chiark / gitweb /
Fix borders on the HTML menu bar.
[sgt-puzzles.git] / loopy.c
diff --git a/loopy.c b/loopy.c
index 85590fae986b9a47ea6ec6031943a65c949952a0..7d3436aacbc5862ab2d657f93c4b0c828143e4f6 100644 (file)
--- a/loopy.c
+++ b/loopy.c
@@ -118,6 +118,7 @@ struct game_state {
     char *lines;
 
     unsigned char *line_errors;
+    int exactly_one_loop;
 
     int solved;
     int cheated;
@@ -231,7 +232,7 @@ struct game_drawstate {
     char *clue_satisfied;
 };
 
-static char *validate_desc(game_params *params, char *desc);
+static char *validate_desc(const game_params *params, const char *desc);
 static int dot_order(const game_state* state, int i, char line_type);
 static int face_order(const game_state* state, int i, char line_type);
 static solver_state *solve_game_rec(const solver_state *sstate);
@@ -242,32 +243,55 @@ static void check_caches(const solver_state* sstate);
 #define check_caches(s)
 #endif
 
-/* ------- List of grid generators ------- */
-#define GRIDLIST(A) \
-    A(Squares,GRID_SQUARE,3,3) \
-    A(Triangular,GRID_TRIANGULAR,3,3) \
-    A(Honeycomb,GRID_HONEYCOMB,3,3) \
-    A(Snub-Square,GRID_SNUBSQUARE,3,3) \
-    A(Cairo,GRID_CAIRO,3,4) \
-    A(Great-Hexagonal,GRID_GREATHEXAGONAL,3,3) \
-    A(Octagonal,GRID_OCTAGONAL,3,3) \
-    A(Kites,GRID_KITE,3,3) \
-    A(Floret,GRID_FLORET,1,2) \
-    A(Dodecagonal,GRID_DODECAGONAL,2,2) \
-    A(Great-Dodecagonal,GRID_GREATDODECAGONAL,2,2) \
-    A(Penrose (kite/dart),GRID_PENROSE_P2,3,3) \
-    A(Penrose (rhombs),GRID_PENROSE_P3,3,3)
-
-#define GRID_NAME(title,type,amin,omin) #title,
-#define GRID_CONFIG(title,type,amin,omin) ":" #title
-#define GRID_TYPE(title,type,amin,omin) type,
+/*
+ * Grid type config options available in Loopy.
+ *
+ * Annoyingly, we have to use an enum here which doesn't match up
+ * exactly to the grid-type enum in grid.h. Values in params->types
+ * are given by names such as LOOPY_GRID_SQUARE, which shouldn't be
+ * confused with GRID_SQUARE which is the value you pass to grid_new()
+ * and friends. So beware!
+ *
+ * (This is partly for historical reasons - Loopy's version of the
+ * enum is encoded in game parameter strings, so we keep it for
+ * backwards compatibility. But also, we need to store additional data
+ * here alongside each enum value, such as names for the presets menu,
+ * which isn't stored in grid.h; so we have to have our own list macro
+ * here anyway, and C doesn't make it easy to enforce that that lines
+ * up exactly with grid.h.)
+ *
+ * Do not add values to this list _except_ at the end, or old game ids
+ * will stop working!
+ */
+#define GRIDLIST(A)                                             \
+    A("Squares",SQUARE,3,3)                                     \
+    A("Triangular",TRIANGULAR,3,3)                              \
+    A("Honeycomb",HONEYCOMB,3,3)                                \
+    A("Snub-Square",SNUBSQUARE,3,3)                             \
+    A("Cairo",CAIRO,3,4)                                        \
+    A("Great-Hexagonal",GREATHEXAGONAL,3,3)                     \
+    A("Octagonal",OCTAGONAL,3,3)                                \
+    A("Kites",KITE,3,3)                                         \
+    A("Floret",FLORET,1,2)                                      \
+    A("Dodecagonal",DODECAGONAL,2,2)                            \
+    A("Great-Dodecagonal",GREATDODECAGONAL,2,2)                 \
+    A("Penrose (kite/dart)",PENROSE_P2,3,3)                     \
+    A("Penrose (rhombs)",PENROSE_P3,3,3)                        \
+    A("Great-Great-Dodecagonal",GREATGREATDODECAGONAL,2,2)      \
+    /* end of list */
+
+#define GRID_NAME(title,type,amin,omin) title,
+#define GRID_CONFIG(title,type,amin,omin) ":" title
+#define GRID_LOOPYTYPE(title,type,amin,omin) LOOPY_GRID_ ## type,
+#define GRID_GRIDTYPE(title,type,amin,omin) GRID_ ## type,
 #define GRID_SIZES(title,type,amin,omin) \
     {amin, omin, \
      "Width and height for this grid type must both be at least " #amin, \
      "At least one of width and height for this grid type must be at least " #omin,},
+enum { GRIDLIST(GRID_LOOPYTYPE) };
 static char const *const gridnames[] = { GRIDLIST(GRID_NAME) };
 #define GRID_CONFIGS GRIDLIST(GRID_CONFIG)
-static grid_type grid_types[] = { GRIDLIST(GRID_TYPE) };
+static grid_type grid_types[] = { GRIDLIST(GRID_GRIDTYPE) };
 #define NUM_GRID_TYPES (sizeof(grid_types) / sizeof(grid_types[0]))
 static const struct {
     int amin, omin;
@@ -277,7 +301,8 @@ static const struct {
 /* Generates a (dynamically allocated) new grid, according to the
  * type and size requested in params.  Does nothing if the grid is already
  * generated. */
-static grid *loopy_generate_grid(game_params *params, char *grid_desc)
+static grid *loopy_generate_grid(const game_params *params,
+                                 const char *grid_desc)
 {
     return grid_new(grid_types[params->type], params->w, params->h, grid_desc);
 }
@@ -306,7 +331,7 @@ static grid *loopy_generate_grid(game_params *params, char *grid_desc)
  * General struct manipulation and other straightforward code
  */
 
-static game_state *dup_game(game_state *state)
+static game_state *dup_game(const game_state *state)
 {
     game_state *ret = snew(game_state);
 
@@ -324,6 +349,7 @@ static game_state *dup_game(game_state *state)
 
     ret->line_errors = snewn(state->game_grid->num_edges, unsigned char);
     memcpy(ret->line_errors, state->line_errors, state->game_grid->num_edges);
+    ret->exactly_one_loop = state->exactly_one_loop;
 
     ret->grid_type = state->grid_type;
     return ret;
@@ -340,7 +366,7 @@ static void free_game(game_state *state)
     }
 }
 
-static solver_state *new_solver_state(game_state *state, int diff) {
+static solver_state *new_solver_state(const game_state *state, int diff) {
     int i;
     int num_dots = state->game_grid->num_dots;
     int num_faces = state->game_grid->num_faces;
@@ -479,7 +505,7 @@ static game_params *default_params(void)
     return ret;
 }
 
-static game_params *dup_params(game_params *params)
+static game_params *dup_params(const game_params *params)
 {
     game_params *ret = snew(game_params);
 
@@ -487,61 +513,82 @@ static game_params *dup_params(game_params *params)
     return ret;
 }
 
-static const game_params presets[] = {
+static const game_params loopy_presets_top[] = {
 #ifdef SMALL_SCREEN
-    {  7,  7, DIFF_EASY, 0 },
-    {  7,  7, DIFF_NORMAL, 0 },
-    {  7,  7, DIFF_HARD, 0 },
-    {  7,  7, DIFF_HARD, 1 },
-    {  7,  7, DIFF_HARD, 2 },
-    {  5,  5, DIFF_HARD, 3 },
-    {  7,  7, DIFF_HARD, 4 },
-    {  5,  4, DIFF_HARD, 5 },
-    {  5,  5, DIFF_HARD, 6 },
-    {  5,  5, DIFF_HARD, 7 },
-    {  3,  3, DIFF_HARD, 8 },
-    {  3,  3, DIFF_HARD, 9 },
-    {  3,  3, DIFF_HARD, 10 },
-    {  6,  6, DIFF_HARD, 11 },
-    {  6,  6, DIFF_HARD, 12 },
+    {  7,  7, DIFF_EASY,   LOOPY_GRID_SQUARE },
+    {  7,  7, DIFF_NORMAL, LOOPY_GRID_SQUARE },
+    {  7,  7, DIFF_HARD,   LOOPY_GRID_SQUARE },
+    {  7,  7, DIFF_HARD,   LOOPY_GRID_TRIANGULAR },
+    {  5,  5, DIFF_HARD,   LOOPY_GRID_SNUBSQUARE },
+    {  7,  7, DIFF_HARD,   LOOPY_GRID_CAIRO },
+    {  5,  5, DIFF_HARD,   LOOPY_GRID_KITE },
+    {  6,  6, DIFF_HARD,   LOOPY_GRID_PENROSE_P2 },
+    {  6,  6, DIFF_HARD,   LOOPY_GRID_PENROSE_P3 },
 #else
-    {  7,  7, DIFF_EASY, 0 },
-    {  10,  10, DIFF_EASY, 0 },
-    {  7,  7, DIFF_NORMAL, 0 },
-    {  10,  10, DIFF_NORMAL, 0 },
-    {  7,  7, DIFF_HARD, 0 },
-    {  10,  10, DIFF_HARD, 0 },
-    {  10,  10, DIFF_HARD, 1 },
-    {  12,  10, DIFF_HARD, 2 },
-    {  7,  7, DIFF_HARD, 3 },
-    {  9,  9, DIFF_HARD, 4 },
-    {  5,  4, DIFF_HARD, 5 },
-    {  7,  7, DIFF_HARD, 6 },
-    {  5,  5, DIFF_HARD, 7 },
-    {  5,  5, DIFF_HARD, 8 },
-    {  5,  4, DIFF_HARD, 9 },
-    {  5,  4, DIFF_HARD, 10 },
-    {  10, 10, DIFF_HARD, 11 },
-    {  10, 10, DIFF_HARD, 12 }
+    {  7,  7, DIFF_EASY,   LOOPY_GRID_SQUARE },
+    { 10, 10, DIFF_EASY,   LOOPY_GRID_SQUARE },
+    {  7,  7, DIFF_NORMAL, LOOPY_GRID_SQUARE },
+    { 10, 10, DIFF_NORMAL, LOOPY_GRID_SQUARE },
+    {  7,  7, DIFF_HARD,   LOOPY_GRID_SQUARE },
+    { 10, 10, DIFF_HARD,   LOOPY_GRID_SQUARE },
+    { 12, 10, DIFF_HARD,   LOOPY_GRID_TRIANGULAR },
+    {  7,  7, DIFF_HARD,   LOOPY_GRID_SNUBSQUARE },
+    {  9,  9, DIFF_HARD,   LOOPY_GRID_CAIRO },
+    {  5,  5, DIFF_HARD,   LOOPY_GRID_KITE },
+    { 10, 10, DIFF_HARD,   LOOPY_GRID_PENROSE_P2 },
+    { 10, 10, DIFF_HARD,   LOOPY_GRID_PENROSE_P3 },
 #endif
 };
 
-static int game_fetch_preset(int i, char **name, game_params **params)
+static const game_params loopy_presets_more[] = {
+#ifdef SMALL_SCREEN
+    {  7,  7, DIFF_HARD,   LOOPY_GRID_HONEYCOMB },
+    {  5,  4, DIFF_HARD,   LOOPY_GRID_GREATHEXAGONAL },
+    {  5,  5, DIFF_HARD,   LOOPY_GRID_OCTAGONAL },
+    {  3,  3, DIFF_HARD,   LOOPY_GRID_FLORET },
+    {  3,  3, DIFF_HARD,   LOOPY_GRID_DODECAGONAL },
+    {  3,  3, DIFF_HARD,   LOOPY_GRID_GREATDODECAGONAL },
+    {  3,  2, DIFF_HARD,   LOOPY_GRID_GREATGREATDODECAGONAL },
+#else
+    { 10, 10, DIFF_HARD,   LOOPY_GRID_HONEYCOMB },
+    {  5,  4, DIFF_HARD,   LOOPY_GRID_GREATHEXAGONAL },
+    {  7,  7, DIFF_HARD,   LOOPY_GRID_OCTAGONAL },
+    {  5,  5, DIFF_HARD,   LOOPY_GRID_FLORET },
+    {  5,  4, DIFF_HARD,   LOOPY_GRID_DODECAGONAL },
+    {  5,  4, DIFF_HARD,   LOOPY_GRID_GREATDODECAGONAL },
+    {  5,  3, DIFF_HARD,   LOOPY_GRID_GREATGREATDODECAGONAL },
+#endif
+};
+
+static void preset_menu_add_preset_with_title(struct preset_menu *menu,
+                                              const game_params *params)
 {
-    game_params *tmppar;
     char buf[80];
+    game_params *dup_params;
 
-    if (i < 0 || i >= lenof(presets))
-        return FALSE;
+    sprintf(buf, "%dx%d %s - %s", params->h, params->w,
+            gridnames[params->type], diffnames[params->diff]);
 
-    tmppar = snew(game_params);
-    *tmppar = presets[i];
-    *params = tmppar;
-    sprintf(buf, "%dx%d %s - %s", tmppar->h, tmppar->w,
-            gridnames[tmppar->type], diffnames[tmppar->diff]);
-    *name = dupstr(buf);
+    dup_params = snew(game_params);
+    *dup_params = *params;
 
-    return TRUE;
+    preset_menu_add_preset(menu, dupstr(buf), dup_params);
+}
+
+static struct preset_menu *game_preset_menu(void)
+{
+    struct preset_menu *top, *more;
+    int i;
+
+    top = preset_menu_new();
+    for (i = 0; i < lenof(loopy_presets_top); i++)
+        preset_menu_add_preset_with_title(top, &loopy_presets_top[i]);
+
+    more = preset_menu_add_submenu(top, dupstr("More..."));
+    for (i = 0; i < lenof(loopy_presets_more); i++)
+        preset_menu_add_preset_with_title(more, &loopy_presets_more[i]);
+
+    return top;
 }
 
 static void free_params(game_params *params)
@@ -574,7 +621,7 @@ static void decode_params(game_params *params, char const *string)
     }
 }
 
-static char *encode_params(game_params *params, int full)
+static char *encode_params(const game_params *params, int full)
 {
     char str[80];
     sprintf(str, "%dx%dt%d", params->w, params->h, params->type);
@@ -583,7 +630,7 @@ static char *encode_params(game_params *params, int full)
     return dupstr(str);
 }
 
-static config_item *game_configure(game_params *params)
+static config_item *game_configure(const game_params *params)
 {
     config_item *ret;
     char buf[80];
@@ -620,7 +667,7 @@ static config_item *game_configure(game_params *params)
     return ret;
 }
 
-static game_params *custom_params(config_item *cfg)
+static game_params *custom_params(const config_item *cfg)
 {
     game_params *ret = snew(game_params);
 
@@ -632,7 +679,7 @@ static game_params *custom_params(config_item *cfg)
     return ret;
 }
 
-static char *validate_params(game_params *params, int full)
+static char *validate_params(const game_params *params, int full)
 {
     if (params->type < 0 || params->type >= NUM_GRID_TYPES)
         return "Illegal grid type";
@@ -694,7 +741,7 @@ static char *state_to_text(const game_state *state)
 /* Splits up a (optional) grid_desc from the game desc. Returns the
  * grid_desc (which needs freeing) and updates the desc pointer to
  * start of real desc, or returns NULL if no desc. */
-static char *extract_grid_desc(char **desc)
+static char *extract_grid_desc(const char **desc)
 {
     char *sep = strchr(*desc, GRID_DESC_SEP), *gd;
     int gd_len;
@@ -713,7 +760,7 @@ static char *extract_grid_desc(char **desc)
 
 /* We require that the params pass the test in validate_params and that the
  * description fills the entire game area */
-static char *validate_desc(game_params *params, char *desc)
+static char *validate_desc(const game_params *params, const char *desc)
 {
     int count = 0;
     grid *g;
@@ -802,7 +849,7 @@ static char *encode_solve_move(const game_state *state)
     return ret;
 }
 
-static game_ui *new_ui(game_state *state)
+static game_ui *new_ui(const game_state *state)
 {
     return NULL;
 }
@@ -811,21 +858,21 @@ static void free_ui(game_ui *ui)
 {
 }
 
-static char *encode_ui(game_ui *ui)
+static char *encode_ui(const game_ui *ui)
 {
     return NULL;
 }
 
-static void decode_ui(game_ui *ui, char *encoding)
+static void decode_ui(game_ui *ui, const char *encoding)
 {
 }
 
-static void game_changed_state(game_ui *ui, game_state *oldstate,
-                               game_state *newstate)
+static void game_changed_state(game_ui *ui, const game_state *oldstate,
+                               const game_state *newstate)
 {
 }
 
-static void game_compute_size(game_params *params, int tilesize,
+static void game_compute_size(const game_params *params, int tilesize,
                               int *x, int *y)
 {
     int grid_width, grid_height, rendered_width, rendered_height;
@@ -842,14 +889,14 @@ static void game_compute_size(game_params *params, int tilesize,
 }
 
 static void game_set_size(drawing *dr, game_drawstate *ds,
-                         game_params *params, int tilesize)
+                          const game_params *params, int tilesize)
 {
     ds->tilesize = tilesize;
 }
 
 static float *game_colours(frontend *fe, int *ncolours)
 {
-    float *ret = snewn(4 * NCOLOURS, float);
+    float *ret = snewn(3 * NCOLOURS, float);
 
     frontend_default_colour(fe, &ret[COL_BACKGROUND * 3]);
 
@@ -891,7 +938,7 @@ static float *game_colours(frontend *fe, int *ncolours)
     return ret;
 }
 
-static game_drawstate *game_new_drawstate(drawing *dr, game_state *state)
+static game_drawstate *game_new_drawstate(drawing *dr, const game_state *state)
 {
     struct game_drawstate *ds = snew(struct game_drawstate);
     int num_faces = state->game_grid->num_faces;
@@ -926,25 +973,25 @@ static void game_free_drawstate(drawing *dr, game_drawstate *ds)
     sfree(ds);
 }
 
-static int game_timing_state(game_state *state, game_ui *ui)
+static int game_timing_state(const game_state *state, game_ui *ui)
 {
     return TRUE;
 }
 
-static float game_anim_length(game_state *oldstate, game_state *newstate,
-                              int dir, game_ui *ui)
+static float game_anim_length(const game_state *oldstate,
+                              const game_state *newstate, int dir, game_ui *ui)
 {
     return 0.0F;
 }
 
-static int game_can_format_as_text_now(game_params *params)
+static int game_can_format_as_text_now(const game_params *params)
 {
     if (params->type != 0)
         return FALSE;
     return TRUE;
 }
 
-static char *game_text_format(game_state *state)
+static char *game_text_format(const game_state *state)
 {
     int w, h, W, H;
     int x, y, i;
@@ -1364,7 +1411,7 @@ static game_state *remove_clues(game_state *state, random_state *rs,
 }
 
 
-static char *new_game_desc(game_params *params, random_state *rs,
+static char *new_game_desc(const game_params *params, random_state *rs,
                            char **aux, int interactive)
 {
     /* solution and description both use run-length encoding in obvious ways */
@@ -1379,6 +1426,7 @@ static char *new_game_desc(game_params *params, random_state *rs,
     state->clues = snewn(g->num_faces, signed char);
     state->lines = snewn(g->num_edges, char);
     state->line_errors = snewn(g->num_edges, unsigned char);
+    state->exactly_one_loop = FALSE;
 
     state->grid_type = params->type;
 
@@ -1426,7 +1474,8 @@ static char *new_game_desc(game_params *params, random_state *rs,
     return retval;
 }
 
-static game_state *new_game(midend *me, game_params *params, char *desc)
+static game_state *new_game(midend *me, const game_params *params,
+                            const char *desc)
 {
     int i;
     game_state *state = snew(game_state);
@@ -1449,6 +1498,7 @@ static game_state *new_game(midend *me, game_params *params, char *desc)
     state->clues = snewn(num_faces, signed char);
     state->lines = snewn(num_edges, char);
     state->line_errors = snewn(num_edges, unsigned char);
+    state->exactly_one_loop = FALSE;
 
     state->solved = state->cheated = FALSE;
 
@@ -1487,141 +1537,119 @@ static game_state *new_game(midend *me, game_params *params, char *desc)
 static int check_completion(game_state *state)
 {
     grid *g = state->game_grid;
-    int *dsf;
-    int num_faces = g->num_faces;
-    int i;
-    int infinite_area, finite_area;
-    int loops_found = 0;
-    int found_edge_not_in_loop = FALSE;
+    int i, ret;
+    int *dsf, *component_state;
+    int nsilly, nloop, npath, largest_comp, largest_size, total_pathsize;
+    enum { COMP_NONE, COMP_LOOP, COMP_PATH, COMP_SILLY, COMP_EMPTY };
 
     memset(state->line_errors, 0, g->num_edges);
 
-    /* LL implementation of SGT's idea:
-     * A loop will partition the grid into an inside and an outside.
-     * If there is more than one loop, the grid will be partitioned into
-     * even more distinct regions.  We can therefore track equivalence of
-     * faces, by saying that two faces are equivalent when there is a non-YES
-     * edge between them.
-     * We could keep track of the number of connected components, by counting
-     * the number of dsf-merges that aren't no-ops.
-     * But we're only interested in 3 separate cases:
-     * no loops, one loop, more than one loop.
+    /*
+     * Find loops in the grid, and determine whether the puzzle is
+     * solved.
+     *
+     * Loopy is a bit more complicated than most puzzles that care
+     * about loop detection. In most of them, loops are simply
+     * _forbidden_; so the obviously right way to do
+     * error-highlighting during play is to light up a graph edge red
+     * iff it is part of a loop, which is exactly what the centralised
+     * findloop.c makes easy.
+     *
+     * But Loopy is unusual in that you're _supposed_ to be making a
+     * loop - and yet _some_ loops are not the right loop. So we need
+     * to be more discriminating, by identifying loops one by one and
+     * then thinking about which ones to highlight, and so findloop.c
+     * isn't quite the right tool for the job in this case.
+     *
+     * Worse still, consider situations in which the grid contains a
+     * loop and also some non-loop edges: there are some cases like
+     * this in which the user's intuitive expectation would be to
+     * highlight the loop (if you're only about half way through the
+     * puzzle and have accidentally made a little loop in some corner
+     * of the grid), and others in which they'd be more likely to
+     * expect you to highlight the non-loop edges (if you've just
+     * closed off a whole loop that you thought was the entire
+     * solution, but forgot some disconnected edges in a corner
+     * somewhere). So while it's easy enough to check whether the
+     * solution is _right_, highlighting the wrong parts is a tricky
+     * problem for this puzzle!
      *
-     * No loops: all faces are equivalent to the infinite face.
-     * One loop: only two equivalence classes - finite and infinite.
-     * >= 2 loops: there are 2 distinct finite regions.
+     * I'd quite like, in some situations, to identify the largest
+     * loop among the player's YES edges, and then light up everything
+     * other than that. But finding the longest cycle in a graph is an
+     * NP-complete problem (because, in particular, it must return a
+     * Hamilton cycle if one exists).
      *
-     * So we simply make two passes through all the edges.
-     * In the first pass, we dsf-merge the two faces bordering each non-YES
-     * edge.
-     * In the second pass, we look for YES-edges bordering:
-     * a) two non-equivalent faces.
-     * b) two non-equivalent faces, and one of them is part of a different
-     *    finite area from the first finite area we've seen.
+     * However, I think we can make the problem tractable by
+     * exercising the Puzzles principle that it isn't absolutely
+     * necessary to highlight _all_ errors: the key point is that by
+     * the time the user has filled in the whole grid, they should
+     * either have seen a completion flash, or have _some_ error
+     * highlight showing them why the solution isn't right. So in
+     * principle it would be *just about* good enough to highlight
+     * just one error in the whole grid, if there was really no better
+     * way. But we'd like to highlight as many errors as possible.
      *
-     * An occurrence of a) means there is at least one loop.
-     * An occurrence of b) means there is more than one loop.
-     * Edges satisfying a) are marked as errors.
+     * In this case, I think the simple approach is to make use of the
+     * fact that no vertex may have degree > 2, and that's really
+     * simple to detect. So the plan goes like this:
      *
-     * While we're at it, we set a flag if we find a YES edge that is not
-     * part of a loop.
-     * This information will help decide, if there's a single loop, whether it
-     * is a candidate for being a solution (that is, all YES edges are part of
-     * this loop).
+     *  - Form the dsf of connected components of the graph vertices.
      *
-     * If there is a candidate loop, we then go through all clues and check
-     * they are all satisfied.  If so, we have found a solution and we can
-     * unmark all line_errors.
+     *  - Highlight an error at any vertex with degree > 2. (It so
+     *    happens that we do this by lighting up all the edges
+     *    incident to that vertex, but that's an output detail.)
+     *
+     *  - Any component that contains such a vertex is now excluded
+     *    from further consideration, because it already has a
+     *    highlight.
+     *
+     *  - The remaining components have no vertex with degree > 2, and
+     *    hence they all consist of either a simple loop, or a simple
+     *    path with two endpoints.
+     *
+     *  - For these purposes, group together all the paths and imagine
+     *    them to be a single component (because in most normal
+     *    situations the player will gradually build up the solution
+     *    _not_ all in one connected segment, but as lots of separate
+     *    little path pieces that gradually connect to each other).
+     *
+     *  - After doing that, if there is exactly one (sensible)
+     *    component - be it a collection of paths or a loop - then
+     *    highlight no further edge errors. (The former case is normal
+     *    during play, and the latter is a potentially solved puzzle.)
+     *
+     *  - Otherwise, find the largest of the sensible components,
+     *    leave that one unhighlighted, and light the rest up in red.
      */
-    
-    /* Infinite face is at the end - its index is num_faces.
-     * This macro is just to make this obvious! */
-    #define INF_FACE num_faces
-    dsf = snewn(num_faces + 1, int);
-    dsf_init(dsf, num_faces + 1);
-    
-    /* First pass */
-    for (i = 0; i < g->num_edges; i++) {
-        grid_edge *e = g->edges + i;
-        int f1 = e->face1 ? e->face1 - g->faces : INF_FACE;
-        int f2 = e->face2 ? e->face2 - g->faces : INF_FACE;
-        if (state->lines[i] != LINE_YES)
-            dsf_merge(dsf, f1, f2);
-    }
-    
-    /* Second pass */
-    infinite_area = dsf_canonify(dsf, INF_FACE);
-    finite_area = -1;
-    for (i = 0; i < g->num_edges; i++) {
-        grid_edge *e = g->edges + i;
-        int f1 = e->face1 ? e->face1 - g->faces : INF_FACE;
-        int can1 = dsf_canonify(dsf, f1);
-        int f2 = e->face2 ? e->face2 - g->faces : INF_FACE;
-        int can2 = dsf_canonify(dsf, f2);
-        if (state->lines[i] != LINE_YES) continue;
-
-        if (can1 == can2) {
-            /* Faces are equivalent, so this edge not part of a loop */
-            found_edge_not_in_loop = TRUE;
-            continue;
-        }
-        state->line_errors[i] = TRUE;
-        if (loops_found == 0) loops_found = 1;
 
-        /* Don't bother with further checks if we've already found 2 loops */
-        if (loops_found == 2) continue;
+    dsf = snew_dsf(g->num_dots);
 
-        if (finite_area == -1) {
-            /* Found our first finite area */
-            if (can1 != infinite_area)
-                finite_area = can1;
-            else
-                finite_area = can2;
-        }
-
-        /* Have we found a second area? */
-        if (finite_area != -1) {
-            if (can1 != infinite_area && can1 != finite_area) {
-                loops_found = 2;
-                continue;
-            }
-            if (can2 != infinite_area && can2 != finite_area) {
-                loops_found = 2;
-            }
+    /* Build the dsf. */
+    for (i = 0; i < g->num_edges; i++) {
+        if (state->lines[i] == LINE_YES) {
+            grid_edge *e = g->edges + i;
+            int d1 = e->dot1 - g->dots, d2 = e->dot2 - g->dots;
+            dsf_merge(dsf, d1, d2);
         }
     }
 
-/*
-    printf("loops_found = %d\n", loops_found);
-    printf("found_edge_not_in_loop = %s\n",
-        found_edge_not_in_loop ? "TRUE" : "FALSE");
-*/
-
-    sfree(dsf); /* No longer need the dsf */
-    
-    /* Have we found a candidate loop? */
-    if (loops_found == 1 && !found_edge_not_in_loop) {
-        /* Yes, so check all clues are satisfied */
-        int found_clue_violation = FALSE;
-        for (i = 0; i < num_faces; i++) {
-            int c = state->clues[i];
-            if (c >= 0) {
-                if (face_order(state, i, LINE_YES) != c) {
-                    found_clue_violation = TRUE;
-                    break;
-                }
-            }
-        }
-        
-        if (!found_clue_violation) {
-            /* The loop is good */
-            memset(state->line_errors, 0, g->num_edges);
-            return TRUE; /* No need to bother checking for dot violations */
-        }
+    /* Initialise a state variable for each connected component. */
+    component_state = snewn(g->num_dots, int);
+    for (i = 0; i < g->num_dots; i++) {
+        if (dsf_canonify(dsf, i) == i)
+            component_state[i] = COMP_LOOP;
+        else
+            component_state[i] = COMP_NONE;
     }
 
-    /* Check for dot violations */
+    /* Check for dots with degree > 3. Here we also spot dots of
+     * degree 1 in which the user has marked all the non-edges as
+     * LINE_NO, because those are also clear vertex-level errors, so
+     * we give them the same treatment of excluding their connected
+     * component from the subsequent loop analysis. */
     for (i = 0; i < g->num_dots; i++) {
+        int comp = dsf_canonify(dsf, i);
         int yes = dot_order(state, i, LINE_YES);
         int unknown = dot_order(state, i, LINE_UNKNOWN);
         if ((yes == 1 && unknown == 0) || (yes >= 3)) {
@@ -1633,9 +1661,108 @@ static int check_completion(game_state *state)
                 if (state->lines[e] == LINE_YES)
                     state->line_errors[e] = TRUE;
             }
+            /* And mark this component as not worthy of further
+             * consideration. */
+            component_state[comp] = COMP_SILLY;
+
+        } else if (yes == 0) {
+            /* A completely isolated dot must also be excluded it from
+             * the subsequent loop highlighting pass, but we tag it
+             * with a different enum value to avoid it counting
+             * towards the components that inhibit returning a win
+             * status. */
+            component_state[comp] = COMP_EMPTY;
+        } else if (yes == 1) {
+            /* A dot with degree 1 that didn't fall into the 'clearly
+             * erroneous' case above indicates that this connected
+             * component will be a path rather than a loop - unless
+             * something worse elsewhere in the component has
+             * classified it as silly. */
+            if (component_state[comp] != COMP_SILLY)
+                component_state[comp] = COMP_PATH;
         }
     }
-    return FALSE;
+
+    /* Count up the components. Also, find the largest sensible
+     * component. (Tie-breaking condition is derived from the order of
+     * vertices in the grid data structure, which is fairly arbitrary
+     * but at least stays stable throughout the game.) */
+    nsilly = nloop = npath = 0;
+    total_pathsize = 0;
+    largest_comp = largest_size = -1;
+    for (i = 0; i < g->num_dots; i++) {
+        if (component_state[i] == COMP_SILLY) {
+            nsilly++;
+        } else if (component_state[i] == COMP_PATH) {
+            total_pathsize += dsf_size(dsf, i);
+            npath = 1;
+        } else if (component_state[i] == COMP_LOOP) {
+            int this_size;
+
+            nloop++;
+
+            if ((this_size = dsf_size(dsf, i)) > largest_size) {
+                largest_comp = i;
+                largest_size = this_size;
+            }
+        }
+    }
+    if (largest_size < total_pathsize) {
+        largest_comp = -1;             /* means the paths */
+        largest_size = total_pathsize;
+    }
+
+    if (nloop > 0 && nloop + npath > 1) {
+        /*
+         * If there are at least two sensible components including at
+         * least one loop, highlight all edges in every sensible
+         * component that is not the largest one.
+         */
+        for (i = 0; i < g->num_edges; i++) {
+            if (state->lines[i] == LINE_YES) {
+                grid_edge *e = g->edges + i;
+                int d1 = e->dot1 - g->dots; /* either endpoint is good enough */
+                int comp = dsf_canonify(dsf, d1);
+                if ((component_state[comp] == COMP_PATH &&
+                     -1 != largest_comp) ||
+                    (component_state[comp] == COMP_LOOP &&
+                     comp != largest_comp))
+                    state->line_errors[i] = TRUE;
+            }
+        }
+    }
+
+    if (nloop == 1 && npath == 0 && nsilly == 0) {
+        /*
+         * If there is exactly one component and it is a loop, then
+         * the puzzle is potentially complete, so check the clues.
+         */
+        ret = TRUE;
+
+        for (i = 0; i < g->num_faces; i++) {
+            int c = state->clues[i];
+            if (c >= 0 && face_order(state, i, LINE_YES) != c) {
+                ret = FALSE;
+                break;
+            }
+        }
+
+        /*
+         * Also, whether or not the puzzle is actually complete, set
+         * the flag that says this game_state has exactly one loop and
+         * nothing else, which will be used to vary the semantics of
+         * clue highlighting at display time.
+         */
+        state->exactly_one_loop = TRUE;
+    } else {
+        ret = FALSE;
+        state->exactly_one_loop = FALSE;
+    }
+
+    sfree(component_state);
+    sfree(dsf);
+
+    return ret;
 }
 
 /* ----------------------------------------------------------------------
@@ -2784,8 +2911,8 @@ static solver_state *solve_game_rec(const solver_state *sstate_start)
     return sstate;
 }
 
-static char *solve_game(game_state *state, game_state *currstate,
-                        char *aux, char **error)
+static char *solve_game(const game_state *state, const game_state *currstate,
+                        const char *aux, char **error)
 {
     char *soln = NULL;
     solver_state *sstate, *new_sstate;
@@ -2813,13 +2940,15 @@ static char *solve_game(game_state *state, game_state *currstate,
  * Drawing and mouse-handling
  */
 
-static char *interpret_move(game_state *state, game_ui *ui, game_drawstate *ds,
+static char *interpret_move(const game_state *state, game_ui *ui,
+                            const game_drawstate *ds,
                             int x, int y, int button)
 {
     grid *g = state->game_grid;
     grid_edge *e;
     int i;
-    char *ret, buf[80];
+    char *movebuf;
+    int movelen, movesize;
     char button_char = ' ';
     enum line_state old_state;
 
@@ -2881,14 +3010,86 @@ static char *interpret_move(game_state *state, game_ui *ui, game_drawstate *ds,
        return NULL;
     }
 
+    movelen = 0;
+    movesize = 80;
+    movebuf = snewn(movesize, char);
+    movelen = sprintf(movebuf, "%d%c", i, (int)button_char);
+    {
+        static enum { OFF, FIXED, ADAPTIVE, DUNNO } autofollow = DUNNO;
+        if (autofollow == DUNNO) {
+            const char *env = getenv("LOOPY_AUTOFOLLOW");
+            if (env && !strcmp(env, "off"))
+                autofollow = OFF;
+            else if (env && !strcmp(env, "fixed"))
+                autofollow = FIXED;
+            else if (env && !strcmp(env, "adaptive"))
+                autofollow = ADAPTIVE;
+            else
+                autofollow = OFF;
+        }
+
+        if (autofollow != OFF) {
+            int dotid;
+            for (dotid = 0; dotid < 2; dotid++) {
+                grid_dot *dot = (dotid == 0 ? e->dot1 : e->dot2);
+                grid_edge *e_this = e;
+
+                while (1) {
+                    int j, n_found;
+                    grid_edge *e_next = NULL;
+
+                    for (j = n_found = 0; j < dot->order; j++) {
+                        grid_edge *e_candidate = dot->edges[j];
+                        int i_candidate = e_candidate - g->edges;
+                        if (e_candidate != e_this &&
+                            (autofollow == FIXED ||
+                             state->lines[i] == LINE_NO ||
+                             state->lines[i_candidate] != LINE_NO)) {
+                            e_next = e_candidate;
+                            n_found++;
+                        }
+                    }
 
-    sprintf(buf, "%d%c", i, (int)button_char);
-    ret = dupstr(buf);
+                    if (n_found != 1 ||
+                        state->lines[e_next - g->edges] != state->lines[i])
+                        break;
+
+                    if (e_next == e) {
+                        /*
+                         * Special case: we might have come all the
+                         * way round a loop and found our way back to
+                         * the same edge we started from. In that
+                         * situation, we must terminate not only this
+                         * while loop, but the 'for' outside it that
+                         * was tracing in both directions from the
+                         * starting edge, because if we let it trace
+                         * in the second direction then we'll only
+                         * find ourself traversing the same loop in
+                         * the other order and generate an encoded
+                         * move string that mentions the same set of
+                         * edges twice.
+                         */
+                        goto autofollow_done;
+                    }
 
-    return ret;
+                    dot = (e_next->dot1 != dot ? e_next->dot1 : e_next->dot2);
+                    if (movelen > movesize - 40) {
+                        movesize = movesize * 5 / 4 + 128;
+                        movebuf = sresize(movebuf, movesize, char);
+                    }
+                    e_this = e_next;
+                    movelen += sprintf(movebuf+movelen, "%d%c",
+                                       (int)(e_this - g->edges), button_char);
+                }
+            }
+          autofollow_done:;
+        }
+    }
+
+    return sresize(movebuf, movelen+1, char);
 }
 
-static game_state *execute_move(game_state *state, char *move)
+static game_state *execute_move(const game_state *state, const char *move)
 {
     int i;
     game_state *newstate = dup_game(state);
@@ -2992,19 +3193,14 @@ static void face_text_bbox(game_drawstate *ds, grid *g, grid_face *f,
 }
 
 static void game_redraw_clue(drawing *dr, game_drawstate *ds,
-                            game_state *state, int i)
+                            const game_state *state, int i)
 {
     grid *g = state->game_grid;
     grid_face *f = g->faces + i;
     int x, y;
-    char c[3];
+    char c[20];
 
-    if (state->clues[i] < 10) {
-        c[0] = CLUE2CHAR(state->clues[i]);
-        c[1] = '\0';
-    } else {
-        sprintf(c, "%d", state->clues[i]);
-    }
+    sprintf(c, "%d", state->clues[i]);
 
     face_text_pos(ds, g, f, &x, &y);
     draw_text(dr, x, y,
@@ -3056,7 +3252,7 @@ static const int loopy_line_redraw_phases[] = {
 #define NPHASES lenof(loopy_line_redraw_phases)
 
 static void game_redraw_line(drawing *dr, game_drawstate *ds,
-                            game_state *state, int i, int phase)
+                            const game_state *state, int i, int phase)
 {
     grid *g = state->game_grid;
     grid_edge *e = g->edges + i;
@@ -3098,7 +3294,7 @@ static void game_redraw_line(drawing *dr, game_drawstate *ds,
 }
 
 static void game_redraw_dot(drawing *dr, game_drawstate *ds,
-                           game_state *state, int i)
+                           const game_state *state, int i)
 {
     grid *g = state->game_grid;
     grid_dot *d = g->dots + i;
@@ -3120,7 +3316,8 @@ static int boxes_intersect(int x0, int y0, int w0, int h0,
 }
 
 static void game_redraw_in_rect(drawing *dr, game_drawstate *ds,
-                                game_state *state, int x, int y, int w, int h)
+                                const game_state *state,
+                                int x, int y, int w, int h)
 {
     grid *g = state->game_grid;
     int i, phase;
@@ -3153,8 +3350,9 @@ static void game_redraw_in_rect(drawing *dr, game_drawstate *ds,
     draw_update(dr, x, y, w, h);
 }
 
-static void game_redraw(drawing *dr, game_drawstate *ds, game_state *oldstate,
-                        game_state *state, int dir, game_ui *ui,
+static void game_redraw(drawing *dr, game_drawstate *ds,
+                        const game_state *oldstate, const game_state *state,
+                        int dir, const game_ui *ui,
                         float animtime, float flashtime)
 {
 #define REDRAW_OBJECTS_LIMIT 16                /* Somewhat arbitrary tradeoff */
@@ -3193,60 +3391,100 @@ static void game_redraw(drawing *dr, game_drawstate *ds, game_state *oldstate,
      * what needs doing, and the second actually does it.
      */
 
-    if (!ds->started)
+    if (!ds->started) {
        redraw_everything = TRUE;
-    else {
-
-       /* First, trundle through the faces. */
-       for (i = 0; i < g->num_faces; i++) {
-           grid_face *f = g->faces + i;
-           int sides = f->order;
-           int clue_mistake;
-           int clue_satisfied;
-           int n = state->clues[i];
-           if (n < 0)
-               continue;
-
-           clue_mistake = (face_order(state, i, LINE_YES) > n ||
-                           face_order(state, i, LINE_NO ) > (sides-n));
-           clue_satisfied = (face_order(state, i, LINE_YES) == n &&
-                             face_order(state, i, LINE_NO ) == (sides-n));
-
-           if (clue_mistake != ds->clue_error[i] ||
-               clue_satisfied != ds->clue_satisfied[i]) {
-               ds->clue_error[i] = clue_mistake;
-               ds->clue_satisfied[i] = clue_satisfied;
-               if (nfaces == REDRAW_OBJECTS_LIMIT)
-                   redraw_everything = TRUE;
-               else
-                   faces[nfaces++] = i;
-           }
-       }
+        /*
+         * But we must still go through the upcoming loops, so that we
+         * set up stuff in ds correctly for the initial redraw.
+         */
+    }
 
-       /* Work out what the flash state needs to be. */
-       if (flashtime > 0 &&
-           (flashtime <= FLASH_TIME/3 ||
-            flashtime >= FLASH_TIME*2/3)) {
-           flash_changed = !ds->flashing;
-           ds->flashing = TRUE;
-       } else {
-           flash_changed = ds->flashing;
-           ds->flashing = FALSE;
-       }
+    /* First, trundle through the faces. */
+    for (i = 0; i < g->num_faces; i++) {
+        grid_face *f = g->faces + i;
+        int sides = f->order;
+        int yes_order, no_order;
+        int clue_mistake;
+        int clue_satisfied;
+        int n = state->clues[i];
+        if (n < 0)
+            continue;
 
-       /* Now, trundle through the edges. */
-       for (i = 0; i < g->num_edges; i++) {
-           char new_ds =
-               state->line_errors[i] ? DS_LINE_ERROR : state->lines[i];
-           if (new_ds != ds->lines[i] ||
-               (flash_changed && state->lines[i] == LINE_YES)) {
-               ds->lines[i] = new_ds;
-               if (nedges == REDRAW_OBJECTS_LIMIT)
-                   redraw_everything = TRUE;
-               else
-                   edges[nedges++] = i;
-           }
-       }
+        yes_order = face_order(state, i, LINE_YES);
+        if (state->exactly_one_loop) {
+            /*
+             * Special case: if the set of LINE_YES edges in the grid
+             * consists of exactly one loop and nothing else, then we
+             * switch to treating LINE_UNKNOWN the same as LINE_NO for
+             * purposes of clue checking.
+             *
+             * This is because some people like to play Loopy without
+             * using the right-click, i.e. never setting anything to
+             * LINE_NO. Without this special case, if a person playing
+             * in that style fills in what they think is a correct
+             * solution loop but in fact it has an underfilled clue,
+             * then we will display no victory flash and also no error
+             * highlight explaining why not. With this special case,
+             * we light up underfilled clues at the instant the loop
+             * is closed. (Of course, *overfilled* clues are fine
+             * either way.)
+             *
+             * (It might still be considered unfortunate that we can't
+             * warn this style of player any earlier, if they make a
+             * mistake very near the beginning which doesn't show up
+             * until they close the last edge of the loop. One other
+             * thing we _could_ do here is to treat any LINE_UNKNOWN
+             * as LINE_NO if either of its endpoints has yes-degree 2,
+             * reflecting the fact that setting that line to YES would
+             * be an obvious error. But I don't think even that could
+             * catch _all_ clue errors in a timely manner; I think
+             * there are some that won't be displayed until the loop
+             * is filled in, even so, and there's no way to avoid that
+             * with complete reliability except to switch to being a
+             * player who sets things to LINE_NO.)
+             */
+            no_order = sides - yes_order;
+        } else {
+            no_order = face_order(state, i, LINE_NO);
+        }
+
+        clue_mistake = (yes_order > n || no_order > (sides-n));
+        clue_satisfied = (yes_order == n && no_order == (sides-n));
+
+        if (clue_mistake != ds->clue_error[i] ||
+            clue_satisfied != ds->clue_satisfied[i]) {
+            ds->clue_error[i] = clue_mistake;
+            ds->clue_satisfied[i] = clue_satisfied;
+            if (nfaces == REDRAW_OBJECTS_LIMIT)
+                redraw_everything = TRUE;
+            else
+                faces[nfaces++] = i;
+        }
+    }
+
+    /* Work out what the flash state needs to be. */
+    if (flashtime > 0 &&
+        (flashtime <= FLASH_TIME/3 ||
+         flashtime >= FLASH_TIME*2/3)) {
+        flash_changed = !ds->flashing;
+        ds->flashing = TRUE;
+    } else {
+        flash_changed = ds->flashing;
+        ds->flashing = FALSE;
+    }
+
+    /* Now, trundle through the edges. */
+    for (i = 0; i < g->num_edges; i++) {
+        char new_ds =
+            state->line_errors[i] ? DS_LINE_ERROR : state->lines[i];
+        if (new_ds != ds->lines[i] ||
+            (flash_changed && state->lines[i] == LINE_YES)) {
+            ds->lines[i] = new_ds;
+            if (nedges == REDRAW_OBJECTS_LIMIT)
+                redraw_everything = TRUE;
+            else
+                edges[nedges++] = i;
+        }
     }
 
     /* Pass one is now done.  Now we do the actual drawing. */
@@ -3282,8 +3520,8 @@ static void game_redraw(drawing *dr, game_drawstate *ds, game_state *oldstate,
     ds->started = TRUE;
 }
 
-static float game_flash_length(game_state *oldstate, game_state *newstate,
-                               int dir, game_ui *ui)
+static float game_flash_length(const game_state *oldstate,
+                               const game_state *newstate, int dir, game_ui *ui)
 {
     if (!oldstate->solved  &&  newstate->solved &&
         !oldstate->cheated && !newstate->cheated) {
@@ -3293,12 +3531,12 @@ static float game_flash_length(game_state *oldstate, game_state *newstate,
     return 0.0F;
 }
 
-static int game_status(game_state *state)
+static int game_status(const game_state *state)
 {
     return state->solved ? +1 : 0;
 }
 
-static void game_print_size(game_params *params, float *x, float *y)
+static void game_print_size(const game_params *params, float *x, float *y)
 {
     int pw, ph;
 
@@ -3310,7 +3548,7 @@ static void game_print_size(game_params *params, float *x, float *y)
     *y = ph / 100.0F;
 }
 
-static void game_print(drawing *dr, game_state *state, int tilesize)
+static void game_print(drawing *dr, const game_state *state, int tilesize)
 {
     int ink = print_mono_colour(dr, 0);
     int i;
@@ -3336,10 +3574,9 @@ static void game_print(drawing *dr, game_state *state, int tilesize)
         grid_face *f = g->faces + i;
         int clue = state->clues[i];
         if (clue >= 0) {
-            char c[2];
+            char c[20];
             int x, y;
-            c[0] = CLUE2CHAR(clue);
-            c[1] = '\0';
+            sprintf(c, "%d", state->clues[i]);
             face_text_pos(ds, g, f, &x, &y);
             draw_text(dr, x, y,
                       FONT_VARIABLE, ds->tilesize / 2,
@@ -3403,7 +3640,7 @@ static void game_print(drawing *dr, game_state *state, int tilesize)
 const struct game thegame = {
     "Loopy", "games.loopy", "loopy",
     default_params,
-    game_fetch_preset,
+    NULL, game_preset_menu,
     decode_params,
     encode_params,
     free_params,