| 1 | /* -*-javascript-*- |
| 2 | * |
| 3 | * Dependency-based computation. |
| 4 | * |
| 5 | * (c) 2013 Mark Wooding |
| 6 | */ |
| 7 | |
| 8 | /*----- Licensing notice --------------------------------------------------* |
| 9 | * |
| 10 | * This program is free software; you can redistribute it and/or |
| 11 | * modify it under the terms of the GNU General Public License as |
| 12 | * published by the Free Software Foundation; either version 2 of the |
| 13 | * License, or (at your option) any later version. |
| 14 | * |
| 15 | * This program is distributed in the hope that it will be useful, |
| 16 | * but WITHOUT ANY WARRANTY; without even the implied warranty of |
| 17 | * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the |
| 18 | * GNU Library General Public License for more details. |
| 19 | * |
| 20 | * You should have received a copy of the GNU General Public License |
| 21 | * along with this program; if not, see <http://www.gnu.org/licenses/>. |
| 22 | */ |
| 23 | |
| 24 | var DEP = { }; (function () { with (DEP) { |
| 25 | |
| 26 | /*----- Utility functions and classes -------------------------------------*/ |
| 27 | |
| 28 | DEP.dolist = function (list, func) { |
| 29 | /* Apply FUNC to each element of LIST, discarding the results. |
| 30 | * |
| 31 | * JavaScript's looping iterates over indices rather than values, which is |
| 32 | * consistent with what you want from a mapping, but inconvenient for |
| 33 | * sequences. |
| 34 | */ |
| 35 | |
| 36 | var i; |
| 37 | for (i in list) func(list[i]); |
| 38 | } |
| 39 | |
| 40 | function eql(x, y) { |
| 41 | /* A standard equality function, suitable for detecting changes to `Dep' |
| 42 | * objects. This may be needlessly sensitive for some cases, but at least |
| 43 | * that's slow rather than wrong. |
| 44 | */ |
| 45 | |
| 46 | return x === y; |
| 47 | } |
| 48 | |
| 49 | /* A Tag is an object which is interesting only because of its identity, and |
| 50 | * that the set of Tags is basically determined by the static structure of |
| 51 | * the program. |
| 52 | */ |
| 53 | DEP.Tag = function (what) { |
| 54 | this.what = what; |
| 55 | } |
| 56 | Tag.prototype = { |
| 57 | toString: function () { return '#{Tag ' + this.what + '}'; } |
| 58 | } |
| 59 | |
| 60 | /* A Generation is like a Tag, except that it's expected that a program will |
| 61 | * manufacture Generations repeatedly, and perhaps use them to determine |
| 62 | * whether an object is `up-to-date' in some sense. |
| 63 | */ |
| 64 | DEP.Generation = function (what) { |
| 65 | this.what = what; |
| 66 | this.seq = Generation._next++; |
| 67 | } |
| 68 | Generation.prototype = { |
| 69 | toString: function () { |
| 70 | return '#{Generation ' + this.what + ' #' + this.seq.toString() + '}'; |
| 71 | } |
| 72 | }; |
| 73 | Generation._next = 0; |
| 74 | DEP.Generation = Generation; |
| 75 | |
| 76 | /*----- The system state --------------------------------------------------*/ |
| 77 | |
| 78 | var GENERATION = new Generation('dep-generation'); |
| 79 | // Current recomputation generation. |
| 80 | |
| 81 | var EVALUATING = null; // The dep being re-evaluated. |
| 82 | var STATE = 'ready'; // The current state. |
| 83 | |
| 84 | /* Flags for Dep objects. */ |
| 85 | var F_VALUE = 1; // Value is up-to-date. |
| 86 | var F_DEPS = 2; // Known as a dependent. |
| 87 | var F_CHANGED = 4; // Changed in current phase. |
| 88 | var F_RECOMPUTING = 8; // Currently being recomputed. |
| 89 | var F_QUEUED = 16; // Queued for recomputation. |
| 90 | |
| 91 | var BAD = Tag('BAD') // Used for the value of `bad' deps. |
| 92 | |
| 93 | var DELAYED = []; // Actions delayed by `with_frozen'. |
| 94 | var PENDING = []; // Deps awaiting recomputation. |
| 95 | |
| 96 | /*----- Exceptions --------------------------------------------------------*/ |
| 97 | |
| 98 | DEP.CircularDependency = new Tag('CircularDependency'); |
| 99 | DEP.BusyRecomputing = new Tag('BusyRecomputing'); |
| 100 | |
| 101 | /*----- Main code ---------------------------------------------------------*/ |
| 102 | |
| 103 | /* A `Dep' object maintains a value, either because it was provided |
| 104 | * explicitly by the programmer, or computed in terms of other `Dep' objects. |
| 105 | * In the latter case, its value will be recomputed if any of its |
| 106 | * dependencies change value. |
| 107 | * |
| 108 | * A `Dep' object may have listener functions attached to it. These |
| 109 | * functions will bw called when its value changes. |
| 110 | * |
| 111 | * A `Dep' may be `bad'. In this case, use of its value by a dependent |
| 112 | * causes that `Dep' to become bad in turn. |
| 113 | */ |
| 114 | |
| 115 | DEP.Dep = function (value, maybefunc) { |
| 116 | /* Initialize a new `Dep' object. |
| 117 | * |
| 118 | * Handling of the arguments is a little fiddly. If two arguments are |
| 119 | * provided then the first is set as the value and the second as the |
| 120 | * recomputation function. Otherwise, the first argument is interpreted as |
| 121 | * the recomputation function if it has `function' type, or as an initial |
| 122 | * explicit value otherwise. To force a function to be interpreted as an |
| 123 | * initial value, pass a second argument of `null'. |
| 124 | * |
| 125 | * Some properties can be fiddled with by client programs (rather than |
| 126 | * trying to invent some crazy option-parsing protocol in the constructor). |
| 127 | * |
| 128 | * `name' A string name for this `Dep', used when printing. |
| 129 | * |
| 130 | * `equivp' A predicate for deciding whether two value assigned |
| 131 | * to this `Dep' are equivalent. Used to decide whether |
| 132 | * a change should be propagated. |
| 133 | */ |
| 134 | |
| 135 | var val, func, f = F_CHANGED; |
| 136 | var me = this; |
| 137 | |
| 138 | // Work out what's going on with our crazy argument convention. |
| 139 | if (maybefunc !== undefined) { |
| 140 | val = value; |
| 141 | func = maybefunc; |
| 142 | f |= F_VALUE | F_QUEUED; |
| 143 | } else if (typeof value === 'function') { |
| 144 | val = BAD; |
| 145 | func = value; |
| 146 | f |= F_QUEUED; |
| 147 | } else { |
| 148 | val = value; |
| 149 | func = null; |
| 150 | f |= F_VALUE | F_DEPS; |
| 151 | } |
| 152 | |
| 153 | // Initialize the various slots. |
| 154 | this._value_function = func; // Recomputation function. |
| 155 | this._value = val; // Current value. |
| 156 | |
| 157 | this.name = undefined; // Name, for printing. |
| 158 | this.equivp = eql; // Equivalent-value predicate. |
| 159 | this.__flags = f; // Current flags. |
| 160 | this._generation = GENERATION; // Have we done this one already?. |
| 161 | this._listeners = []; // List of listener functions. |
| 162 | |
| 163 | // We need to maintain the dependency graph. In order to prevent duplicate |
| 164 | // entries in the various sets, we use maps rather than lists, but this |
| 165 | // presents a problem with coming up with keys: JavaScript stringifies |
| 166 | // objects rather than using them as-is, which is obviously hopeless. So |
| 167 | // assign each `Dep' a unique sequence number and use that as the key. |
| 168 | this._seq = Dep._seq++; // For identifying this object. |
| 169 | this._dependents = { }; // Set of dependent objects. |
| 170 | this._dependencies = { }; // Set of objects we depend on. |
| 171 | |
| 172 | // If we have a recomputation function then exercise it. |
| 173 | if (func !== null) { |
| 174 | with_frozen(function () { |
| 175 | PENDING.push(me); |
| 176 | }); |
| 177 | } |
| 178 | } |
| 179 | |
| 180 | Dep._seq = 0; // Next sequence number to allocate. |
| 181 | |
| 182 | Dep.prototype = { |
| 183 | |
| 184 | toString: function () { |
| 185 | /* Convert the receiver to a string. |
| 186 | * |
| 187 | * The printed representation includes a number of bits of information |
| 188 | * which are probably meaningless to most readers but are handy for |
| 189 | * debugging this library if it's misbehaving. |
| 190 | */ |
| 191 | |
| 192 | // Basic stuff. |
| 193 | var s = '#{Dep'; |
| 194 | var f = this._flags(); |
| 195 | var v = this._value; |
| 196 | |
| 197 | // The dep's name. |
| 198 | if (this.name !== null) s += ' "' + this.name + '"'; |
| 199 | |
| 200 | // Sequence number -- distinguishes the dep if nothing else will. |
| 201 | s += ' #' + this._seq; |
| 202 | |
| 203 | // The value, or some kind of marker that it doesn't have one. |
| 204 | if (!(f & F_VALUE)) s += ' #{out-of-date}'; |
| 205 | else if (v === BAD) s += ' #{bad}'; |
| 206 | else s += ' ' + v.toString(); |
| 207 | |
| 208 | // Various flags. |
| 209 | if (!(f & F_DEPS)) s += ' :recompute-deps'; |
| 210 | if (f & F_QUEUED) s += ' :queued'; |
| 211 | if (f & F_CHANGED) s += ' :changed'; |
| 212 | |
| 213 | // Done. |
| 214 | s += '}'; |
| 215 | return s; |
| 216 | }, |
| 217 | |
| 218 | _flags: function () { |
| 219 | /* Return the receiver's flags. |
| 220 | * |
| 221 | * If the receiver isn't up-to-date then we synthesize the flags. |
| 222 | */ |
| 223 | |
| 224 | if (this.state === 'ready') return F_VALUE | F_DEPS; |
| 225 | else if (this._generation === GENERATION) return this.__flags; |
| 226 | else if (this._value_function === null) return F_VALUE | F_DEPS; |
| 227 | else return 0; |
| 228 | }, |
| 229 | |
| 230 | _update: function (value) { |
| 231 | /* Store VALUE as the receiver's value. |
| 232 | * |
| 233 | * Return whether the value has changed as a result. This is a low-level |
| 234 | * function which doesn't handle propagation or anything like that. |
| 235 | */ |
| 236 | |
| 237 | if (value === BAD ? |
| 238 | this._value === BAD : |
| 239 | (this._value !== BAD && |
| 240 | this.equivp(value, this._value))) |
| 241 | return false; |
| 242 | else { |
| 243 | this._value = value; |
| 244 | return true; |
| 245 | } |
| 246 | }, |
| 247 | |
| 248 | _new_value: function () { |
| 249 | /* Run the `_value_function' of the receiver, returning the result. |
| 250 | * |
| 251 | * If a bad dep is read during this then return `BAD'. Other exceptions |
| 252 | * are propagated in the usual way. |
| 253 | */ |
| 254 | |
| 255 | var old = EVALUATING; |
| 256 | var val, e; |
| 257 | |
| 258 | this._dependencies = { }; |
| 259 | try { |
| 260 | EVALUATING = this; |
| 261 | try { |
| 262 | return this._value_function(); |
| 263 | } catch (e) { |
| 264 | if (e === BAD) return BAD; |
| 265 | else throw e; |
| 266 | } |
| 267 | } finally { |
| 268 | EVALUATING = old; |
| 269 | } |
| 270 | }, |
| 271 | |
| 272 | _propagate: function () { |
| 273 | /* Propagate a change in the receiver to dependents and listeners. */ |
| 274 | |
| 275 | var d, di, f; |
| 276 | |
| 277 | // Iterate over each dependent, pushing each one onto the `PENDING' |
| 278 | // queue, bringing it into the current generation, and marking it as |
| 279 | // having no current value. |
| 280 | for (di in this._dependents) { |
| 281 | d = this._dependents[di]; |
| 282 | f = d._flags(); |
| 283 | if (!(f & (F_QUEUED | F_DEPS))) { |
| 284 | PENDING.push(d); |
| 285 | d._generation = GENERATION; |
| 286 | d.__flags = (f & ~F_VALUE) | F_QUEUED; |
| 287 | } |
| 288 | } |
| 289 | |
| 290 | // We no longer have any dependents. Maybe we'll acquire more when the |
| 291 | // old dependents recompute themselves. |
| 292 | this._dependents = { }; |
| 293 | |
| 294 | // Tell the listeners that something interesting happened. |
| 295 | dolist(this._listeners, function (l) { l(); }); |
| 296 | }, |
| 297 | |
| 298 | _recompute: function () { |
| 299 | /* Recompute the receiver, propagating changes and so on. |
| 300 | * |
| 301 | * Return whether we actually needed to change anything. |
| 302 | */ |
| 303 | |
| 304 | var queued = this.__flags & F_QUEUED; |
| 305 | var e; |
| 306 | var me = this; |
| 307 | |
| 308 | // Update us with the given VALUE. |
| 309 | function update(value) { |
| 310 | if (me._update(value)) { |
| 311 | me.__flags = queued | F_VALUE | F_DEPS | F_CHANGED; |
| 312 | me._propagate(); |
| 313 | return true; |
| 314 | } else { |
| 315 | me.__flags = queued | F_VALUE | F_DEPS; |
| 316 | return false; |
| 317 | } |
| 318 | }; |
| 319 | |
| 320 | // Try to recompute our value. If that doesn't work then mark us as bad |
| 321 | // and propagate that. |
| 322 | try { |
| 323 | return update(this._new_value()); |
| 324 | } catch (e) { |
| 325 | update(BAD); |
| 326 | throw e; |
| 327 | } |
| 328 | }, |
| 329 | |
| 330 | _force: function () { |
| 331 | /* Make sure the receiver has a current value. |
| 332 | * |
| 333 | * Return true if the receiver's value has changed in this recomputation |
| 334 | * phase. |
| 335 | * |
| 336 | * If it already has a good value then just return. Otherwise mark it |
| 337 | * as recomputing (to trap circularities) and poke our dependencies to |
| 338 | * ensure that they're up-to-date. If they weren't, then we recompute |
| 339 | * our own value before returning. |
| 340 | */ |
| 341 | |
| 342 | var f = this._flags(); |
| 343 | var d, any = false; |
| 344 | |
| 345 | if (f & F_RECOMPUTING) throw CircularDependency; |
| 346 | else if (f & F_VALUE) return f & F_CHANGED; |
| 347 | else { |
| 348 | this._generation = GENERATION; |
| 349 | this.__flags = (f & ~F_QUEUED) | F_RECOMPUTING; |
| 350 | for (d in this._dependencies) |
| 351 | if (this._dependencies[d]._force()) any = true; |
| 352 | if (any) |
| 353 | return this._recompute(); |
| 354 | else { |
| 355 | this.__flags = f; |
| 356 | return false; |
| 357 | } |
| 358 | } |
| 359 | }, |
| 360 | |
| 361 | value: function () { |
| 362 | /* Fetch and return the receiver's current value. |
| 363 | * |
| 364 | * If the receiver is bad then an exception is thrown. This exception |
| 365 | * can be caught using `orelse'; a dep recomputation function can let the |
| 366 | * exception propagate, and be marked as bad in turn. |
| 367 | */ |
| 368 | |
| 369 | var val; |
| 370 | |
| 371 | if (state === 'recomputing') { |
| 372 | if (EVALUATING) { |
| 373 | this._dependents[EVALUATING._seq] = EVALUATING; |
| 374 | EVALUATING._dependencies[this._seq] = this; |
| 375 | } |
| 376 | this._force(); |
| 377 | } |
| 378 | val = this._value; |
| 379 | if (val === BAD) throw BAD; |
| 380 | return val; |
| 381 | }, |
| 382 | |
| 383 | set_value: function (value) { |
| 384 | /* Set the receiver's value to VALUE, and propagate. */ |
| 385 | |
| 386 | var me = this; |
| 387 | |
| 388 | with_frozen(function () { |
| 389 | if (me._update(value)) { |
| 390 | me._generation = GENERATION; |
| 391 | me.__flags = F_VALUE | F_CHANGED; |
| 392 | me._propagate(); |
| 393 | } |
| 394 | }); |
| 395 | }, |
| 396 | |
| 397 | make_bad: function () { |
| 398 | /* Mark the receiver as being bad, and propagate. */ |
| 399 | this.set_value(BAD); |
| 400 | }, |
| 401 | |
| 402 | add_listener: function (func) { |
| 403 | /* Add FUNC to the receiver's list of listeners. |
| 404 | * |
| 405 | * Listener functions are called without arguments, and any values |
| 406 | * returned are ignored. |
| 407 | */ |
| 408 | |
| 409 | this._listeners.push(func); |
| 410 | }, |
| 411 | |
| 412 | goodp: function () { |
| 413 | /* Return whether the receiver is good (i.e., not marked as bad). */ |
| 414 | |
| 415 | return this._value !== BAD; |
| 416 | } |
| 417 | }; |
| 418 | |
| 419 | DEP.orelse = function (thunk, errthunk) { |
| 420 | /* Call THUNK. If it succeeds, then return its result. If THUNK |
| 421 | * reads a bad dep then call ERRTHINK and return its result instead. |
| 422 | */ |
| 423 | |
| 424 | var e; |
| 425 | try { return thunk(); } |
| 426 | catch (e) { |
| 427 | if (e === BAD) { return errthunk(); } |
| 428 | else throw e; |
| 429 | } |
| 430 | } |
| 431 | |
| 432 | DEP.bad = function () { |
| 433 | /* For use in a value-function: make the dep be bad. */ |
| 434 | throw BAD; |
| 435 | } |
| 436 | |
| 437 | function recompute_pending() { |
| 438 | /* Recompute any pending dependencies. |
| 439 | * |
| 440 | * The state is `recomputing' during this. |
| 441 | */ |
| 442 | |
| 443 | var d, f; |
| 444 | |
| 445 | state = 'recomputing'; |
| 446 | try { |
| 447 | while (PENDING.length) { |
| 448 | d = PENDING.shift(); |
| 449 | f = d.__flags; |
| 450 | d.__flags = f & ~F_QUEUED; |
| 451 | if (!(f & F_VALUE)) |
| 452 | d._recompute(); |
| 453 | else if (!(f & F_DEPS)) { |
| 454 | d.new_value(); |
| 455 | d.__flags = f | F_DEPS; |
| 456 | } |
| 457 | } |
| 458 | } finally { |
| 459 | while (PENDING.length) { |
| 460 | d = PENDING.shift(); |
| 461 | d._value = BAD; |
| 462 | } |
| 463 | } |
| 464 | } |
| 465 | |
| 466 | DEP.with_frozen = function (body, delay) { |
| 467 | /* Call BODY with dependencies frozen. |
| 468 | * |
| 469 | * If the BODY function changes any dep values (using D.set_value(...)) |
| 470 | * then dependents won't be updated until the BODY completes. |
| 471 | * |
| 472 | * It's very |
| 473 | * bad to do this during a recomputation phase. If DELAY is true, then the |
| 474 | * BODY is delayed until the recomputation completes; otherwise you get an |
| 475 | * exception. |
| 476 | */ |
| 477 | |
| 478 | var op, val; |
| 479 | var old_delayed, old_pending; |
| 480 | |
| 481 | switch (STATE) { |
| 482 | case 'frozen': |
| 483 | body(); |
| 484 | break; |
| 485 | case 'recomputing': |
| 486 | if (!delay) throw BusyRecomputing; |
| 487 | DELAYED.push(body); |
| 488 | break; |
| 489 | case 'ready': |
| 490 | old_delayed = DELAYED; |
| 491 | old_pending = PENDING; |
| 492 | try { |
| 493 | DELAYED = []; |
| 494 | PENDING = []; |
| 495 | GENERATION = new Generation('dep-generation'); |
| 496 | val = body(); |
| 497 | for (;;) { |
| 498 | recompute_pending(); |
| 499 | if (!DELAYED.length) break; |
| 500 | op = DELAYED.shift(); |
| 501 | op(); |
| 502 | } |
| 503 | } finally { |
| 504 | DELAYED = old_delayed; |
| 505 | PENDING = old_pending; |
| 506 | } |
| 507 | break; |
| 508 | } |
| 509 | } |
| 510 | |
| 511 | /*----- That's all, folks -------------------------------------------------*/ |
| 512 | } })(); |