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1 | /* Byte-wise substring search, using the Two-Way algorithm. |
2 | Copyright (C) 2008 Free Software Foundation, Inc. | |
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 | |
8 | the Free Software Foundation; either version 3, or (at your option) | |
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 | |
17 | with this program; if not, write to the Free Software Foundation, | |
18 | Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA. */ | |
19 | ||
20 | /* Before including this file, you need to include <config.h> and | |
21 | <string.h>, 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> | |
46 | ||
47 | /* We use the Two-Way string matching algorithm, which guarantees | |
48 | linear complexity with constant space. Additionally, for long | |
49 | needles, we also use a bad character shift table similar to the | |
50 | Boyer-Moore algorithm to achieve improved (potentially sub-linear) | |
51 | performance. | |
52 | ||
53 | See http://www-igm.univ-mlv.fr/~lecroq/string/node26.html#SECTION00260 | |
54 | and http://en.wikipedia.org/wiki/Boyer-Moore_string_search_algorithm | |
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 | ||
70 | #define MAX(a, b) ((a < b) ? (b) : (a)) | |
71 | ||
72 | #ifndef CANON_ELEMENT | |
73 | # define CANON_ELEMENT(c) c | |
74 | #endif | |
75 | #ifndef CMP_FUNC | |
76 | # define CMP_FUNC memcmp | |
77 | #endif | |
78 | ||
79 | /* Perform a critical factorization of NEEDLE, of length NEEDLE_LEN. | |
80 | Return the index of the first byte in the right half, and set | |
81 | *PERIOD to the global period of the right half. | |
82 | ||
83 | The global period of a string is the smallest index (possibly its | |
84 | length) at which all remaining bytes in the string are repetitions | |
85 | of the prefix (the last repetition may be a subset of the prefix). | |
86 | ||
87 | When NEEDLE is factored into two halves, a local period is the | |
88 | length of the smallest word that shares a suffix with the left half | |
89 | and shares a prefix with the right half. All factorizations of a | |
90 | non-empty NEEDLE have a local period of at least 1 and no greater | |
91 | than NEEDLE_LEN. | |
92 | ||
93 | A critical factorization has the property that the local period | |
94 | equals the global period. All strings have at least one critical | |
95 | factorization with the left half smaller than the global period. | |
96 | ||
97 | Given an ordered alphabet, a critical factorization can be computed | |
98 | in linear time, with 2 * NEEDLE_LEN comparisons, by computing the | |
99 | larger of two ordered maximal suffixes. The ordered maximal | |
100 | suffixes are determined by lexicographic comparison of | |
101 | periodicity. */ | |
102 | static size_t | |
103 | critical_factorization (const unsigned char *needle, size_t needle_len, | |
104 | size_t *period) | |
105 | { | |
106 | /* Index of last byte of left half, or SIZE_MAX. */ | |
107 | size_t max_suffix, max_suffix_rev; | |
108 | size_t j; /* Index into NEEDLE for current candidate suffix. */ | |
109 | size_t k; /* Offset into current period. */ | |
110 | size_t p; /* Intermediate period. */ | |
111 | unsigned char a, b; /* Current comparison bytes. */ | |
112 | ||
113 | /* Invariants: | |
114 | 0 <= j < NEEDLE_LEN - 1 | |
115 | -1 <= max_suffix{,_rev} < j (treating SIZE_MAX as if it were signed) | |
116 | min(max_suffix, max_suffix_rev) < global period of NEEDLE | |
117 | 1 <= p <= global period of NEEDLE | |
118 | p == global period of the substring NEEDLE[max_suffix{,_rev}+1...j] | |
119 | 1 <= k <= p | |
120 | */ | |
121 | ||
122 | /* Perform lexicographic search. */ | |
123 | max_suffix = SIZE_MAX; | |
124 | j = 0; | |
125 | k = p = 1; | |
126 | while (j + k < needle_len) | |
127 | { | |
128 | a = CANON_ELEMENT (needle[j + k]); | |
129 | b = CANON_ELEMENT (needle[max_suffix + k]); | |
130 | if (a < b) | |
131 | { | |
132 | /* Suffix is smaller, period is entire prefix so far. */ | |
133 | j += k; | |
134 | k = 1; | |
135 | p = j - max_suffix; | |
136 | } | |
137 | else if (a == b) | |
138 | { | |
139 | /* Advance through repetition of the current period. */ | |
140 | if (k != p) | |
141 | ++k; | |
142 | else | |
143 | { | |
144 | j += p; | |
145 | k = 1; | |
146 | } | |
147 | } | |
148 | else /* b < a */ | |
149 | { | |
150 | /* Suffix is larger, start over from current location. */ | |
151 | max_suffix = j++; | |
152 | k = p = 1; | |
153 | } | |
154 | } | |
155 | *period = p; | |
156 | ||
157 | /* Perform reverse lexicographic search. */ | |
158 | max_suffix_rev = SIZE_MAX; | |
159 | j = 0; | |
160 | k = p = 1; | |
161 | while (j + k < needle_len) | |
162 | { | |
163 | a = CANON_ELEMENT (needle[j + k]); | |
164 | b = CANON_ELEMENT (needle[max_suffix_rev + k]); | |
165 | if (b < a) | |
166 | { | |
167 | /* Suffix is smaller, period is entire prefix so far. */ | |
168 | j += k; | |
169 | k = 1; | |
170 | p = j - max_suffix_rev; | |
171 | } | |
172 | else if (a == b) | |
173 | { | |
174 | /* Advance through repetition of the current period. */ | |
175 | if (k != p) | |
176 | ++k; | |
177 | else | |
178 | { | |
179 | j += p; | |
180 | k = 1; | |
181 | } | |
182 | } | |
183 | else /* a < b */ | |
184 | { | |
185 | /* Suffix is larger, start over from current location. */ | |
186 | max_suffix_rev = j++; | |
187 | k = p = 1; | |
188 | } | |
189 | } | |
190 | ||
191 | /* Choose the longer suffix. Return the first byte of the right | |
192 | half, rather than the last byte of the left half. */ | |
193 | if (max_suffix_rev + 1 < max_suffix + 1) | |
194 | return max_suffix + 1; | |
195 | *period = p; | |
196 | return max_suffix_rev + 1; | |
197 | } | |
198 | ||
199 | /* Return the first location of non-empty NEEDLE within HAYSTACK, or | |
200 | NULL. HAYSTACK_LEN is the minimum known length of HAYSTACK. This | |
201 | method is optimized for NEEDLE_LEN < LONG_NEEDLE_THRESHOLD. | |
202 | Performance is guaranteed to be linear, with an initialization cost | |
203 | of 2 * NEEDLE_LEN comparisons. | |
204 | ||
205 | If AVAILABLE does not modify HAYSTACK_LEN (as in memmem), then at | |
206 | most 2 * HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching. | |
207 | If AVAILABLE modifies HAYSTACK_LEN (as in strstr), then at most 3 * | |
208 | HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching. */ | |
209 | static RETURN_TYPE | |
210 | two_way_short_needle (const unsigned char *haystack, size_t haystack_len, | |
211 | const unsigned char *needle, size_t needle_len) | |
212 | { | |
213 | size_t i; /* Index into current byte of NEEDLE. */ | |
214 | size_t j; /* Index into current window of HAYSTACK. */ | |
215 | size_t period; /* The period of the right half of needle. */ | |
216 | size_t suffix; /* The index of the right half of needle. */ | |
217 | ||
218 | /* Factor the needle into two halves, such that the left half is | |
219 | smaller than the global period, and the right half is | |
220 | periodic (with a period as large as NEEDLE_LEN - suffix). */ | |
221 | suffix = critical_factorization (needle, needle_len, &period); | |
222 | ||
223 | /* Perform the search. Each iteration compares the right half | |
224 | first. */ | |
225 | if (CMP_FUNC (needle, needle + period, suffix) == 0) | |
226 | { | |
227 | /* Entire needle is periodic; a mismatch can only advance by the | |
228 | period, so use memory to avoid rescanning known occurrences | |
229 | of the period. */ | |
230 | size_t memory = 0; | |
231 | j = 0; | |
232 | while (AVAILABLE (haystack, haystack_len, j, needle_len)) | |
233 | { | |
234 | /* Scan for matches in right half. */ | |
235 | i = MAX (suffix, memory); | |
236 | while (i < needle_len && (CANON_ELEMENT (needle[i]) | |
237 | == CANON_ELEMENT (haystack[i + j]))) | |
238 | ++i; | |
239 | if (needle_len <= i) | |
240 | { | |
241 | /* Scan for matches in left half. */ | |
242 | i = suffix - 1; | |
243 | while (memory < i + 1 && (CANON_ELEMENT (needle[i]) | |
244 | == CANON_ELEMENT (haystack[i + j]))) | |
245 | --i; | |
246 | if (i + 1 < memory + 1) | |
247 | return (RETURN_TYPE) (haystack + j); | |
248 | /* No match, so remember how many repetitions of period | |
249 | on the right half were scanned. */ | |
250 | j += period; | |
251 | memory = needle_len - period; | |
252 | } | |
253 | else | |
254 | { | |
255 | j += i - suffix + 1; | |
256 | memory = 0; | |
257 | } | |
258 | } | |
259 | } | |
260 | else | |
261 | { | |
262 | /* The two halves of needle are distinct; no extra memory is | |
263 | required, and any mismatch results in a maximal shift. */ | |
264 | period = MAX (suffix, needle_len - suffix) + 1; | |
265 | j = 0; | |
266 | while (AVAILABLE (haystack, haystack_len, j, needle_len)) | |
267 | { | |
268 | /* Scan for matches in right half. */ | |
269 | i = suffix; | |
270 | while (i < needle_len && (CANON_ELEMENT (needle[i]) | |
271 | == CANON_ELEMENT (haystack[i + j]))) | |
272 | ++i; | |
273 | if (needle_len <= i) | |
274 | { | |
275 | /* Scan for matches in left half. */ | |
276 | i = suffix - 1; | |
277 | while (i != SIZE_MAX && (CANON_ELEMENT (needle[i]) | |
278 | == CANON_ELEMENT (haystack[i + j]))) | |
279 | --i; | |
280 | if (i == SIZE_MAX) | |
281 | return (RETURN_TYPE) (haystack + j); | |
282 | j += period; | |
283 | } | |
284 | else | |
285 | j += i - suffix + 1; | |
286 | } | |
287 | } | |
288 | return NULL; | |
289 | } | |
290 | ||
291 | /* Return the first location of non-empty NEEDLE within HAYSTACK, or | |
292 | NULL. HAYSTACK_LEN is the minimum known length of HAYSTACK. This | |
293 | method is optimized for LONG_NEEDLE_THRESHOLD <= NEEDLE_LEN. | |
294 | Performance is guaranteed to be linear, with an initialization cost | |
295 | of 3 * NEEDLE_LEN + (1 << CHAR_BIT) operations. | |
296 | ||
297 | If AVAILABLE does not modify HAYSTACK_LEN (as in memmem), then at | |
298 | most 2 * HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching, | |
299 | and sublinear performance O(HAYSTACK_LEN / NEEDLE_LEN) is possible. | |
300 | If AVAILABLE modifies HAYSTACK_LEN (as in strstr), then at most 3 * | |
301 | HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching, and | |
302 | sublinear performance is not possible. */ | |
303 | static RETURN_TYPE | |
304 | two_way_long_needle (const unsigned char *haystack, size_t haystack_len, | |
305 | const unsigned char *needle, size_t needle_len) | |
306 | { | |
307 | size_t i; /* Index into current byte of NEEDLE. */ | |
308 | size_t j; /* Index into current window of HAYSTACK. */ | |
309 | size_t period; /* The period of the right half of needle. */ | |
310 | size_t suffix; /* The index of the right half of needle. */ | |
311 | size_t shift_table[1U << CHAR_BIT]; /* See below. */ | |
312 | ||
313 | /* Factor the needle into two halves, such that the left half is | |
314 | smaller than the global period, and the right half is | |
315 | periodic (with a period as large as NEEDLE_LEN - suffix). */ | |
316 | suffix = critical_factorization (needle, needle_len, &period); | |
317 | ||
318 | /* Populate shift_table. For each possible byte value c, | |
319 | shift_table[c] is the distance from the last occurrence of c to | |
320 | the end of NEEDLE, or NEEDLE_LEN if c is absent from the NEEDLE. | |
321 | shift_table[NEEDLE[NEEDLE_LEN - 1]] contains the only 0. */ | |
322 | for (i = 0; i < 1U << CHAR_BIT; i++) | |
323 | shift_table[i] = needle_len; | |
324 | for (i = 0; i < needle_len; i++) | |
325 | shift_table[CANON_ELEMENT (needle[i])] = needle_len - i - 1; | |
326 | ||
327 | /* Perform the search. Each iteration compares the right half | |
328 | first. */ | |
329 | if (CMP_FUNC (needle, needle + period, suffix) == 0) | |
330 | { | |
331 | /* Entire needle is periodic; a mismatch can only advance by the | |
332 | period, so use memory to avoid rescanning known occurrences | |
333 | of the period. */ | |
334 | size_t memory = 0; | |
335 | size_t shift; | |
336 | j = 0; | |
337 | while (AVAILABLE (haystack, haystack_len, j, needle_len)) | |
338 | { | |
339 | /* Check the last byte first; if it does not match, then | |
340 | shift to the next possible match location. */ | |
341 | shift = shift_table[CANON_ELEMENT (haystack[j + needle_len - 1])]; | |
342 | if (0 < shift) | |
343 | { | |
344 | if (memory && shift < period) | |
345 | { | |
346 | /* Since needle is periodic, but the last period has | |
347 | a byte out of place, there can be no match until | |
348 | after the mismatch. */ | |
349 | shift = needle_len - period; | |
350 | memory = 0; | |
351 | } | |
352 | j += shift; | |
353 | continue; | |
354 | } | |
355 | /* Scan for matches in right half. The last byte has | |
356 | already been matched, by virtue of the shift table. */ | |
357 | i = MAX (suffix, memory); | |
358 | while (i < needle_len - 1 && (CANON_ELEMENT (needle[i]) | |
359 | == CANON_ELEMENT (haystack[i + j]))) | |
360 | ++i; | |
361 | if (needle_len - 1 <= i) | |
362 | { | |
363 | /* Scan for matches in left half. */ | |
364 | i = suffix - 1; | |
365 | while (memory < i + 1 && (CANON_ELEMENT (needle[i]) | |
366 | == CANON_ELEMENT (haystack[i + j]))) | |
367 | --i; | |
368 | if (i + 1 < memory + 1) | |
369 | return (RETURN_TYPE) (haystack + j); | |
370 | /* No match, so remember how many repetitions of period | |
371 | on the right half were scanned. */ | |
372 | j += period; | |
373 | memory = needle_len - period; | |
374 | } | |
375 | else | |
376 | { | |
377 | j += i - suffix + 1; | |
378 | memory = 0; | |
379 | } | |
380 | } | |
381 | } | |
382 | else | |
383 | { | |
384 | /* The two halves of needle are distinct; no extra memory is | |
385 | required, and any mismatch results in a maximal shift. */ | |
386 | size_t shift; | |
387 | period = MAX (suffix, needle_len - suffix) + 1; | |
388 | j = 0; | |
389 | while (AVAILABLE (haystack, haystack_len, j, needle_len)) | |
390 | { | |
391 | /* Check the last byte first; if it does not match, then | |
392 | shift to the next possible match location. */ | |
393 | shift = shift_table[CANON_ELEMENT (haystack[j + needle_len - 1])]; | |
394 | if (0 < shift) | |
395 | { | |
396 | j += shift; | |
397 | continue; | |
398 | } | |
399 | /* Scan for matches in right half. The last byte has | |
400 | already been matched, by virtue of the shift table. */ | |
401 | i = suffix; | |
402 | while (i < needle_len - 1 && (CANON_ELEMENT (needle[i]) | |
403 | == CANON_ELEMENT (haystack[i + j]))) | |
404 | ++i; | |
405 | if (needle_len - 1 <= i) | |
406 | { | |
407 | /* Scan for matches in left half. */ | |
408 | i = suffix - 1; | |
409 | while (i != SIZE_MAX && (CANON_ELEMENT (needle[i]) | |
410 | == CANON_ELEMENT (haystack[i + j]))) | |
411 | --i; | |
412 | if (i == SIZE_MAX) | |
413 | return (RETURN_TYPE) (haystack + j); | |
414 | j += period; | |
415 | } | |
416 | else | |
417 | j += i - suffix + 1; | |
418 | } | |
419 | } | |
420 | return NULL; | |
421 | } | |
422 | ||
423 | #undef AVAILABLE | |
424 | #undef CANON_ELEMENT | |
425 | #undef CMP_FUNC | |
426 | #undef MAX | |
427 | #undef RETURN_TYPE |