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0caca71a | 1 | /* Byte-wise substring search, using the Two-Way algorithm. |
be75d758 | 2 | Copyright (C) 2008-2012 Free Software Foundation, Inc. |
0caca71a UD |
3 | This file is part of the GNU C Library. |
4 | Written by Eric Blake <ebb9@byu.net>, 2008. | |
5 | ||
6 | The GNU C Library is free software; you can redistribute it and/or | |
7 | modify it under the terms of the GNU Lesser General Public | |
8 | License as published by the Free Software Foundation; either | |
9 | version 2.1 of the License, or (at your option) any later version. | |
10 | ||
11 | The GNU C Library 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 GNU | |
14 | Lesser General Public License for more details. | |
15 | ||
16 | You should have received a copy of the GNU Lesser General Public | |
59ba27a6 PE |
17 | License along with the GNU C Library; if not, see |
18 | <http://www.gnu.org/licenses/>. */ | |
0caca71a UD |
19 | |
20 | /* Before including this file, you need to include <string.h> (and | |
21 | <config.h> before that, if not part of libc), and define: | |
22 | RESULT_TYPE A macro that expands to the return type. | |
23 | AVAILABLE(h, h_l, j, n_l) | |
24 | A macro that returns nonzero if there are | |
25 | at least N_L bytes left starting at H[J]. | |
26 | H is 'unsigned char *', H_L, J, and N_L | |
27 | are 'size_t'; H_L is an lvalue. For | |
28 | NUL-terminated searches, H_L can be | |
29 | modified each iteration to avoid having | |
30 | to compute the end of H up front. | |
31 | ||
32 | For case-insensitivity, you may optionally define: | |
33 | CMP_FUNC(p1, p2, l) A macro that returns 0 iff the first L | |
34 | characters of P1 and P2 are equal. | |
35 | CANON_ELEMENT(c) A macro that canonicalizes an element right after | |
36 | it has been fetched from one of the two strings. | |
37 | The argument is an 'unsigned char'; the result | |
38 | must be an 'unsigned char' as well. | |
39 | ||
40 | This file undefines the macros documented above, and defines | |
41 | LONG_NEEDLE_THRESHOLD. | |
42 | */ | |
43 | ||
44 | #include <limits.h> | |
45 | #include <stdint.h> | |
be75d758 | 46 | #include <sys/param.h> /* Defines MAX. */ |
0caca71a UD |
47 | |
48 | /* We use the Two-Way string matching algorithm, which guarantees | |
49 | linear complexity with constant space. Additionally, for long | |
50 | needles, we also use a bad character shift table similar to the | |
51 | Boyer-Moore algorithm to achieve improved (potentially sub-linear) | |
52 | performance. | |
53 | ||
54 | See http://www-igm.univ-mlv.fr/~lecroq/string/node26.html#SECTION00260 | |
55 | and http://en.wikipedia.org/wiki/Boyer-Moore_string_search_algorithm | |
56 | */ | |
57 | ||
58 | /* Point at which computing a bad-byte shift table is likely to be | |
59 | worthwhile. Small needles should not compute a table, since it | |
60 | adds (1 << CHAR_BIT) + NEEDLE_LEN computations of preparation for a | |
61 | speedup no greater than a factor of NEEDLE_LEN. The larger the | |
62 | needle, the better the potential performance gain. On the other | |
63 | hand, on non-POSIX systems with CHAR_BIT larger than eight, the | |
64 | memory required for the table is prohibitive. */ | |
65 | #if CHAR_BIT < 10 | |
66 | # define LONG_NEEDLE_THRESHOLD 32U | |
67 | #else | |
68 | # define LONG_NEEDLE_THRESHOLD SIZE_MAX | |
69 | #endif | |
70 | ||
0caca71a UD |
71 | #ifndef CANON_ELEMENT |
72 | # define CANON_ELEMENT(c) c | |
73 | #endif | |
74 | #ifndef CMP_FUNC | |
75 | # define CMP_FUNC memcmp | |
76 | #endif | |
77 | ||
400726de MK |
78 | #ifndef AVAILABLE1 |
79 | # define AVAILABLE1(h, h_l, j, n_l) AVAILABLE (h, h_l, j, n_l) | |
80 | #endif | |
81 | #ifndef AVAILABLE2 | |
82 | # define AVAILABLE2(h, h_l, j, n_l) (1) | |
83 | #endif | |
84 | #ifndef RET0_IF_0 | |
85 | # define RET0_IF_0(a) /* nothing */ | |
86 | #endif | |
87 | ||
0caca71a UD |
88 | /* Perform a critical factorization of NEEDLE, of length NEEDLE_LEN. |
89 | Return the index of the first byte in the right half, and set | |
90 | *PERIOD to the global period of the right half. | |
91 | ||
92 | The global period of a string is the smallest index (possibly its | |
93 | length) at which all remaining bytes in the string are repetitions | |
94 | of the prefix (the last repetition may be a subset of the prefix). | |
95 | ||
96 | When NEEDLE is factored into two halves, a local period is the | |
97 | length of the smallest word that shares a suffix with the left half | |
98 | and shares a prefix with the right half. All factorizations of a | |
99 | non-empty NEEDLE have a local period of at least 1 and no greater | |
100 | than NEEDLE_LEN. | |
101 | ||
102 | A critical factorization has the property that the local period | |
103 | equals the global period. All strings have at least one critical | |
104 | factorization with the left half smaller than the global period. | |
105 | ||
106 | Given an ordered alphabet, a critical factorization can be computed | |
107 | in linear time, with 2 * NEEDLE_LEN comparisons, by computing the | |
108 | larger of two ordered maximal suffixes. The ordered maximal | |
109 | suffixes are determined by lexicographic comparison of | |
110 | periodicity. */ | |
111 | static size_t | |
112 | critical_factorization (const unsigned char *needle, size_t needle_len, | |
113 | size_t *period) | |
114 | { | |
115 | /* Index of last byte of left half, or SIZE_MAX. */ | |
116 | size_t max_suffix, max_suffix_rev; | |
117 | size_t j; /* Index into NEEDLE for current candidate suffix. */ | |
118 | size_t k; /* Offset into current period. */ | |
119 | size_t p; /* Intermediate period. */ | |
120 | unsigned char a, b; /* Current comparison bytes. */ | |
121 | ||
122 | /* Invariants: | |
123 | 0 <= j < NEEDLE_LEN - 1 | |
124 | -1 <= max_suffix{,_rev} < j (treating SIZE_MAX as if it were signed) | |
125 | min(max_suffix, max_suffix_rev) < global period of NEEDLE | |
126 | 1 <= p <= global period of NEEDLE | |
127 | p == global period of the substring NEEDLE[max_suffix{,_rev}+1...j] | |
128 | 1 <= k <= p | |
129 | */ | |
130 | ||
131 | /* Perform lexicographic search. */ | |
132 | max_suffix = SIZE_MAX; | |
133 | j = 0; | |
134 | k = p = 1; | |
135 | while (j + k < needle_len) | |
136 | { | |
137 | a = CANON_ELEMENT (needle[j + k]); | |
138 | b = CANON_ELEMENT (needle[max_suffix + k]); | |
139 | if (a < b) | |
140 | { | |
141 | /* Suffix is smaller, period is entire prefix so far. */ | |
142 | j += k; | |
143 | k = 1; | |
144 | p = j - max_suffix; | |
145 | } | |
146 | else if (a == b) | |
147 | { | |
148 | /* Advance through repetition of the current period. */ | |
149 | if (k != p) | |
150 | ++k; | |
151 | else | |
152 | { | |
153 | j += p; | |
154 | k = 1; | |
155 | } | |
156 | } | |
157 | else /* b < a */ | |
158 | { | |
159 | /* Suffix is larger, start over from current location. */ | |
160 | max_suffix = j++; | |
161 | k = p = 1; | |
162 | } | |
163 | } | |
164 | *period = p; | |
165 | ||
166 | /* Perform reverse lexicographic search. */ | |
167 | max_suffix_rev = SIZE_MAX; | |
168 | j = 0; | |
169 | k = p = 1; | |
170 | while (j + k < needle_len) | |
171 | { | |
172 | a = CANON_ELEMENT (needle[j + k]); | |
173 | b = CANON_ELEMENT (needle[max_suffix_rev + k]); | |
174 | if (b < a) | |
175 | { | |
176 | /* Suffix is smaller, period is entire prefix so far. */ | |
177 | j += k; | |
178 | k = 1; | |
179 | p = j - max_suffix_rev; | |
180 | } | |
181 | else if (a == b) | |
182 | { | |
183 | /* Advance through repetition of the current period. */ | |
184 | if (k != p) | |
185 | ++k; | |
186 | else | |
187 | { | |
188 | j += p; | |
189 | k = 1; | |
190 | } | |
191 | } | |
192 | else /* a < b */ | |
193 | { | |
194 | /* Suffix is larger, start over from current location. */ | |
195 | max_suffix_rev = j++; | |
196 | k = p = 1; | |
197 | } | |
198 | } | |
199 | ||
200 | /* Choose the longer suffix. Return the first byte of the right | |
201 | half, rather than the last byte of the left half. */ | |
202 | if (max_suffix_rev + 1 < max_suffix + 1) | |
203 | return max_suffix + 1; | |
204 | *period = p; | |
205 | return max_suffix_rev + 1; | |
206 | } | |
207 | ||
208 | /* Return the first location of non-empty NEEDLE within HAYSTACK, or | |
209 | NULL. HAYSTACK_LEN is the minimum known length of HAYSTACK. This | |
210 | method is optimized for NEEDLE_LEN < LONG_NEEDLE_THRESHOLD. | |
211 | Performance is guaranteed to be linear, with an initialization cost | |
212 | of 2 * NEEDLE_LEN comparisons. | |
213 | ||
214 | If AVAILABLE does not modify HAYSTACK_LEN (as in memmem), then at | |
215 | most 2 * HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching. | |
216 | If AVAILABLE modifies HAYSTACK_LEN (as in strstr), then at most 3 * | |
217 | HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching. */ | |
218 | static RETURN_TYPE | |
219 | two_way_short_needle (const unsigned char *haystack, size_t haystack_len, | |
220 | const unsigned char *needle, size_t needle_len) | |
221 | { | |
222 | size_t i; /* Index into current byte of NEEDLE. */ | |
223 | size_t j; /* Index into current window of HAYSTACK. */ | |
224 | size_t period; /* The period of the right half of needle. */ | |
225 | size_t suffix; /* The index of the right half of needle. */ | |
226 | ||
227 | /* Factor the needle into two halves, such that the left half is | |
228 | smaller than the global period, and the right half is | |
229 | periodic (with a period as large as NEEDLE_LEN - suffix). */ | |
230 | suffix = critical_factorization (needle, needle_len, &period); | |
231 | ||
232 | /* Perform the search. Each iteration compares the right half | |
233 | first. */ | |
234 | if (CMP_FUNC (needle, needle + period, suffix) == 0) | |
235 | { | |
236 | /* Entire needle is periodic; a mismatch can only advance by the | |
237 | period, so use memory to avoid rescanning known occurrences | |
238 | of the period. */ | |
239 | size_t memory = 0; | |
240 | j = 0; | |
241 | while (AVAILABLE (haystack, haystack_len, j, needle_len)) | |
242 | { | |
243 | /* Scan for matches in right half. */ | |
244 | i = MAX (suffix, memory); | |
245 | while (i < needle_len && (CANON_ELEMENT (needle[i]) | |
246 | == CANON_ELEMENT (haystack[i + j]))) | |
247 | ++i; | |
248 | if (needle_len <= i) | |
249 | { | |
250 | /* Scan for matches in left half. */ | |
251 | i = suffix - 1; | |
252 | while (memory < i + 1 && (CANON_ELEMENT (needle[i]) | |
253 | == CANON_ELEMENT (haystack[i + j]))) | |
254 | --i; | |
255 | if (i + 1 < memory + 1) | |
256 | return (RETURN_TYPE) (haystack + j); | |
257 | /* No match, so remember how many repetitions of period | |
258 | on the right half were scanned. */ | |
259 | j += period; | |
260 | memory = needle_len - period; | |
261 | } | |
262 | else | |
263 | { | |
264 | j += i - suffix + 1; | |
265 | memory = 0; | |
266 | } | |
267 | } | |
268 | } | |
269 | else | |
270 | { | |
20a71f2c MK |
271 | /* The comparison always starts from needle[suffix], so cache it |
272 | and use an optimized first-character loop. */ | |
273 | unsigned char needle_suffix = CANON_ELEMENT (needle[suffix]); | |
274 | ||
0caca71a UD |
275 | /* The two halves of needle are distinct; no extra memory is |
276 | required, and any mismatch results in a maximal shift. */ | |
277 | period = MAX (suffix, needle_len - suffix) + 1; | |
278 | j = 0; | |
400726de | 279 | while (AVAILABLE1 (haystack, haystack_len, j, needle_len)) |
0caca71a | 280 | { |
400726de MK |
281 | unsigned char haystack_char; |
282 | ||
20a71f2c MK |
283 | /* TODO: The first-character loop can be sped up by adapting |
284 | longword-at-a-time implementation of memchr/strchr. */ | |
285 | if (needle_suffix | |
400726de | 286 | != (haystack_char = CANON_ELEMENT (haystack[suffix + j]))) |
20a71f2c | 287 | { |
400726de | 288 | RET0_IF_0 (haystack_char); |
20a71f2c MK |
289 | ++j; |
290 | continue; | |
291 | } | |
292 | ||
0caca71a | 293 | /* Scan for matches in right half. */ |
20a71f2c | 294 | i = suffix + 1; |
400726de MK |
295 | while (i < needle_len) |
296 | { | |
297 | if (CANON_ELEMENT (needle[i]) | |
298 | != (haystack_char = CANON_ELEMENT (haystack[i + j]))) | |
299 | { | |
300 | RET0_IF_0 (haystack_char); | |
301 | break; | |
302 | } | |
303 | ++i; | |
304 | } | |
0caca71a UD |
305 | if (needle_len <= i) |
306 | { | |
307 | /* Scan for matches in left half. */ | |
308 | i = suffix - 1; | |
400726de MK |
309 | while (i != SIZE_MAX) |
310 | { | |
311 | if (CANON_ELEMENT (needle[i]) | |
312 | != (haystack_char = CANON_ELEMENT (haystack[i + j]))) | |
313 | { | |
314 | RET0_IF_0 (haystack_char); | |
315 | break; | |
316 | } | |
317 | --i; | |
318 | } | |
0caca71a UD |
319 | if (i == SIZE_MAX) |
320 | return (RETURN_TYPE) (haystack + j); | |
321 | j += period; | |
322 | } | |
323 | else | |
324 | j += i - suffix + 1; | |
400726de MK |
325 | |
326 | if (!AVAILABLE2 (haystack, haystack_len, j, needle_len)) | |
327 | break; | |
0caca71a UD |
328 | } |
329 | } | |
400726de | 330 | ret0: __attribute__ ((unused)) |
0caca71a UD |
331 | return NULL; |
332 | } | |
333 | ||
334 | /* Return the first location of non-empty NEEDLE within HAYSTACK, or | |
335 | NULL. HAYSTACK_LEN is the minimum known length of HAYSTACK. This | |
336 | method is optimized for LONG_NEEDLE_THRESHOLD <= NEEDLE_LEN. | |
337 | Performance is guaranteed to be linear, with an initialization cost | |
338 | of 3 * NEEDLE_LEN + (1 << CHAR_BIT) operations. | |
339 | ||
340 | If AVAILABLE does not modify HAYSTACK_LEN (as in memmem), then at | |
341 | most 2 * HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching, | |
342 | and sublinear performance O(HAYSTACK_LEN / NEEDLE_LEN) is possible. | |
343 | If AVAILABLE modifies HAYSTACK_LEN (as in strstr), then at most 3 * | |
344 | HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching, and | |
345 | sublinear performance is not possible. */ | |
346 | static RETURN_TYPE | |
347 | two_way_long_needle (const unsigned char *haystack, size_t haystack_len, | |
348 | const unsigned char *needle, size_t needle_len) | |
349 | { | |
350 | size_t i; /* Index into current byte of NEEDLE. */ | |
351 | size_t j; /* Index into current window of HAYSTACK. */ | |
352 | size_t period; /* The period of the right half of needle. */ | |
353 | size_t suffix; /* The index of the right half of needle. */ | |
354 | size_t shift_table[1U << CHAR_BIT]; /* See below. */ | |
355 | ||
356 | /* Factor the needle into two halves, such that the left half is | |
357 | smaller than the global period, and the right half is | |
358 | periodic (with a period as large as NEEDLE_LEN - suffix). */ | |
359 | suffix = critical_factorization (needle, needle_len, &period); | |
360 | ||
361 | /* Populate shift_table. For each possible byte value c, | |
362 | shift_table[c] is the distance from the last occurrence of c to | |
363 | the end of NEEDLE, or NEEDLE_LEN if c is absent from the NEEDLE. | |
364 | shift_table[NEEDLE[NEEDLE_LEN - 1]] contains the only 0. */ | |
365 | for (i = 0; i < 1U << CHAR_BIT; i++) | |
366 | shift_table[i] = needle_len; | |
367 | for (i = 0; i < needle_len; i++) | |
368 | shift_table[CANON_ELEMENT (needle[i])] = needle_len - i - 1; | |
369 | ||
370 | /* Perform the search. Each iteration compares the right half | |
371 | first. */ | |
372 | if (CMP_FUNC (needle, needle + period, suffix) == 0) | |
373 | { | |
374 | /* Entire needle is periodic; a mismatch can only advance by the | |
375 | period, so use memory to avoid rescanning known occurrences | |
376 | of the period. */ | |
377 | size_t memory = 0; | |
378 | size_t shift; | |
379 | j = 0; | |
380 | while (AVAILABLE (haystack, haystack_len, j, needle_len)) | |
381 | { | |
382 | /* Check the last byte first; if it does not match, then | |
383 | shift to the next possible match location. */ | |
384 | shift = shift_table[CANON_ELEMENT (haystack[j + needle_len - 1])]; | |
385 | if (0 < shift) | |
386 | { | |
387 | if (memory && shift < period) | |
388 | { | |
389 | /* Since needle is periodic, but the last period has | |
390 | a byte out of place, there can be no match until | |
391 | after the mismatch. */ | |
392 | shift = needle_len - period; | |
0caca71a | 393 | } |
5fb308bc | 394 | memory = 0; |
0caca71a UD |
395 | j += shift; |
396 | continue; | |
397 | } | |
398 | /* Scan for matches in right half. The last byte has | |
399 | already been matched, by virtue of the shift table. */ | |
400 | i = MAX (suffix, memory); | |
401 | while (i < needle_len - 1 && (CANON_ELEMENT (needle[i]) | |
402 | == CANON_ELEMENT (haystack[i + j]))) | |
403 | ++i; | |
404 | if (needle_len - 1 <= i) | |
405 | { | |
406 | /* Scan for matches in left half. */ | |
407 | i = suffix - 1; | |
408 | while (memory < i + 1 && (CANON_ELEMENT (needle[i]) | |
409 | == CANON_ELEMENT (haystack[i + j]))) | |
410 | --i; | |
411 | if (i + 1 < memory + 1) | |
412 | return (RETURN_TYPE) (haystack + j); | |
413 | /* No match, so remember how many repetitions of period | |
414 | on the right half were scanned. */ | |
415 | j += period; | |
416 | memory = needle_len - period; | |
417 | } | |
418 | else | |
419 | { | |
420 | j += i - suffix + 1; | |
421 | memory = 0; | |
422 | } | |
423 | } | |
424 | } | |
425 | else | |
426 | { | |
427 | /* The two halves of needle are distinct; no extra memory is | |
428 | required, and any mismatch results in a maximal shift. */ | |
429 | size_t shift; | |
430 | period = MAX (suffix, needle_len - suffix) + 1; | |
431 | j = 0; | |
432 | while (AVAILABLE (haystack, haystack_len, j, needle_len)) | |
433 | { | |
434 | /* Check the last byte first; if it does not match, then | |
435 | shift to the next possible match location. */ | |
436 | shift = shift_table[CANON_ELEMENT (haystack[j + needle_len - 1])]; | |
437 | if (0 < shift) | |
438 | { | |
439 | j += shift; | |
440 | continue; | |
441 | } | |
442 | /* Scan for matches in right half. The last byte has | |
443 | already been matched, by virtue of the shift table. */ | |
444 | i = suffix; | |
445 | while (i < needle_len - 1 && (CANON_ELEMENT (needle[i]) | |
446 | == CANON_ELEMENT (haystack[i + j]))) | |
447 | ++i; | |
448 | if (needle_len - 1 <= i) | |
449 | { | |
450 | /* Scan for matches in left half. */ | |
451 | i = suffix - 1; | |
452 | while (i != SIZE_MAX && (CANON_ELEMENT (needle[i]) | |
453 | == CANON_ELEMENT (haystack[i + j]))) | |
454 | --i; | |
455 | if (i == SIZE_MAX) | |
456 | return (RETURN_TYPE) (haystack + j); | |
457 | j += period; | |
458 | } | |
459 | else | |
460 | j += i - suffix + 1; | |
461 | } | |
462 | } | |
463 | return NULL; | |
464 | } | |
465 | ||
466 | #undef AVAILABLE | |
400726de MK |
467 | #undef AVAILABLE1 |
468 | #undef AVAILABLE2 | |
0caca71a UD |
469 | #undef CANON_ELEMENT |
470 | #undef CMP_FUNC | |
400726de | 471 | #undef RET0_IF_0 |
0caca71a | 472 | #undef RETURN_TYPE |