1 /* Analyze differences between two vectors.
2
3 Copyright (C) 1988-1989, 1992-1995, 2001-2004, 2006-2020 Free Software
4 Foundation, Inc.
5
6 This program is free software: you can redistribute it and/or modify
7 it under the terms of the GNU General Public License as published by
8 the Free Software Foundation; either version 3 of the License, or
9 (at your option) any later version.
10
11 This program is distributed in the hope that it will be useful,
12 but WITHOUT ANY WARRANTY; without even the implied warranty of
13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 GNU General Public License for more details.
15
16 You should have received a copy of the GNU General Public License
17 along with this program. If not, see <https://www.gnu.org/licenses/>. */
18
19
20 /* The basic idea is to consider two vectors as similar if, when
21 transforming the first vector into the second vector through a
22 sequence of edits (inserts and deletes of one element each),
23 this sequence is short - or equivalently, if the ordered list
24 of elements that are untouched by these edits is long. For a
25 good introduction to the subject, read about the "Levenshtein
26 distance" in Wikipedia.
27
28 The basic algorithm is described in:
29 "An O(ND) Difference Algorithm and its Variations", Eugene W. Myers,
30 Algorithmica Vol. 1, 1986, pp. 251-266,
31 <https://doi.org/10.1007/BF01840446>.
32 See especially section 4.2, which describes the variation used below.
33
34 The basic algorithm was independently discovered as described in:
35 "Algorithms for Approximate String Matching", Esko Ukkonen,
36 Information and Control Vol. 64, 1985, pp. 100-118,
37 <https://doi.org/10.1016/S0019-9958(85)80046-2>.
38
39 Unless the 'find_minimal' flag is set, this code uses the TOO_EXPENSIVE
40 heuristic, by Paul Eggert, to limit the cost to O(N**1.5 log N)
41 at the price of producing suboptimal output for large inputs with
42 many differences. */
43
44 /* Before including this file, you need to define:
45 ELEMENT The element type of the vectors being compared.
46 EQUAL A two-argument macro that tests two elements for
47 equality.
48 OFFSET A signed integer type sufficient to hold the
49 difference between two indices. Usually
50 something like ptrdiff_t.
51 EXTRA_CONTEXT_FIELDS Declarations of fields for 'struct context'.
52 NOTE_DELETE(ctxt, xoff) Record the removal of the object xvec[xoff].
53 NOTE_INSERT(ctxt, yoff) Record the insertion of the object yvec[yoff].
54 EARLY_ABORT(ctxt) (Optional) A boolean expression that triggers an
55 early abort of the computation.
56 USE_HEURISTIC (Optional) Define if you want to support the
57 heuristic for large vectors.
58 It is also possible to use this file with abstract arrays. In this case,
59 xvec and yvec are not represented in memory. They only exist conceptually.
60 In this case, the list of defines above is amended as follows:
61 ELEMENT Undefined.
62 EQUAL Undefined.
63 XVECREF_YVECREF_EQUAL(ctxt, xoff, yoff)
64 A three-argument macro: References xvec[xoff] and
65 yvec[yoff] and tests these elements for equality.
66 Before including this file, you also need to include:
67 #include <limits.h>
68 #include <stdbool.h>
69 #include "minmax.h"
70 */
71
72 /* Maximum value of type OFFSET. */
73 #define OFFSET_MAX \
74 ((((OFFSET)1 << (sizeof (OFFSET) * CHAR_BIT - 2)) - 1) * 2 + 1)
75
76 /* Default to no early abort. */
77 #ifndef EARLY_ABORT
78 # define EARLY_ABORT(ctxt) false
79 #endif
80
81 /* Use this to suppress gcc's "...may be used before initialized" warnings.
82 Beware: The Code argument must not contain commas. */
83 #ifndef IF_LINT
84 # if defined GCC_LINT || defined lint
85 # define IF_LINT(Code) Code
86 # else
87 # define IF_LINT(Code) /* empty */
88 # endif
89 #endif
90
91 /* As above, but when Code must contain one comma. */
92 #ifndef IF_LINT2
93 # if defined GCC_LINT || defined lint
94 # define IF_LINT2(Code1, Code2) Code1, Code2
95 # else
96 # define IF_LINT2(Code1, Code2) /* empty */
97 # endif
98 #endif
99
100 /*
101 * Context of comparison operation.
102 */
103 struct context
104 {
105 #ifdef ELEMENT
106 /* Vectors being compared. */
107 ELEMENT const *xvec;
108 ELEMENT const *yvec;
109 #endif
110
111 /* Extra fields. */
112 EXTRA_CONTEXT_FIELDS
113
114 /* Vector, indexed by diagonal, containing 1 + the X coordinate of the point
115 furthest along the given diagonal in the forward search of the edit
116 matrix. */
117 OFFSET *fdiag;
118
119 /* Vector, indexed by diagonal, containing the X coordinate of the point
120 furthest along the given diagonal in the backward search of the edit
121 matrix. */
122 OFFSET *bdiag;
123
124 #ifdef USE_HEURISTIC
125 /* This corresponds to the diff --speed-large-files flag. With this
126 heuristic, for vectors with a constant small density of changes,
127 the algorithm is linear in the vector size. */
128 bool heuristic;
129 #endif
130
131 /* Edit scripts longer than this are too expensive to compute. */
132 OFFSET too_expensive;
133
134 /* Snakes bigger than this are considered "big". */
135 #define SNAKE_LIMIT 20
136 };
137
138 struct partition
139 {
140 /* Midpoints of this partition. */
141 OFFSET xmid;
142 OFFSET ymid;
143
144 /* True if low half will be analyzed minimally. */
145 bool lo_minimal;
146
147 /* Likewise for high half. */
148 bool hi_minimal;
149 };
150
151
152 /* Find the midpoint of the shortest edit script for a specified portion
153 of the two vectors.
154
155 Scan from the beginnings of the vectors, and simultaneously from the ends,
156 doing a breadth-first search through the space of edit-sequence.
157 When the two searches meet, we have found the midpoint of the shortest
158 edit sequence.
159
160 If FIND_MINIMAL is true, find the minimal edit script regardless of
161 expense. Otherwise, if the search is too expensive, use heuristics to
162 stop the search and report a suboptimal answer.
163
164 Set PART->(xmid,ymid) to the midpoint (XMID,YMID). The diagonal number
165 XMID - YMID equals the number of inserted elements minus the number
166 of deleted elements (counting only elements before the midpoint).
167
168 Set PART->lo_minimal to true iff the minimal edit script for the
169 left half of the partition is known; similarly for PART->hi_minimal.
170
171 This function assumes that the first elements of the specified portions
172 of the two vectors do not match, and likewise that the last elements do not
173 match. The caller must trim matching elements from the beginning and end
174 of the portions it is going to specify.
175
176 If we return the "wrong" partitions, the worst this can do is cause
177 suboptimal diff output. It cannot cause incorrect diff output. */
178
179 static void
diag(OFFSET xoff,OFFSET xlim,OFFSET yoff,OFFSET ylim,bool find_minimal,struct partition * part,struct context * ctxt)180 diag (OFFSET xoff, OFFSET xlim, OFFSET yoff, OFFSET ylim, bool find_minimal,
181 struct partition *part, struct context *ctxt)
182 {
183 OFFSET *const fd = ctxt->fdiag; /* Give the compiler a chance. */
184 OFFSET *const bd = ctxt->bdiag; /* Additional help for the compiler. */
185 #ifdef ELEMENT
186 ELEMENT const *const xv = ctxt->xvec; /* Still more help for the compiler. */
187 ELEMENT const *const yv = ctxt->yvec; /* And more and more . . . */
188 #define XREF_YREF_EQUAL(x,y) EQUAL (xv[x], yv[y])
189 #else
190 #define XREF_YREF_EQUAL(x,y) XVECREF_YVECREF_EQUAL (ctxt, x, y)
191 #endif
192 const OFFSET dmin = xoff - ylim; /* Minimum valid diagonal. */
193 const OFFSET dmax = xlim - yoff; /* Maximum valid diagonal. */
194 const OFFSET fmid = xoff - yoff; /* Center diagonal of top-down search. */
195 const OFFSET bmid = xlim - ylim; /* Center diagonal of bottom-up search. */
196 OFFSET fmin = fmid;
197 OFFSET fmax = fmid; /* Limits of top-down search. */
198 OFFSET bmin = bmid;
199 OFFSET bmax = bmid; /* Limits of bottom-up search. */
200 OFFSET c; /* Cost. */
201 bool odd = (fmid - bmid) & 1; /* True if southeast corner is on an odd
202 diagonal with respect to the northwest. */
203
204 fd[fmid] = xoff;
205 bd[bmid] = xlim;
206
207 for (c = 1;; ++c)
208 {
209 OFFSET d; /* Active diagonal. */
210 bool big_snake _GL_UNUSED = false;
211
212 /* Extend the top-down search by an edit step in each diagonal. */
213 if (fmin > dmin)
214 fd[--fmin - 1] = -1;
215 else
216 ++fmin;
217 if (fmax < dmax)
218 fd[++fmax + 1] = -1;
219 else
220 --fmax;
221 for (d = fmax; d >= fmin; d -= 2)
222 {
223 OFFSET x;
224 OFFSET y;
225 OFFSET tlo = fd[d - 1];
226 OFFSET thi = fd[d + 1];
227 OFFSET x0 = tlo < thi ? thi : tlo + 1;
228
229 for (x = x0, y = x0 - d;
230 x < xlim && y < ylim && XREF_YREF_EQUAL (x, y);
231 x++, y++)
232 continue;
233 if (x - x0 > SNAKE_LIMIT)
234 big_snake = true;
235 fd[d] = x;
236 if (odd && bmin <= d && d <= bmax && bd[d] <= x)
237 {
238 part->xmid = x;
239 part->ymid = y;
240 part->lo_minimal = part->hi_minimal = true;
241 return;
242 }
243 }
244
245 /* Similarly extend the bottom-up search. */
246 if (bmin > dmin)
247 bd[--bmin - 1] = OFFSET_MAX;
248 else
249 ++bmin;
250 if (bmax < dmax)
251 bd[++bmax + 1] = OFFSET_MAX;
252 else
253 --bmax;
254 for (d = bmax; d >= bmin; d -= 2)
255 {
256 OFFSET x;
257 OFFSET y;
258 OFFSET tlo = bd[d - 1];
259 OFFSET thi = bd[d + 1];
260 OFFSET x0 = tlo < thi ? tlo : thi - 1;
261
262 for (x = x0, y = x0 - d;
263 xoff < x && yoff < y && XREF_YREF_EQUAL (x - 1, y - 1);
264 x--, y--)
265 continue;
266 if (x0 - x > SNAKE_LIMIT)
267 big_snake = true;
268 bd[d] = x;
269 if (!odd && fmin <= d && d <= fmax && x <= fd[d])
270 {
271 part->xmid = x;
272 part->ymid = y;
273 part->lo_minimal = part->hi_minimal = true;
274 return;
275 }
276 }
277
278 if (find_minimal)
279 continue;
280
281 #ifdef USE_HEURISTIC
282 bool heuristic = ctxt->heuristic;
283 #else
284 bool heuristic = false;
285 #endif
286
287 /* Heuristic: check occasionally for a diagonal that has made lots
288 of progress compared with the edit distance. If we have any
289 such, find the one that has made the most progress and return it
290 as if it had succeeded.
291
292 With this heuristic, for vectors with a constant small density
293 of changes, the algorithm is linear in the vector size. */
294
295 if (200 < c && big_snake && heuristic)
296 {
297 {
298 OFFSET best = 0;
299
300 for (d = fmax; d >= fmin; d -= 2)
301 {
302 OFFSET dd = d - fmid;
303 OFFSET x = fd[d];
304 OFFSET y = x - d;
305 OFFSET v = (x - xoff) * 2 - dd;
306
307 if (v > 12 * (c + (dd < 0 ? -dd : dd)))
308 {
309 if (v > best
310 && xoff + SNAKE_LIMIT <= x && x < xlim
311 && yoff + SNAKE_LIMIT <= y && y < ylim)
312 {
313 /* We have a good enough best diagonal; now insist
314 that it end with a significant snake. */
315 int k;
316
317 for (k = 1; XREF_YREF_EQUAL (x - k, y - k); k++)
318 if (k == SNAKE_LIMIT)
319 {
320 best = v;
321 part->xmid = x;
322 part->ymid = y;
323 break;
324 }
325 }
326 }
327 }
328 if (best > 0)
329 {
330 part->lo_minimal = true;
331 part->hi_minimal = false;
332 return;
333 }
334 }
335
336 {
337 OFFSET best = 0;
338
339 for (d = bmax; d >= bmin; d -= 2)
340 {
341 OFFSET dd = d - bmid;
342 OFFSET x = bd[d];
343 OFFSET y = x - d;
344 OFFSET v = (xlim - x) * 2 + dd;
345
346 if (v > 12 * (c + (dd < 0 ? -dd : dd)))
347 {
348 if (v > best
349 && xoff < x && x <= xlim - SNAKE_LIMIT
350 && yoff < y && y <= ylim - SNAKE_LIMIT)
351 {
352 /* We have a good enough best diagonal; now insist
353 that it end with a significant snake. */
354 int k;
355
356 for (k = 0; XREF_YREF_EQUAL (x + k, y + k); k++)
357 if (k == SNAKE_LIMIT - 1)
358 {
359 best = v;
360 part->xmid = x;
361 part->ymid = y;
362 break;
363 }
364 }
365 }
366 }
367 if (best > 0)
368 {
369 part->lo_minimal = false;
370 part->hi_minimal = true;
371 return;
372 }
373 }
374 }
375
376 /* Heuristic: if we've gone well beyond the call of duty, give up
377 and report halfway between our best results so far. */
378 if (c >= ctxt->too_expensive)
379 {
380 OFFSET fxybest;
381 OFFSET fxbest IF_LINT (= 0);
382 OFFSET bxybest;
383 OFFSET bxbest IF_LINT (= 0);
384
385 /* Find forward diagonal that maximizes X + Y. */
386 fxybest = -1;
387 for (d = fmax; d >= fmin; d -= 2)
388 {
389 OFFSET x = MIN (fd[d], xlim);
390 OFFSET y = x - d;
391 if (ylim < y)
392 {
393 x = ylim + d;
394 y = ylim;
395 }
396 if (fxybest < x + y)
397 {
398 fxybest = x + y;
399 fxbest = x;
400 }
401 }
402
403 /* Find backward diagonal that minimizes X + Y. */
404 bxybest = OFFSET_MAX;
405 for (d = bmax; d >= bmin; d -= 2)
406 {
407 OFFSET x = MAX (xoff, bd[d]);
408 OFFSET y = x - d;
409 if (y < yoff)
410 {
411 x = yoff + d;
412 y = yoff;
413 }
414 if (x + y < bxybest)
415 {
416 bxybest = x + y;
417 bxbest = x;
418 }
419 }
420
421 /* Use the better of the two diagonals. */
422 if ((xlim + ylim) - bxybest < fxybest - (xoff + yoff))
423 {
424 part->xmid = fxbest;
425 part->ymid = fxybest - fxbest;
426 part->lo_minimal = true;
427 part->hi_minimal = false;
428 }
429 else
430 {
431 part->xmid = bxbest;
432 part->ymid = bxybest - bxbest;
433 part->lo_minimal = false;
434 part->hi_minimal = true;
435 }
436 return;
437 }
438 }
439 #undef XREF_YREF_EQUAL
440 }
441
442
443 /* Compare in detail contiguous subsequences of the two vectors
444 which are known, as a whole, to match each other.
445
446 The subsequence of vector 0 is [XOFF, XLIM) and likewise for vector 1.
447
448 Note that XLIM, YLIM are exclusive bounds. All indices into the vectors
449 are origin-0.
450
451 If FIND_MINIMAL, find a minimal difference no matter how
452 expensive it is.
453
454 The results are recorded by invoking NOTE_DELETE and NOTE_INSERT.
455
456 Return false if terminated normally, or true if terminated through early
457 abort. */
458
459 static bool
compareseq(OFFSET xoff,OFFSET xlim,OFFSET yoff,OFFSET ylim,bool find_minimal,struct context * ctxt)460 compareseq (OFFSET xoff, OFFSET xlim, OFFSET yoff, OFFSET ylim,
461 bool find_minimal, struct context *ctxt)
462 {
463 #ifdef ELEMENT
464 ELEMENT const *xv = ctxt->xvec; /* Help the compiler. */
465 ELEMENT const *yv = ctxt->yvec;
466 #define XREF_YREF_EQUAL(x,y) EQUAL (xv[x], yv[y])
467 #else
468 #define XREF_YREF_EQUAL(x,y) XVECREF_YVECREF_EQUAL (ctxt, x, y)
469 #endif
470
471 /* Slide down the bottom initial diagonal. */
472 while (xoff < xlim && yoff < ylim && XREF_YREF_EQUAL (xoff, yoff))
473 {
474 xoff++;
475 yoff++;
476 }
477
478 /* Slide up the top initial diagonal. */
479 while (xoff < xlim && yoff < ylim && XREF_YREF_EQUAL (xlim - 1, ylim - 1))
480 {
481 xlim--;
482 ylim--;
483 }
484
485 /* Handle simple cases. */
486 if (xoff == xlim)
487 while (yoff < ylim)
488 {
489 NOTE_INSERT (ctxt, yoff);
490 if (EARLY_ABORT (ctxt))
491 return true;
492 yoff++;
493 }
494 else if (yoff == ylim)
495 while (xoff < xlim)
496 {
497 NOTE_DELETE (ctxt, xoff);
498 if (EARLY_ABORT (ctxt))
499 return true;
500 xoff++;
501 }
502 else
503 {
504 struct partition part IF_LINT2 (= { .xmid = 0, .ymid = 0 });
505
506 /* Find a point of correspondence in the middle of the vectors. */
507 diag (xoff, xlim, yoff, ylim, find_minimal, &part, ctxt);
508
509 /* Use the partitions to split this problem into subproblems. */
510 if (compareseq (xoff, part.xmid, yoff, part.ymid, part.lo_minimal, ctxt))
511 return true;
512 if (compareseq (part.xmid, xlim, part.ymid, ylim, part.hi_minimal, ctxt))
513 return true;
514 }
515
516 return false;
517 #undef XREF_YREF_EQUAL
518 }
519
520 #undef ELEMENT
521 #undef EQUAL
522 #undef OFFSET
523 #undef EXTRA_CONTEXT_FIELDS
524 #undef NOTE_DELETE
525 #undef NOTE_INSERT
526 #undef EARLY_ABORT
527 #undef USE_HEURISTIC
528 #undef XVECREF_YVECREF_EQUAL
529 #undef OFFSET_MAX
530