4 * Suffix array match-finder for Lempel-Ziv compression.
8 * Copyright (c) 2013, 2014 Eric Biggers. All rights reserved.
10 * Redistribution and use in source and binary forms, with or without
11 * modification, are permitted provided that the following conditions
14 * 1. Redistributions of source code must retain the above copyright
15 * notice, this list of conditions and the following disclaimer.
17 * 2. Redistributions in binary form must reproduce the above copyright
18 * notice, this list of conditions and the following disclaimer in the
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21 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS "AS IS" AND
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24 * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE
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34 #ifndef _WIMLIB_LZ_SARRAY_H
35 #define _WIMLIB_LZ_SARRAY_H
37 #include "wimlib/compiler.h" /* must define '_always_inline_attribute' */
38 #include "wimlib/lz.h" /* must define 'struct raw_match' and LZ_ASSERT() */
39 #include "wimlib/types.h" /* must define 'bool', 'u8', 'u16, and 'u32' */
43 /* Position type --- must be an unsigned type large enough to hold the length of
44 * longest window for which the suffix array match-finder will be used. */
45 typedef u32 lz_sarray_pos_t;
47 /* Length type --- must be an unsigned type large enough to hold the maximum
49 typedef u16 lz_sarray_len_t;
51 /* Cost type, for the user-provided match cost evaluation function. */
52 typedef lz_sarray_pos_t lz_sarray_cost_t;
54 /* Type of distances in suffix array links. A larger type would allow skipping
55 * irrelevant suffixes more quickly, which is especially helpful towards the
56 * start of the window. However, even a single byte allows skipping 255 at a
57 * time, which where it matters is already a big improvement over the
58 * alternative of searching the suffixes consecutively. */
59 typedef u8 lz_sarray_delta_t;
61 #define LZ_SARRAY_LEN_MAX ((lz_sarray_len_t)~0UL)
62 #define LZ_SARRAY_POS_MAX ((lz_sarray_pos_t)~0UL)
63 #define LZ_SARRAY_DELTA_MAX ((lz_sarray_delta_t)~0UL)
64 #define LZ_SARRAY_INFINITE_COST ((lz_sarray_cost_t)~0UL)
66 /* State of the suffix array LZ (Lempel-Ziv) match-finder.
68 * This is defined here for benefit of the inlined code. It's not intended for
69 * code outside the match-finder itself to read or write members from this
72 /* Allocated window size for the match-finder.
74 * Note: this match-finder does not store the window itself, as the
75 * suffix array (@SA) and associated arrays (@ISA, @LCP, @salink) are
76 * sufficient to find matches. This number is the maximum length of
77 * those arrays, or also the maximum window (block) size that can be
78 * passed to lz_sarray_load_window(). */
79 lz_sarray_pos_t max_window_size;
81 /* Minimum length of matches to return. */
82 lz_sarray_len_t min_match_len;
84 /* Maximum length of matches to return. */
85 lz_sarray_len_t max_match_len;
87 /* Maximum matches to consider at each position (max search depth). */
88 u32 max_matches_to_consider;
90 /* Maximum number of matches to return at each position. */
91 u32 max_matches_to_return;
93 /* Current position in the window. */
94 lz_sarray_pos_t cur_pos;
96 /* Current window size. */
97 lz_sarray_pos_t window_size;
99 /* Suffix array for the current window.
100 * This is a mapping from suffix rank to suffix position. */
103 /* Inverse suffix array for the current window.
104 * This is a mapping from suffix position to suffix rank.
105 * If 0 <= r < window_size, then ISA[SA[r]] == r. */
106 lz_sarray_pos_t *ISA;
108 /* Suffix array links.
110 * During a linear scan of the input string to find matches, this array
111 * used to keep track of which rank suffixes in the suffix array appear
112 * before the current position. Instead of searching in the original
113 * suffix array, scans for matches at a given position traverse a linked
114 * list containing (usually) only suffixes that appear before that
116 struct salink *salink;
119 /* Suffix array link. An array of these structures, one per suffix rank, is
120 * used as a replacement for the raw LCP (Longest Common Prefix) array to allow
121 * skipping over suffixes that appear later in the window and hence cannot be
122 * used as LZ77 matches. */
125 /* Temporary fields used while this structure is being
128 * Note: we want the entire `struct salink' to be only 6 bytes,
129 * even though this makes "next_initial" unaligned. */
131 lz_sarray_pos_t next_initial;
132 lz_sarray_len_t lcpnext_initial;
136 /* Intially, the length, in bytes, of the longest common
137 * prefix (LCP) between the suffix having this rank and
138 * the suffix with the the smallest larger rank that
139 * starts earlier in the window than the suffix having
142 * Later, during match-finding, after the corresponding
143 * suffix has entered the LZ77 dictionary, this value
144 * may be updated by lz_sarray_update_salink() to refer
145 * instead to a lexicographically closer (but still
146 * larger) suffix that begins at a later position that
147 * has entered the LZ77 dictionary. */
148 lz_sarray_len_t lcpnext;
150 /* Initially, the length, in bytes, of the longest
151 * common prefix (LCP) between the suffix having this
152 * rank and the suffix with the the largest smaller rank
153 * that starts earlier in the window than the suffix
156 * Later, during match-finding, after the corresponding
157 * suffix has entered the LZ77 dictionary, this value
158 * may be updated by lz_sarray_update_salink() to refer
159 * instead to a lexicographically closer (but still
160 * smaller) suffix that begins at a later position that
161 * has entered the LZ77 dictionary. */
162 lz_sarray_len_t lcpprev;
164 /* Distance to the suffix referred to in the description
165 * of "lcpnext" above, but capped to a maximum value to
166 * save memory. If the true distance was truncated,
167 * this will give the distance to the rank of a suffix
168 * that is lexicographically closer to the current
169 * suffix than the desired suffix, but appears *later*
170 * in the window and hence cannot be used as the basis
171 * for a LZ77 match. */
172 lz_sarray_delta_t dist_to_next;
174 /* Distance to the suffix referred to in the description
175 * of "lcpprev" above, but capped to a maximum value to
176 * save memory. If the true distance was truncated,
177 * this will give the distance to the rank of a suffix
178 * that is lexicographically closer to the current
179 * suffix than the desired suffix, but appears *later*
180 * in the window and hence cannot be used as the basis
181 * for a LZ77 match. */
182 lz_sarray_delta_t dist_to_prev;
188 /*-----------------------------------*/
189 /* Functions defined in lz_sarray.c */
190 /*-----------------------------------*/
193 lz_sarray_init(struct lz_sarray *mf,
194 lz_sarray_pos_t max_window_size,
195 lz_sarray_len_t min_match_len,
196 lz_sarray_len_t max_match_len,
197 u32 max_matches_to_consider,
198 u32 max_matches_to_return);
201 lz_sarray_get_needed_memory(lz_sarray_pos_t max_window_size);
204 lz_sarray_destroy(struct lz_sarray *mf);
207 lz_sarray_load_window(struct lz_sarray *mf, const u8 T[], lz_sarray_pos_t n);
209 /*-------------------*/
210 /* Inline functions */
211 /*-------------------*/
213 static _always_inline_attribute lz_sarray_pos_t
214 lz_sarray_get_pos(const struct lz_sarray *mf)
219 /* Advance the suffix array match-finder to the next position. */
220 static _always_inline_attribute void
221 lz_sarray_update_salink(const lz_sarray_pos_t r, struct salink link[])
223 const lz_sarray_pos_t next = r + link[r].dist_to_next;
224 const lz_sarray_pos_t prev = r - link[r].dist_to_prev;
226 if (next != r && link[r].dist_to_next < link[next].dist_to_prev) {
227 link[next].dist_to_prev = link[r].dist_to_next;
228 link[next].lcpprev = link[r].lcpnext;
231 if (prev != r && link[r].dist_to_prev < link[prev].dist_to_next) {
232 link[prev].dist_to_next = link[r].dist_to_prev;
233 link[prev].lcpnext = link[r].lcpprev;
237 /* Skip the current position in the suffix array match-finder. */
238 static _always_inline_attribute void
239 lz_sarray_skip_position(struct lz_sarray *mf)
241 LZ_ASSERT(mf->cur_pos < mf->window_size);
242 lz_sarray_update_salink(mf->ISA[mf->cur_pos++], mf->salink);
246 * Use the suffix array match-finder to retrieve a list of matches at the
249 * Returns the number of matches written into @matches. The matches are
250 * returned in decreasing order by length, and each will be of unique length
251 * between the minimum and maximum match lengths (inclusively) passed to
252 * lz_sarray_init(). Up to @max_matches_to_return (passed to lz_sarray_init())
253 * matches will be returned.
255 * @eval_match_cost is a function for evaluating the cost of a match when
256 * deciding which ones to return. It needs to be fast, and need not be exact;
257 * an implementation might simply rank matches by their offset, for example,
258 * although implementations may choose to take into account additional
259 * information such as repeat offsets.
261 static _always_inline_attribute u32
262 lz_sarray_get_matches(struct lz_sarray *mf,
263 struct raw_match matches[],
264 lz_sarray_cost_t (*eval_match_cost)
265 (lz_sarray_pos_t length,
266 lz_sarray_pos_t offset,
268 const void *eval_match_cost_ctx)
270 LZ_ASSERT(mf->cur_pos < mf->window_size);
271 const lz_sarray_pos_t i = mf->cur_pos++;
273 const lz_sarray_pos_t * const restrict SA = mf->SA;
274 const lz_sarray_pos_t * const restrict ISA = mf->ISA;
275 struct salink * const restrict link = mf->salink;
276 const u32 max_matches_to_consider = mf->max_matches_to_consider;
277 const u32 max_matches_to_return = mf->max_matches_to_return;
279 /* r = Rank of the suffix at the current position. */
280 const lz_sarray_pos_t r = ISA[i];
282 /* Prepare for searching the current position. */
283 lz_sarray_update_salink(r, link);
285 /* L = rank of next suffix to the left;
286 * R = rank of next suffix to the right;
287 * lenL = length of match between current position and the suffix with rank L;
288 * lenR = length of match between current position and the suffix with rank R.
290 * This is left and right relative to the rank of the current suffix.
291 * Since the suffixes in the suffix array are sorted, the longest
292 * matches are immediately to the left and right (using the linked list
293 * to ignore all suffixes that occur later in the window). The match
294 * length decreases the farther left and right we go. We shall keep the
295 * length on both sides in sync in order to choose the lowest-cost match
298 lz_sarray_pos_t L = r - link[r].dist_to_prev;
299 lz_sarray_pos_t R = r + link[r].dist_to_next;
300 lz_sarray_pos_t lenL = link[r].lcpprev;
301 lz_sarray_pos_t lenR = link[r].lcpnext;
303 /* nmatches = number of matches found so far. */
306 /* best_cost = cost of lowest-cost match found so far.
308 * Shorter matches that do not have a lower cost than this are
309 * discarded, since presumably it would be cheaper to output the bytes
310 * from the longer match instead. */
311 lz_sarray_cost_t best_cost = LZ_SARRAY_INFINITE_COST;
313 /* count_remaining = maximum number of possible matches remaining to be
315 u32 count_remaining = max_matches_to_consider;
317 /* pending_offset = offset of lowest-cost match found for the current
318 * length, or 0 if no match has been found yet. */
319 lz_sarray_pos_t pending_offset = 0;
321 /* Note: some 'goto' statements are used in the remainder of this
322 * function to remove unnecessary checks and create branches that the
323 * CPU may predict better. (This function is performance critical.) */
325 if (lenL != 0 && lenL >= lenR)
333 /* Search suffixes on the left until the match length has decreased
334 * below the next match length on the right or to below the minimum
337 lz_sarray_pos_t offset;
338 lz_sarray_cost_t cost;
339 lz_sarray_pos_t old_L;
340 lz_sarray_pos_t old_lenL;
342 /* Check for hard cutoff on amount of work done. */
343 if (count_remaining-- == 0) {
344 if (pending_offset != 0) {
345 /* Save pending match. */
346 matches[nmatches++] = (struct raw_match){
348 .offset = pending_offset,
355 /* Suffix is in LZ77 dictionary. (Check was needed
356 * because the salink array caps distances to save
361 /* Save match offset if it results in lower cost. */
362 cost = (*eval_match_cost)(lenL, offset,
363 eval_match_cost_ctx);
364 if (cost < best_cost) {
366 pending_offset = offset;
370 /* Advance left to next suffix. */
375 L -= link[L].dist_to_prev;
377 if (link[old_L].lcpprev < old_lenL) {
378 /* Match length decreased. */
380 lenL = link[old_L].lcpprev;
382 if (old_lenL > lenR) {
383 /* Neither the right side nor the left size has
384 * any more matches of length @old_lenL. If a
385 * pending match exists, save it. */
386 if (pending_offset != 0) {
387 matches[nmatches++] = (struct raw_match){
389 .offset = pending_offset,
391 if (nmatches == max_matches_to_return)
398 /* New match length on left is still at
399 * least as large as the next match
400 * length on the right: Keep extending
401 * left, unless the minimum match length
402 * would be underrun. */
409 /* Here we have lenL < lenR. Extend right, unless the
410 * minimum match length would be underrun. */
418 /* Search suffixes on the right until the match length has decreased to
419 * the next match length on the left or to below the minimum match
422 lz_sarray_pos_t offset;
423 lz_sarray_cost_t cost;
424 lz_sarray_pos_t old_R;
425 lz_sarray_pos_t old_lenR;
427 /* Check for hard cutoff on amount of work done. */
428 if (count_remaining-- == 0) {
429 if (pending_offset != 0) {
430 /* Save pending match. */
431 matches[nmatches++] = (struct raw_match){
433 .offset = pending_offset,
440 /* Suffix is in LZ77 dictionary. (Check was needed
441 * because the salink array caps distances to save
446 /* Save match offset if it results in lower cost. */
447 cost = (*eval_match_cost)(lenR,
449 eval_match_cost_ctx);
450 if (cost < best_cost) {
452 pending_offset = offset;
456 /* Advance right to next suffix. */
461 R += link[R].dist_to_next;
463 if (link[old_R].lcpnext < lenR) {
464 /* Match length decreased. */
466 lenR = link[old_R].lcpnext;
468 /* Neither the right side nor the left size has any more
469 * matches of length @old_lenR. If a pending match
470 * exists, save it. */
471 if (pending_offset != 0) {
472 matches[nmatches++] = (struct raw_match){
474 .offset = pending_offset,
476 if (nmatches == max_matches_to_return)
483 /* lenR > lenL: Keep extending right, unless
484 * the minimum match length would be underrun.
489 /* lenL >= lenR: Extend left, unless the
490 * minimum match length would be underrun, in
491 * which case we are done. */
501 #endif /* _WIMLIB_LZ_SARRAY_H */