Commit | Line | Data |
---|---|---|
f6ea5628 | 1 | /* Byte-wise substring search, using the Two-Way algorithm. |
5df4cba6 | 2 | Copyright (C) 2008-2020 Free Software Foundation, Inc. |
f6ea5628 DJ |
3 | This file is part of the GNU C Library. |
4 | Written by Eric Blake <ebb9@byu.net>, 2008. | |
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 | |
4c38e0a4 | 8 | the Free Software Foundation; either version 3, or (at your option) |
f6ea5628 DJ |
9 | 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 along | |
c0c3707f | 17 | with this program; if not, see <https://www.gnu.org/licenses/>. */ |
f6ea5628 DJ |
18 | |
19 | /* Before including this file, you need to include <config.h> and | |
20 | <string.h>, and define: | |
c0c3707f | 21 | RETURN_TYPE A macro that expands to the return type. |
f6ea5628 | 22 | AVAILABLE(h, h_l, j, n_l) |
f434ba03 PA |
23 | A macro that returns nonzero if there are |
24 | at least N_L bytes left starting at H[J]. | |
25 | H is 'unsigned char *', H_L, J, and N_L | |
26 | are 'size_t'; H_L is an lvalue. For | |
27 | NUL-terminated searches, H_L can be | |
28 | modified each iteration to avoid having | |
29 | to compute the end of H up front. | |
f6ea5628 DJ |
30 | |
31 | For case-insensitivity, you may optionally define: | |
32 | CMP_FUNC(p1, p2, l) A macro that returns 0 iff the first L | |
f434ba03 | 33 | characters of P1 and P2 are equal. |
f6ea5628 | 34 | CANON_ELEMENT(c) A macro that canonicalizes an element right after |
f434ba03 PA |
35 | it has been fetched from one of the two strings. |
36 | The argument is an 'unsigned char'; the result | |
37 | must be an 'unsigned char' as well. | |
f6ea5628 DJ |
38 | |
39 | This file undefines the macros documented above, and defines | |
40 | LONG_NEEDLE_THRESHOLD. | |
41 | */ | |
42 | ||
43 | #include <limits.h> | |
44 | #include <stdint.h> | |
45 | ||
770d76d7 PA |
46 | /* We use the Two-Way string matching algorithm (also known as |
47 | Chrochemore-Perrin), which guarantees linear complexity with | |
48 | constant space. Additionally, for long needles, we also use a bad | |
49 | character shift table similar to the Boyer-Moore algorithm to | |
50 | achieve improved (potentially sub-linear) performance. | |
f6ea5628 | 51 | |
c0c3707f CB |
52 | See https://www-igm.univ-mlv.fr/~lecroq/string/node26.html#SECTION00260, |
53 | https://en.wikipedia.org/wiki/Boyer-Moore_string_search_algorithm, | |
54 | https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.34.6641&rep=rep1&type=pdf | |
f6ea5628 DJ |
55 | */ |
56 | ||
57 | /* Point at which computing a bad-byte shift table is likely to be | |
58 | worthwhile. Small needles should not compute a table, since it | |
59 | adds (1 << CHAR_BIT) + NEEDLE_LEN computations of preparation for a | |
60 | speedup no greater than a factor of NEEDLE_LEN. The larger the | |
61 | needle, the better the potential performance gain. On the other | |
62 | hand, on non-POSIX systems with CHAR_BIT larger than eight, the | |
63 | memory required for the table is prohibitive. */ | |
64 | #if CHAR_BIT < 10 | |
65 | # define LONG_NEEDLE_THRESHOLD 32U | |
66 | #else | |
67 | # define LONG_NEEDLE_THRESHOLD SIZE_MAX | |
68 | #endif | |
69 | ||
f434ba03 PA |
70 | #ifndef MAX |
71 | # define MAX(a, b) ((a < b) ? (b) : (a)) | |
72 | #endif | |
f6ea5628 DJ |
73 | |
74 | #ifndef CANON_ELEMENT | |
75 | # define CANON_ELEMENT(c) c | |
76 | #endif | |
77 | #ifndef CMP_FUNC | |
78 | # define CMP_FUNC memcmp | |
79 | #endif | |
80 | ||
81 | /* Perform a critical factorization of NEEDLE, of length NEEDLE_LEN. | |
82 | Return the index of the first byte in the right half, and set | |
83 | *PERIOD to the global period of the right half. | |
84 | ||
85 | The global period of a string is the smallest index (possibly its | |
86 | length) at which all remaining bytes in the string are repetitions | |
87 | of the prefix (the last repetition may be a subset of the prefix). | |
88 | ||
89 | When NEEDLE is factored into two halves, a local period is the | |
90 | length of the smallest word that shares a suffix with the left half | |
91 | and shares a prefix with the right half. All factorizations of a | |
92 | non-empty NEEDLE have a local period of at least 1 and no greater | |
93 | than NEEDLE_LEN. | |
94 | ||
95 | A critical factorization has the property that the local period | |
96 | equals the global period. All strings have at least one critical | |
97 | factorization with the left half smaller than the global period. | |
770d76d7 PA |
98 | And while some strings have more than one critical factorization, |
99 | it is provable that with an ordered alphabet, at least one of the | |
100 | critical factorizations corresponds to a maximal suffix. | |
f6ea5628 DJ |
101 | |
102 | Given an ordered alphabet, a critical factorization can be computed | |
103 | in linear time, with 2 * NEEDLE_LEN comparisons, by computing the | |
770d76d7 PA |
104 | shorter of two ordered maximal suffixes. The ordered maximal |
105 | suffixes are determined by lexicographic comparison while tracking | |
f6ea5628 DJ |
106 | periodicity. */ |
107 | static size_t | |
108 | critical_factorization (const unsigned char *needle, size_t needle_len, | |
f434ba03 | 109 | size_t *period) |
f6ea5628 DJ |
110 | { |
111 | /* Index of last byte of left half, or SIZE_MAX. */ | |
112 | size_t max_suffix, max_suffix_rev; | |
113 | size_t j; /* Index into NEEDLE for current candidate suffix. */ | |
114 | size_t k; /* Offset into current period. */ | |
115 | size_t p; /* Intermediate period. */ | |
116 | unsigned char a, b; /* Current comparison bytes. */ | |
117 | ||
770d76d7 PA |
118 | /* Special case NEEDLE_LEN of 1 or 2 (all callers already filtered |
119 | out 0-length needles. */ | |
120 | if (needle_len < 3) | |
121 | { | |
122 | *period = 1; | |
123 | return needle_len - 1; | |
124 | } | |
125 | ||
f6ea5628 DJ |
126 | /* Invariants: |
127 | 0 <= j < NEEDLE_LEN - 1 | |
128 | -1 <= max_suffix{,_rev} < j (treating SIZE_MAX as if it were signed) | |
129 | min(max_suffix, max_suffix_rev) < global period of NEEDLE | |
130 | 1 <= p <= global period of NEEDLE | |
131 | p == global period of the substring NEEDLE[max_suffix{,_rev}+1...j] | |
132 | 1 <= k <= p | |
133 | */ | |
134 | ||
135 | /* Perform lexicographic search. */ | |
136 | max_suffix = SIZE_MAX; | |
137 | j = 0; | |
138 | k = p = 1; | |
139 | while (j + k < needle_len) | |
140 | { | |
141 | a = CANON_ELEMENT (needle[j + k]); | |
142 | b = CANON_ELEMENT (needle[max_suffix + k]); | |
143 | if (a < b) | |
f434ba03 PA |
144 | { |
145 | /* Suffix is smaller, period is entire prefix so far. */ | |
146 | j += k; | |
147 | k = 1; | |
148 | p = j - max_suffix; | |
149 | } | |
f6ea5628 | 150 | else if (a == b) |
f434ba03 PA |
151 | { |
152 | /* Advance through repetition of the current period. */ | |
153 | if (k != p) | |
154 | ++k; | |
155 | else | |
156 | { | |
157 | j += p; | |
158 | k = 1; | |
159 | } | |
160 | } | |
f6ea5628 | 161 | else /* b < a */ |
f434ba03 PA |
162 | { |
163 | /* Suffix is larger, start over from current location. */ | |
164 | max_suffix = j++; | |
165 | k = p = 1; | |
166 | } | |
f6ea5628 DJ |
167 | } |
168 | *period = p; | |
169 | ||
170 | /* Perform reverse lexicographic search. */ | |
171 | max_suffix_rev = SIZE_MAX; | |
172 | j = 0; | |
173 | k = p = 1; | |
174 | while (j + k < needle_len) | |
175 | { | |
176 | a = CANON_ELEMENT (needle[j + k]); | |
177 | b = CANON_ELEMENT (needle[max_suffix_rev + k]); | |
178 | if (b < a) | |
f434ba03 PA |
179 | { |
180 | /* Suffix is smaller, period is entire prefix so far. */ | |
181 | j += k; | |
182 | k = 1; | |
183 | p = j - max_suffix_rev; | |
184 | } | |
f6ea5628 | 185 | else if (a == b) |
f434ba03 PA |
186 | { |
187 | /* Advance through repetition of the current period. */ | |
188 | if (k != p) | |
189 | ++k; | |
190 | else | |
191 | { | |
192 | j += p; | |
193 | k = 1; | |
194 | } | |
195 | } | |
f6ea5628 | 196 | else /* a < b */ |
f434ba03 PA |
197 | { |
198 | /* Suffix is larger, start over from current location. */ | |
199 | max_suffix_rev = j++; | |
200 | k = p = 1; | |
201 | } | |
f6ea5628 DJ |
202 | } |
203 | ||
770d76d7 PA |
204 | /* Choose the shorter suffix. Return the index of the first byte of |
205 | the right half, rather than the last byte of the left half. | |
206 | ||
207 | For some examples, 'banana' has two critical factorizations, both | |
208 | exposed by the two lexicographic extreme suffixes of 'anana' and | |
209 | 'nana', where both suffixes have a period of 2. On the other | |
210 | hand, with 'aab' and 'bba', both strings have a single critical | |
211 | factorization of the last byte, with the suffix having a period | |
212 | of 1. While the maximal lexicographic suffix of 'aab' is 'b', | |
213 | the maximal lexicographic suffix of 'bba' is 'ba', which is not a | |
214 | critical factorization. Conversely, the maximal reverse | |
215 | lexicographic suffix of 'a' works for 'bba', but not 'ab' for | |
216 | 'aab'. The shorter suffix of the two will always be a critical | |
217 | factorization. */ | |
f6ea5628 DJ |
218 | if (max_suffix_rev + 1 < max_suffix + 1) |
219 | return max_suffix + 1; | |
220 | *period = p; | |
221 | return max_suffix_rev + 1; | |
222 | } | |
223 | ||
224 | /* Return the first location of non-empty NEEDLE within HAYSTACK, or | |
225 | NULL. HAYSTACK_LEN is the minimum known length of HAYSTACK. This | |
226 | method is optimized for NEEDLE_LEN < LONG_NEEDLE_THRESHOLD. | |
227 | Performance is guaranteed to be linear, with an initialization cost | |
228 | of 2 * NEEDLE_LEN comparisons. | |
229 | ||
230 | If AVAILABLE does not modify HAYSTACK_LEN (as in memmem), then at | |
231 | most 2 * HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching. | |
232 | If AVAILABLE modifies HAYSTACK_LEN (as in strstr), then at most 3 * | |
233 | HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching. */ | |
234 | static RETURN_TYPE | |
235 | two_way_short_needle (const unsigned char *haystack, size_t haystack_len, | |
f434ba03 | 236 | const unsigned char *needle, size_t needle_len) |
f6ea5628 DJ |
237 | { |
238 | size_t i; /* Index into current byte of NEEDLE. */ | |
239 | size_t j; /* Index into current window of HAYSTACK. */ | |
240 | size_t period; /* The period of the right half of needle. */ | |
241 | size_t suffix; /* The index of the right half of needle. */ | |
242 | ||
243 | /* Factor the needle into two halves, such that the left half is | |
244 | smaller than the global period, and the right half is | |
245 | periodic (with a period as large as NEEDLE_LEN - suffix). */ | |
246 | suffix = critical_factorization (needle, needle_len, &period); | |
247 | ||
248 | /* Perform the search. Each iteration compares the right half | |
249 | first. */ | |
250 | if (CMP_FUNC (needle, needle + period, suffix) == 0) | |
251 | { | |
770d76d7 PA |
252 | /* Entire needle is periodic; a mismatch in the left half can |
253 | only advance by the period, so use memory to avoid rescanning | |
254 | known occurrences of the period in the right half. */ | |
f6ea5628 DJ |
255 | size_t memory = 0; |
256 | j = 0; | |
257 | while (AVAILABLE (haystack, haystack_len, j, needle_len)) | |
f434ba03 PA |
258 | { |
259 | /* Scan for matches in right half. */ | |
260 | i = MAX (suffix, memory); | |
261 | while (i < needle_len && (CANON_ELEMENT (needle[i]) | |
262 | == CANON_ELEMENT (haystack[i + j]))) | |
263 | ++i; | |
264 | if (needle_len <= i) | |
265 | { | |
266 | /* Scan for matches in left half. */ | |
267 | i = suffix - 1; | |
268 | while (memory < i + 1 && (CANON_ELEMENT (needle[i]) | |
269 | == CANON_ELEMENT (haystack[i + j]))) | |
270 | --i; | |
271 | if (i + 1 < memory + 1) | |
272 | return (RETURN_TYPE) (haystack + j); | |
273 | /* No match, so remember how many repetitions of period | |
274 | on the right half were scanned. */ | |
275 | j += period; | |
276 | memory = needle_len - period; | |
277 | } | |
278 | else | |
279 | { | |
280 | j += i - suffix + 1; | |
281 | memory = 0; | |
282 | } | |
283 | } | |
f6ea5628 DJ |
284 | } |
285 | else | |
286 | { | |
287 | /* The two halves of needle are distinct; no extra memory is | |
f434ba03 | 288 | required, and any mismatch results in a maximal shift. */ |
f6ea5628 DJ |
289 | period = MAX (suffix, needle_len - suffix) + 1; |
290 | j = 0; | |
291 | while (AVAILABLE (haystack, haystack_len, j, needle_len)) | |
f434ba03 PA |
292 | { |
293 | /* Scan for matches in right half. */ | |
294 | i = suffix; | |
295 | while (i < needle_len && (CANON_ELEMENT (needle[i]) | |
296 | == CANON_ELEMENT (haystack[i + j]))) | |
297 | ++i; | |
298 | if (needle_len <= i) | |
299 | { | |
300 | /* Scan for matches in left half. */ | |
301 | i = suffix - 1; | |
302 | while (i != SIZE_MAX && (CANON_ELEMENT (needle[i]) | |
303 | == CANON_ELEMENT (haystack[i + j]))) | |
304 | --i; | |
305 | if (i == SIZE_MAX) | |
306 | return (RETURN_TYPE) (haystack + j); | |
307 | j += period; | |
308 | } | |
309 | else | |
310 | j += i - suffix + 1; | |
311 | } | |
f6ea5628 DJ |
312 | } |
313 | return NULL; | |
314 | } | |
315 | ||
316 | /* Return the first location of non-empty NEEDLE within HAYSTACK, or | |
317 | NULL. HAYSTACK_LEN is the minimum known length of HAYSTACK. This | |
318 | method is optimized for LONG_NEEDLE_THRESHOLD <= NEEDLE_LEN. | |
319 | Performance is guaranteed to be linear, with an initialization cost | |
320 | of 3 * NEEDLE_LEN + (1 << CHAR_BIT) operations. | |
321 | ||
322 | If AVAILABLE does not modify HAYSTACK_LEN (as in memmem), then at | |
323 | most 2 * HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching, | |
324 | and sublinear performance O(HAYSTACK_LEN / NEEDLE_LEN) is possible. | |
325 | If AVAILABLE modifies HAYSTACK_LEN (as in strstr), then at most 3 * | |
326 | HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching, and | |
327 | sublinear performance is not possible. */ | |
328 | static RETURN_TYPE | |
329 | two_way_long_needle (const unsigned char *haystack, size_t haystack_len, | |
f434ba03 | 330 | const unsigned char *needle, size_t needle_len) |
f6ea5628 DJ |
331 | { |
332 | size_t i; /* Index into current byte of NEEDLE. */ | |
333 | size_t j; /* Index into current window of HAYSTACK. */ | |
334 | size_t period; /* The period of the right half of needle. */ | |
335 | size_t suffix; /* The index of the right half of needle. */ | |
336 | size_t shift_table[1U << CHAR_BIT]; /* See below. */ | |
337 | ||
338 | /* Factor the needle into two halves, such that the left half is | |
339 | smaller than the global period, and the right half is | |
340 | periodic (with a period as large as NEEDLE_LEN - suffix). */ | |
341 | suffix = critical_factorization (needle, needle_len, &period); | |
342 | ||
343 | /* Populate shift_table. For each possible byte value c, | |
344 | shift_table[c] is the distance from the last occurrence of c to | |
345 | the end of NEEDLE, or NEEDLE_LEN if c is absent from the NEEDLE. | |
346 | shift_table[NEEDLE[NEEDLE_LEN - 1]] contains the only 0. */ | |
347 | for (i = 0; i < 1U << CHAR_BIT; i++) | |
348 | shift_table[i] = needle_len; | |
349 | for (i = 0; i < needle_len; i++) | |
350 | shift_table[CANON_ELEMENT (needle[i])] = needle_len - i - 1; | |
351 | ||
352 | /* Perform the search. Each iteration compares the right half | |
353 | first. */ | |
354 | if (CMP_FUNC (needle, needle + period, suffix) == 0) | |
355 | { | |
770d76d7 PA |
356 | /* Entire needle is periodic; a mismatch in the left half can |
357 | only advance by the period, so use memory to avoid rescanning | |
358 | known occurrences of the period in the right half. */ | |
f6ea5628 DJ |
359 | size_t memory = 0; |
360 | size_t shift; | |
361 | j = 0; | |
362 | while (AVAILABLE (haystack, haystack_len, j, needle_len)) | |
f434ba03 PA |
363 | { |
364 | /* Check the last byte first; if it does not match, then | |
365 | shift to the next possible match location. */ | |
366 | shift = shift_table[CANON_ELEMENT (haystack[j + needle_len - 1])]; | |
367 | if (0 < shift) | |
368 | { | |
369 | if (memory && shift < period) | |
370 | { | |
371 | /* Since needle is periodic, but the last period has | |
372 | a byte out of place, there can be no match until | |
373 | after the mismatch. */ | |
374 | shift = needle_len - period; | |
f434ba03 | 375 | } |
770d76d7 | 376 | memory = 0; |
f434ba03 PA |
377 | j += shift; |
378 | continue; | |
379 | } | |
380 | /* Scan for matches in right half. The last byte has | |
381 | already been matched, by virtue of the shift table. */ | |
382 | i = MAX (suffix, memory); | |
383 | while (i < needle_len - 1 && (CANON_ELEMENT (needle[i]) | |
384 | == CANON_ELEMENT (haystack[i + j]))) | |
385 | ++i; | |
386 | if (needle_len - 1 <= i) | |
387 | { | |
388 | /* Scan for matches in left half. */ | |
389 | i = suffix - 1; | |
390 | while (memory < i + 1 && (CANON_ELEMENT (needle[i]) | |
391 | == CANON_ELEMENT (haystack[i + j]))) | |
392 | --i; | |
393 | if (i + 1 < memory + 1) | |
394 | return (RETURN_TYPE) (haystack + j); | |
395 | /* No match, so remember how many repetitions of period | |
396 | on the right half were scanned. */ | |
397 | j += period; | |
398 | memory = needle_len - period; | |
399 | } | |
400 | else | |
401 | { | |
402 | j += i - suffix + 1; | |
403 | memory = 0; | |
404 | } | |
405 | } | |
f6ea5628 DJ |
406 | } |
407 | else | |
408 | { | |
409 | /* The two halves of needle are distinct; no extra memory is | |
f434ba03 | 410 | required, and any mismatch results in a maximal shift. */ |
f6ea5628 DJ |
411 | size_t shift; |
412 | period = MAX (suffix, needle_len - suffix) + 1; | |
413 | j = 0; | |
414 | while (AVAILABLE (haystack, haystack_len, j, needle_len)) | |
f434ba03 PA |
415 | { |
416 | /* Check the last byte first; if it does not match, then | |
417 | shift to the next possible match location. */ | |
418 | shift = shift_table[CANON_ELEMENT (haystack[j + needle_len - 1])]; | |
419 | if (0 < shift) | |
420 | { | |
421 | j += shift; | |
422 | continue; | |
423 | } | |
424 | /* Scan for matches in right half. The last byte has | |
425 | already been matched, by virtue of the shift table. */ | |
426 | i = suffix; | |
427 | while (i < needle_len - 1 && (CANON_ELEMENT (needle[i]) | |
428 | == CANON_ELEMENT (haystack[i + j]))) | |
429 | ++i; | |
430 | if (needle_len - 1 <= i) | |
431 | { | |
432 | /* Scan for matches in left half. */ | |
433 | i = suffix - 1; | |
434 | while (i != SIZE_MAX && (CANON_ELEMENT (needle[i]) | |
435 | == CANON_ELEMENT (haystack[i + j]))) | |
436 | --i; | |
437 | if (i == SIZE_MAX) | |
438 | return (RETURN_TYPE) (haystack + j); | |
439 | j += period; | |
440 | } | |
441 | else | |
442 | j += i - suffix + 1; | |
443 | } | |
f6ea5628 DJ |
444 | } |
445 | return NULL; | |
446 | } | |
447 | ||
448 | #undef AVAILABLE | |
449 | #undef CANON_ELEMENT | |
450 | #undef CMP_FUNC | |
451 | #undef MAX | |
452 | #undef RETURN_TYPE |