky situation. Using this operation adds a 60-second penalty
4072 to your solution time, so it's to the player's advantage to try to
4073 minimise the chance of having to use it. It's still possible to
4074 render the game uncompletable if you end up with only two tiles
4075 vertically stacked, but that's easy to foresee and avoid using a
4076 shuffle operation. This form of the game \e{is} fair. Implementing
4077 it in Puzzles would require an infrastructure change so that the
4078 back end could communicate time penalties to the mid-end, but that
4079 would be easy enough.)
4081 Providing a \e{unique} solution is a little more negotiable; it
4082 depends on the puzzle. Solo would have been of unacceptably low
4083 quality if it didn't always have a unique solution, whereas Twiddle
4084 inherently has multiple solutions by its very nature and it would
4085 have been meaningless to even \e{suggest} making it uniquely
4086 soluble. Somewhere in between, Flip could reasonably be made to have
4087 unique solutions (by enforcing a zero-dimension kernel in every
4088 generated matrix) but it doesn't seem like a serious quality problem
4091 Of course, you don't \e{have} to care about all this. There's
4092 nothing stopping you implementing any puzzle you want to if you're
4093 happy to maintain your puzzle yourself, distribute it from your own
4094 web site, fork the Puzzles code completely, or anything like that.
4095 It's free software; you can do what you like with it. But any game
4096 that you want to be accepted into \e{my} Puzzles code base has to
4097 satisfy the fairness criterion, which means all randomly generated
4098 puzzles must have a solution (unless the user has deliberately
4099 chosen otherwise) and it must be possible \e{in theory} to find that
4100 solution without having to guess.
4102 \H{writing-gs} Getting started
4104 The simplest way to start writing a new puzzle is to copy
4105 \c{nullgame.c}. This is a template puzzle source file which does
4106 almost nothing, but which contains all the back end function
4107 prototypes and declares the back end data structure correctly. It is
4108 built every time the rest of Puzzles is built, to ensure that it
4109 doesn't get out of sync with the code and remains buildable.
4111 So start by copying \c{nullgame.c} into your new source file. Then
4112 you'll gradually add functionality until the very boring Null Game
4113 turns into your real game.
4115 Next you'll need to add your puzzle to the Makefiles, in order to
4116 compile it conveniently. \e{Do not edit the Makefiles}: they are
4117 created automatically by the script \c{mkfiles.pl}, from the file
4118 called \c{Recipe}. Edit \c{Recipe}, and then re-run \c{mkfiles.pl}.
4120 Also, don't forget to add your puzzle to \c{list.c}: if you don't,
4121 then it will still run fine on platforms which build each puzzle
4122 separately, but Mac OS X and other monolithic platforms will not
4123 include your new puzzle in their single binary.
4125 Once your source file is building, you can move on to the fun bit.
4127 \S{writing-generation} Puzzle generation
4129 Randomly generating instances of your puzzle is almost certain to be
4130 the most difficult part of the code, and also the task with the
4131 highest chance of turning out to be completely infeasible. Therefore
4132 I strongly recommend doing it \e{first}, so that if it all goes
4133 horribly wrong you haven't wasted any more time than you absolutely
4134 had to. What I usually do is to take an unmodified \c{nullgame.c},
4135 and start adding code to \cw{new_game_desc()} which tries to
4136 generate a puzzle instance and print it out using \cw{printf()}.
4137 Once that's working, \e{then} I start connecting it up to the return
4138 value of \cw{new_game_desc()}, populating other structures like
4139 \c{game_params}, and generally writing the rest of the source file.
4141 There are many ways to generate a puzzle which is known to be
4142 soluble. In this section I list all the methods I currently know of,
4143 in case any of them can be applied to your puzzle. (Not all of these
4144 methods will work, or in some cases even make sense, for all
4147 Some puzzles are mathematically tractable, meaning you can work out
4148 in advance which instances are soluble. Sixteen, for example, has a
4149 parity constraint in some settings which renders exactly half the
4150 game space unreachable, but it can be mathematically proved that any
4151 position not in that half \e{is} reachable. Therefore, Sixteen's
4152 grid generation simply consists of selecting at random from a well
4153 defined subset of the game space. Cube in its default state is even
4154 easier: \e{every} possible arrangement of the blue squares and the
4155 cube's starting position is soluble!
4157 Another option is to redefine what you mean by \q{soluble}. Black
4158 Box takes this approach. There are layouts of balls in the box which
4159 are completely indistinguishable from one another no matter how many
4160 beams you fire into the box from which angles, which would normally
4161 be grounds for declaring those layouts unfair; but fortunately,
4162 detecting that indistinguishability is computationally easy. So
4163 Black Box doesn't demand that your ball placements match its own; it
4164 merely demands that your ball placements be \e{indistinguishable}
4165 from the ones it was thinking of. If you have an ambiguous puzzle,
4166 then any of the possible answers is considered to be a solution.
4167 Having redefined the rules in that way, any puzzle is soluble again.
4169 Those are the simple techniques. If they don't work, you have to get
4172 One way to generate a soluble puzzle is to start from the solved
4173 state and make inverse moves until you reach a starting state. Then
4174 you know there's a solution, because you can just list the inverse
4175 moves you made and make them in the opposite order to return to the
4178 This method can be simple and effective for puzzles where you get to
4179 decide what's a starting state and what's not. In Pegs, for example,
4180 the generator begins with one peg in the centre of the board and
4181 makes inverse moves until it gets bored; in this puzzle, valid
4182 inverse moves are easy to detect, and \e{any} state that's reachable
4183 from the solved state by inverse moves is a reasonable starting
4184 position. So Pegs just continues making inverse moves until the
4185 board satisfies some criteria about extent and density, and then
4186 stops and declares itself done.
4188 For other puzzles, it can be a lot more difficult. Same Game uses
4189 this strategy too, and it's lucky to get away with it at all: valid
4190 inverse moves aren't easy to find (because although it's easy to
4191 insert additional squares in a Same Game position, it's difficult to
4192 arrange that \e{after} the insertion they aren't adjacent to any
4193 other squares of the same colour), so you're constantly at risk of
4194 running out of options and having to backtrack or start again. Also,
4195 Same Game grids never start off half-empty, which means you can't
4196 just stop when you run out of moves \dash you have to find a way to
4197 fill the grid up \e{completely}.
4199 The other way to generate a puzzle that's soluble is to start from
4200 the other end, and actually write a \e{solver}. This tends to ensure
4201 that a puzzle has a \e{unique} solution over and above having a
4202 solution at all, so it's a good technique to apply to puzzles for
4203 which that's important.
4205 One theoretical drawback of generating soluble puzzles by using a
4206 solver is that your puzzles are restricted in difficulty to those
4207 which the solver can handle. (Most solvers are not fully general:
4208 many sets of puzzle rules are NP-complete or otherwise nasty, so
4209 most solvers can only handle a subset of the theoretically soluble
4210 puzzles.) It's been my experience in practice, however, that this
4211 usually isn't a problem; computers are good at very different things
4212 from humans, and what the computer thinks is nice and easy might
4213 still be pleasantly challenging for a human. For example, when
4214 solving Dominosa puzzles I frequently find myself using a variety of
4215 reasoning techniques that my solver doesn't know about; in
4216 principle, therefore, I should be able to solve the puzzle using
4217 only those techniques it \e{does} know about, but this would involve
4218 repeatedly searching the entire grid for the one simple deduction I
4219 can make. Computers are good at this sort of exhaustive search, but
4220 it's been my experience that human solvers prefer to do more complex
4221 deductions than to spend ages searching for simple ones. So in many
4222 cases I don't find my own playing experience to be limited by the
4223 restrictions on the solver.
4225 (This isn't \e{always} the case. Solo is a counter-example;
4226 generating Solo puzzles using a simple solver does lead to
4227 qualitatively easier puzzles. Therefore I had to make the Solo
4228 solver rather more advanced than most of them.)
4230 There are several different ways to apply a solver to the problem of
4231 generating a soluble puzzle. I list a few of them below.
4233 The simplest approach is brute force: randomly generate a puzzle,
4234 use the solver to see if it's soluble, and if not, throw it away and
4235 try again until you get lucky. This is often a viable technique if
4236 all else fails, but it tends not to scale well: for many puzzle
4237 types, the probability of finding a uniquely soluble instance
4238 decreases sharply as puzzle size goes up, so this technique might
4239 work reasonably fast for small puzzles but take (almost) forever at
4240 larger sizes. Still, if there's no other alternative it can be
4241 usable: Pattern and Dominosa both use this technique. (However,
4242 Dominosa has a means of tweaking the randomly generated grids to
4243 increase the \e{probability} of them being soluble, by ruling out
4244 one of the most common ambiguous cases. This improved generation
4245 speed by over a factor of 10 on the highest preset!)
4247 An approach which can be more scalable involves generating a grid
4248 and then tweaking it to make it soluble. This is the technique used
4249 by Mines and also by Net: first a random puzzle is generated, and
4250 then the solver is run to see how far it gets. Sometimes the solver
4251 will get stuck; when that happens, examine the area it's having
4252 trouble with, and make a small random change in that area to allow
4253 it to make more progress. Continue solving (possibly even without
4254 restarting the solver), tweaking as necessary, until the solver
4255 finishes. Then restart the solver from the beginning to ensure that
4256 the tweaks haven't caused new problems in the process of solving old
4257 ones (which can sometimes happen).
4259 This strategy works well in situations where the usual solver
4260 failure mode is to get stuck in an easily localised spot. Thus it
4261 works well for Net and Mines, whose most common failure mode tends
4262 to be that most of the grid is fine but there are a few widely
4263 separated ambiguous sections; but it would work less well for
4264 Dominosa, in which the way you get stuck is to have scoured the
4265 whole grid and not found anything you can deduce \e{anywhere}. Also,
4266 it relies on there being a low probability that tweaking the grid
4267 introduces a new problem at the same time as solving the old one;
4268 Mines and Net also have the property that most of their deductions
4269 are local, so that it's very unlikely for a tweak to affect
4270 something half way across the grid from the location where it was
4271 applied. In Dominosa, by contrast, a lot of deductions use
4272 information about half the grid (\q{out of all the sixes, only one
4273 is next to a three}, which can depend on the values of up to 32 of
4274 the 56 squares in the default setting!), so this tweaking strategy
4275 would be rather less likely to work well.
4277 A more specialised strategy is that used in Solo and Slant. These
4278 puzzles have the property that they derive their difficulty from not
4279 presenting all the available clues. (In Solo's case, if all the
4280 possible clues were provided then the puzzle would already be
4281 solved; in Slant it would still require user action to fill in the
4282 lines, but it would present no challenge at all). Therefore, a
4283 simple generation technique is to leave the decision of which clues
4284 to provide until the last minute. In other words, first generate a
4285 random \e{filled} grid with all possible clues present, and then
4286 gradually remove clues for as long as the solver reports that it's
4287 still soluble. Unlike the methods described above, this technique
4288 \e{cannot} fail \dash once you've got a filled grid, nothing can
4289 stop you from being able to convert it into a viable puzzle.
4290 However, it wouldn't even be meaningful to apply this technique to
4291 (say) Pattern, in which clues can never be left out, so the only way
4292 to affect the set of clues is by altering the solution.
4294 (Unfortunately, Solo is complicated by the need to provide puzzles
4295 at varying difficulty levels. It's easy enough to generate a puzzle
4296 of \e{at most} a given level of difficulty; you just have a solver
4297 with configurable intelligence, and you set it to a given level and
4298 apply the above technique, thus guaranteeing that the resulting grid
4299 is solvable by someone with at most that much intelligence. However,
4300 generating a puzzle of \e{at least} a given level of difficulty is
4301 rather harder; if you go for \e{at most} Intermediate level, you're
4302 likely to find that you've accidentally generated a Trivial grid a
4303 lot of the time, because removing just one number is sufficient to
4304 take the puzzle from Trivial straight to Ambiguous. In that
4305 situation Solo has no remaining options but to throw the puzzle away
4308 A final strategy is to use the solver \e{during} puzzle
4309 construction: lay out a bit of the grid, run the solver to see what
4310 it allows you to deduce, and then lay out a bit more to allow the
4311 solver to make more progress. There are articles on the web that
4312 recommend constructing Sudoku puzzles by this method (which is
4313 completely the opposite way round to how Solo does it); for Sudoku
4314 it has the advantage that you get to specify your clue squares in
4315 advance (so you can have them make pretty patterns).
4317 Rectangles uses a strategy along these lines. First it generates a
4318 grid by placing the actual rectangles; then it has to decide where
4319 in each rectangle to place a number. It uses a solver to help it
4320 place the numbers in such a way as to ensure a unique solution. It
4321 does this by means of running a test solver, but it runs the solver
4322 \e{before} it's placed any of the numbers \dash which means the
4323 solver must be capable of coping with uncertainty about exactly
4324 where the numbers are! It runs the solver as far as it can until it
4325 gets stuck; then it narrows down the possible positions of a number
4326 in order to allow the solver to make more progress, and so on. Most
4327 of the time this process terminates with the grid fully solved, at
4328 which point any remaining number-placement decisions can be made at
4329 random from the options not so far ruled out. Note that unlike the
4330 Net/Mines tweaking strategy described above, this algorithm does not
4331 require a checking run after it completes: if it finishes
4332 successfully at all, then it has definitely produced a uniquely
4335 Most of the strategies described above are not 100% reliable. Each
4336 one has a failure rate: every so often it has to throw out the whole
4337 grid and generate a fresh one from scratch. (Solo's strategy would
4338 be the exception, if it weren't for the need to provide configurable
4339 difficulty levels.) Occasional failures are not a fundamental
4340 problem in this sort of work, however: it's just a question of
4341 dividing the grid generation time by the success rate (if it takes
4342 10ms to generate a candidate grid and 1/5 of them work, then it will
4343 take 50ms on average to generate a viable one), and seeing whether
4344 the expected time taken to \e{successfully} generate a puzzle is
4345 unacceptably slow. Dominosa's generator has a very low success rate
4346 (about 1 out of 20 candidate grids turn out to be usable, and if you
4347 think \e{that's} bad then go and look at the source code and find
4348 the comment showing what the figures were before the generation-time
4349 tweaks!), but the generator itself is very fast so this doesn't
4350 matter. Rectangles has a slower generator, but fails well under 50%
4353 So don't be discouraged if you have an algorithm that doesn't always
4354 work: if it \e{nearly} always works, that's probably good enough.
4355 The one place where reliability is important is that your algorithm
4356 must never produce false positives: it must not claim a puzzle is
4357 soluble when it isn't. It can produce false negatives (failing to
4358 notice that a puzzle is soluble), and it can fail to generate a
4359 puzzle at all, provided it doesn't do either so often as to become
4362 One last piece of advice: for grid-based puzzles, when writing and
4363 testing your generation algorithm, it's almost always a good idea
4364 \e{not} to test it initially on a grid that's square (i.e.
4365 \cw{w==h}), because if the grid is square then you won't notice if
4366 you mistakenly write \c{h} instead of \c{w} (or vice versa)
4367 somewhere in the code. Use a rectangular grid for testing, and any
4368 size of grid will be likely to work after that.
4370 \S{writing-textformats} Designing textual description formats
4372 Another aspect of writing a puzzle which is worth putting some
4373 thought into is the design of the various text description formats:
4374 the format of the game parameter encoding, the game description
4375 encoding, and the move encoding.
4377 The first two of these should be reasonably intuitive for a user to
4378 type in; so provide some flexibility where possible. Suppose, for
4379 example, your parameter format consists of two numbers separated by
4380 an \c{x} to specify the grid dimensions (\c{10x10} or \c{20x15}),
4381 and then has some suffixes to specify other aspects of the game
4382 type. It's almost always a good idea in this situation to arrange
4383 that \cw{decode_params()} can handle the suffixes appearing in any
4384 order, even if \cw{encode_params()} only ever generates them in one
4387 These formats will also be expected to be reasonably stable: users
4388 will expect to be able to exchange game IDs with other users who
4389 aren't running exactly the same version of your game. So make them
4390 robust and stable: don't build too many assumptions into the game ID
4391 format which will have to be changed every time something subtle
4392 changes in the puzzle code.
4394 \H{writing-howto} Common how-to questions
4396 This section lists some common things people want to do when writing
4397 a puzzle, and describes how to achieve them within the Puzzles
4400 \S{writing-howto-cursor} Drawing objects at only one position
4402 A common phenomenon is to have an object described in the
4403 \c{game_state} or the \c{game_ui} which can only be at one position.
4404 A cursor \dash probably specified in the \c{game_ui} \dash is a good
4407 In the \c{game_ui}, it would \e{obviously} be silly to have an array
4408 covering the whole game grid with a boolean flag stating whether the
4409 cursor was at each position. Doing that would waste space, would
4410 make it difficult to find the cursor in order to do anything with
4411 it, and would introduce the potential for synchronisation bugs in
4412 which you ended up with two cursors or none. The obviously sensible
4413 way to store a cursor in the \c{game_ui} is to have fields directly
4414 encoding the cursor's coordinates.
4416 However, it is a mistake to assume that the same logic applies to
4417 the \c{game_drawstate}. If you replicate the cursor position fields
4418 in the draw state, the redraw code will get very complicated. In the
4419 draw state, in fact, it \e{is} probably the right thing to have a
4420 cursor flag for every position in the grid. You probably have an
4421 array for the whole grid in the drawstate already (stating what is
4422 currently displayed in the window at each position); the sensible
4423 approach is to add a \q{cursor} flag to each element of that array.
4424 Then the main redraw loop will look something like this
4427 \c for (y = 0; y < h; y++) {
4428 \c for (x = 0; x < w; x++) {
4429 \c int value = state->symbol_at_position[y][x];
4430 \c if (x == ui->cursor_x && y == ui->cursor_y)
4432 \c if (ds->symbol_at_position[y][x] != value) {
4433 \c symbol_drawing_subroutine(dr, ds, x, y, value);
4434 \c ds->symbol_at_position[y][x] = value;
4439 This loop is very simple, pretty hard to get wrong, and
4440 \e{automatically} deals both with erasing the previous cursor and
4441 drawing the new one, with no special case code required.
4443 This type of loop is generally a sensible way to write a redraw
4444 function, in fact. The best thing is to ensure that the information
4445 stored in the draw state for each position tells you \e{everything}
4446 about what was drawn there. A good way to ensure that is to pass
4447 precisely the same information, and \e{only} that information, to a
4448 subroutine that does the actual drawing; then you know there's no
4449 additional information which affects the drawing but which you don't
4452 \S{writing-keyboard-cursor} Implementing a keyboard-controlled cursor
4454 It is often useful to provide a keyboard control method in a
4455 basically mouse-controlled game. A keyboard-controlled cursor is
4456 best implemented by storing its location in the \c{game_ui} (since
4457 if it were in the \c{game_state} then the user would have to
4458 separately undo every cursor move operation). So the procedure would
4461 \b Put cursor position fields in the \c{game_ui}.
4463 \b \cw{interpret_move()} responds to arrow keys by modifying the
4464 cursor position fields and returning \cw{""}.
4466 \b \cw{interpret_move()} responds to some sort of fire button by
4467 actually performing a move based on the current cursor location.
4469 \b You might want an additional \c{game_ui} field stating whether
4470 the cursor is currently visible, and having it disappear when a
4471 mouse action occurs (so that it doesn't clutter the display when not
4474 \b You might also want to automatically hide the cursor in
4475 \cw{changed_state()} when the current game state changes to one in
4476 which there is no move to make (which is the case in some types of
4479 \b \cw{redraw()} draws the cursor using the technique described in
4480 \k{writing-howto-cursor}.
4482 \S{writing-howto-dragging} Implementing draggable sprites
4484 Some games have a user interface which involves dragging some sort
4485 of game element around using the mouse. If you need to show a
4486 graphic moving smoothly over the top of other graphics, use a
4487 blitter (see \k{drawing-blitter} for the blitter API) to save the
4488 background underneath it. The typical scenario goes:
4490 \b Have a blitter field in the \c{game_drawstate}.
4492 \b Set the blitter field to \cw{NULL} in the game's
4493 \cw{new_drawstate()} function, since you don't yet know how big the
4494 piece of saved background needs to be.
4496 \b In the game's \cw{set_size()} function, once you know the size of
4497 the object you'll be dragging around the display and hence the
4498 required size of the blitter, actually allocate the blitter.
4500 \b In \cw{free_drawstate()}, free the blitter if it's not \cw{NULL}.
4502 \b In \cw{interpret_move()}, respond to mouse-down and mouse-drag
4503 events by updating some fields in the \cw{game_ui} which indicate
4504 that a drag is in progress.
4506 \b At the \e{very end} of \cw{redraw()}, after all other drawing has
4507 been done, draw the moving object if there is one. First save the
4508 background under the object in the blitter; then set a clip
4509 rectangle covering precisely the area you just saved (just in case
4510 anti-aliasing or some other error causes your drawing to go beyond
4511 the area you saved). Then draw the object, and call \cw{unclip()}.
4512 Finally, set a flag in the \cw{game_drawstate} that indicates that
4513 the blitter needs restoring.
4515 \b At the very start of \cw{redraw()}, before doing anything else at
4516 all, check the flag in the \cw{game_drawstate}, and if it says the
4517 blitter needs restoring then restore it. (Then clear the flag, so
4518 that this won't happen again in the next redraw if no moving object
4519 is drawn this time.)
4521 This way, you will be able to write the rest of the redraw function
4522 completely ignoring the dragged object, as if it were floating above
4523 your bitmap and being completely separate.
4525 \S{writing-ref-counting} Sharing large invariant data between all
4528 In some puzzles, there is a large amount of data which never changes
4529 between game states. The array of numbers in Dominosa is a good
4532 You \e{could} dynamically allocate a copy of that array in every
4533 \c{game_state}, and have \cw{dup_game()} make a fresh copy of it for
4534 every new \c{game_state}; but it would waste memory and time. A
4535 more efficient way is to use a reference-counted structure.
4537 \b Define a structure type containing the data in question, and also
4538 containing an integer reference count.
4540 \b Have a field in \c{game_state} which is a pointer to this
4543 \b In \cw{new_game()}, when creating a fresh game state at the start
4544 of a new game, create an instance of this structure, initialise it
4545 with the invariant data, and set its reference count to 1.
4547 \b In \cw{dup_game()}, rather than making a copy of the structure
4548 for the new game state, simply set the new game state to point at
4549 the same copy of the structure, and increment its reference count.
4551 \b In \cw{free_game()}, decrement the reference count in the
4552 structure pointed to by the game state; if the count reaches zero,
4555 This way, the invariant data will persist for only as long as it's
4556 genuinely needed; \e{as soon} as the last game state for a
4557 particular puzzle instance is freed, the invariant data for that
4558 puzzle will vanish as well. Reference counting is a very efficient
4559 form of garbage collection, when it works at all. (Which it does in
4560 this instance, of course, because there's no possibility of circular
4563 \S{writing-flash-types} Implementing multiple types of flash
4565 In some games you need to flash in more than one different way.
4566 Mines, for example, flashes white when you win, and flashes red when
4567 you tread on a mine and die.
4569 The simple way to do this is:
4571 \b Have a field in the \c{game_ui} which describes the type of flash.
4573 \b In \cw{flash_length()}, examine the old and new game states to
4574 decide whether a flash is required and what type. Write the type of
4575 flash to the \c{game_ui} field whenever you return non-zero.
4577 \b In \cw{redraw()}, when you detect that \c{flash_time} is
4578 non-zero, examine the field in \c{game_ui} to decide which type of
4581 \cw{redraw()} will never be called with \c{flash_time} non-zero
4582 unless \cw{flash_length()} was first called to tell the mid-end that
4583 a flash was required; so whenever \cw{redraw()} notices that
4584 \c{flash_time} is non-zero, you can be sure that the field in
4585 \c{game_ui} is correctly set.
4587 \S{writing-move-anim} Animating game moves
4589 A number of puzzle types benefit from a quick animation of each move
4592 For some games, such as Fifteen, this is particularly easy. Whenever
4593 \cw{redraw()} is called with \c{oldstate} non-\cw{NULL}, Fifteen
4594 simply compares the position of each tile in the two game states,
4595 and if the tile is not in the same place then it draws it some
4596 fraction of the way from its old position to its new position. This
4597 method copes automatically with undo.
4599 Other games are less obvious. In Sixteen, for example, you can't
4600 just draw each tile a fraction of the way from its old to its new
4601 position: if you did that, the end tile would zip very rapidly past
4602 all the others to get to the other end and that would look silly.
4603 (Worse, it would look inconsistent if the end tile was drawn on top
4604 going one way and on the bottom going the other way.)
4606 A useful trick here is to define a field or two in the game state
4607 that indicates what the last move was.
4609 \b Add a \q{last move} field to the \c{game_state} (or two or more
4610 fields if the move is complex enough to need them).
4612 \b \cw{new_game()} initialises this field to a null value for a new
4615 \b \cw{execute_move()} sets up the field to reflect the move it just
4618 \b \cw{redraw()} now needs to examine its \c{dir} parameter. If
4619 \c{dir} is positive, it determines the move being animated by
4620 looking at the last-move field in \c{newstate}; but if \c{dir} is
4621 negative, it has to look at the last-move field in \c{oldstate}, and
4622 invert whatever move it finds there.
4624 Note also that Sixteen needs to store the \e{direction} of the move,
4625 because you can't quite determine it by examining the row or column
4626 in question. You can in almost all cases, but when the row is
4627 precisely two squares long it doesn't work since a move in either
4628 direction looks the same. (You could argue that since moving a
4629 2-element row left and right has the same effect, it doesn't matter
4630 which one you animate; but in fact it's very disorienting to click
4631 the arrow left and find the row moving right, and almost as bad to
4632 undo a move to the right and find the game animating \e{another}
4635 \S{writing-conditional-anim} Animating drag operations
4637 In Untangle, moves are made by dragging a node from an old position
4638 to a new position. Therefore, at the time when the move is initially
4639 made, it should not be animated, because the node has already been
4640 dragged to the right place and doesn't need moving there. However,
4641 it's nice to animate the same move if it's later undone or redone.
4642 This requires a bit of fiddling.
4644 The obvious approach is to have a flag in the \c{game_ui} which
4645 inhibits move animation, and to set that flag in
4646 \cw{interpret_move()}. The question is, when would the flag be reset
4647 again? The obvious place to do so is \cw{changed_state()}, which
4648 will be called once per move. But it will be called \e{before}
4649 \cw{anim_length()}, so if it resets the flag then \cw{anim_length()}
4650 will never see the flag set at all.
4652 The solution is to have \e{two} flags in a queue.
4654 \b Define two flags in \c{game_ui}; let's call them \q{current} and
4657 \b Set both to \cw{FALSE} in \c{new_ui()}.
4659 \b When a drag operation completes in \cw{interpret_move()}, set the
4660 \q{next} flag to \cw{TRUE}.
4662 \b Every time \cw{changed_state()} is called, set the value of
4663 \q{current} to the value in \q{next}, and then set the value of
4664 \q{next} to \cw{FALSE}.
4666 \b That way, \q{current} will be \cw{TRUE} \e{after} a call to
4667 \cw{changed_state()} if and only if that call to
4668 \cw{changed_state()} was the result of a drag operation processed by
4669 \cw{interpret_move()}. Any other call to \cw{changed_state()}, due
4670 to an Undo or a Redo or a Restart or a Solve, will leave \q{current}
4673 \b So now \cw{anim_length()} can request a move animation if and
4674 only if the \q{current} flag is \e{not} set.
4676 \S{writing-cheating} Inhibiting the victory flash when Solve is used
4678 Many games flash when you complete them, as a visual congratulation
4679 for having got to the end of the puzzle. It often seems like a good
4680 idea to disable that flash when the puzzle is brought to a solved
4681 state by means of the Solve operation.
4683 This is easily done:
4685 \b Add a \q{cheated} flag to the \c{game_state}.
4687 \b Set this flag to \cw{FALSE} in \cw{new_game()}.
4689 \b Have \cw{solve()} return a move description string which clearly
4690 identifies the move as a solve operation.
4692 \b Have \cw{execute_move()} respond to that clear identification by
4693 setting the \q{cheated} flag in the returned \c{game_state}. The
4694 flag will then be propagated to all subsequent game states, even if
4695 the user continues fiddling with the game after it is solved.
4697 \b \cw{flash_length()} now returns non-zero if \c{oldstate} is not
4698 completed and \c{newstate} is, \e{and} neither state has the
4699 \q{cheated} flag set.
4701 \H{writing-testing} Things to test once your puzzle is written
4703 Puzzle implementations written in this framework are self-testing as
4704 far as I could make them.
4706 Textual game and move descriptions, for example, are generated and
4707 parsed as part of the normal process of play. Therefore, if you can
4708 make moves in the game \e{at all} you can be reasonably confident
4709 that the mid-end serialisation interface will function correctly and
4710 you will be able to save your game. (By contrast, if I'd stuck with
4711 a single \cw{make_move()} function performing the jobs of both
4712 \cw{interpret_move()} and \cw{execute_move()}, and had separate
4713 functions to encode and decode a game state in string form, then
4714 those functions would not be used during normal play; so they could
4715 have been completely broken, and you'd never know it until you tried
4716 to save the game \dash which would have meant you'd have to test
4717 game saving \e{extensively} and make sure to test every possible
4718 type of game state. As an added bonus, doing it the way I did leads
4719 to smaller save files.)
4721 There is one exception to this, which is the string encoding of the
4722 \c{game_ui}. Most games do not store anything permanent in the
4723 \c{game_ui}, and hence do not need to put anything in its encode and
4724 decode functions; but if there is anything in there, you do need to
4725 test game loading and saving to ensure those functions work
4728 It's also worth testing undo and redo of all operations, to ensure
4729 that the redraw and the animations (if any) work properly. Failing
4730 to animate undo properly seems to be a common error.
4732 Other than that, just use your common sense.