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f6ea5628 | 1 | /* Byte-wise substring search, using the Two-Way algorithm. |
7b6bb8da | 2 | Copyright (C) 2008, 2009, 2010, 2011 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 | |
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) | |
f434ba03 PA |
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. | |
f6ea5628 DJ |
31 | |
32 | For case-insensitivity, you may optionally define: | |
33 | CMP_FUNC(p1, p2, l) A macro that returns 0 iff the first L | |
f434ba03 | 34 | characters of P1 and P2 are equal. |
f6ea5628 | 35 | CANON_ELEMENT(c) A macro that canonicalizes an element right after |
f434ba03 PA |
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. | |
f6ea5628 DJ |
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 | ||
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. | |
98 | ||
99 | Given an ordered alphabet, a critical factorization can be computed | |
100 | in linear time, with 2 * NEEDLE_LEN comparisons, by computing the | |
101 | larger of two ordered maximal suffixes. The ordered maximal | |
102 | suffixes are determined by lexicographic comparison of | |
103 | periodicity. */ | |
104 | static size_t | |
105 | critical_factorization (const unsigned char *needle, size_t needle_len, | |
f434ba03 | 106 | size_t *period) |
f6ea5628 DJ |
107 | { |
108 | /* Index of last byte of left half, or SIZE_MAX. */ | |
109 | size_t max_suffix, max_suffix_rev; | |
110 | size_t j; /* Index into NEEDLE for current candidate suffix. */ | |
111 | size_t k; /* Offset into current period. */ | |
112 | size_t p; /* Intermediate period. */ | |
113 | unsigned char a, b; /* Current comparison bytes. */ | |
114 | ||
115 | /* Invariants: | |
116 | 0 <= j < NEEDLE_LEN - 1 | |
117 | -1 <= max_suffix{,_rev} < j (treating SIZE_MAX as if it were signed) | |
118 | min(max_suffix, max_suffix_rev) < global period of NEEDLE | |
119 | 1 <= p <= global period of NEEDLE | |
120 | p == global period of the substring NEEDLE[max_suffix{,_rev}+1...j] | |
121 | 1 <= k <= p | |
122 | */ | |
123 | ||
124 | /* Perform lexicographic search. */ | |
125 | max_suffix = SIZE_MAX; | |
126 | j = 0; | |
127 | k = p = 1; | |
128 | while (j + k < needle_len) | |
129 | { | |
130 | a = CANON_ELEMENT (needle[j + k]); | |
131 | b = CANON_ELEMENT (needle[max_suffix + k]); | |
132 | if (a < b) | |
f434ba03 PA |
133 | { |
134 | /* Suffix is smaller, period is entire prefix so far. */ | |
135 | j += k; | |
136 | k = 1; | |
137 | p = j - max_suffix; | |
138 | } | |
f6ea5628 | 139 | else if (a == b) |
f434ba03 PA |
140 | { |
141 | /* Advance through repetition of the current period. */ | |
142 | if (k != p) | |
143 | ++k; | |
144 | else | |
145 | { | |
146 | j += p; | |
147 | k = 1; | |
148 | } | |
149 | } | |
f6ea5628 | 150 | else /* b < a */ |
f434ba03 PA |
151 | { |
152 | /* Suffix is larger, start over from current location. */ | |
153 | max_suffix = j++; | |
154 | k = p = 1; | |
155 | } | |
f6ea5628 DJ |
156 | } |
157 | *period = p; | |
158 | ||
159 | /* Perform reverse lexicographic search. */ | |
160 | max_suffix_rev = SIZE_MAX; | |
161 | j = 0; | |
162 | k = p = 1; | |
163 | while (j + k < needle_len) | |
164 | { | |
165 | a = CANON_ELEMENT (needle[j + k]); | |
166 | b = CANON_ELEMENT (needle[max_suffix_rev + k]); | |
167 | if (b < a) | |
f434ba03 PA |
168 | { |
169 | /* Suffix is smaller, period is entire prefix so far. */ | |
170 | j += k; | |
171 | k = 1; | |
172 | p = j - max_suffix_rev; | |
173 | } | |
f6ea5628 | 174 | else if (a == b) |
f434ba03 PA |
175 | { |
176 | /* Advance through repetition of the current period. */ | |
177 | if (k != p) | |
178 | ++k; | |
179 | else | |
180 | { | |
181 | j += p; | |
182 | k = 1; | |
183 | } | |
184 | } | |
f6ea5628 | 185 | else /* a < b */ |
f434ba03 PA |
186 | { |
187 | /* Suffix is larger, start over from current location. */ | |
188 | max_suffix_rev = j++; | |
189 | k = p = 1; | |
190 | } | |
f6ea5628 DJ |
191 | } |
192 | ||
193 | /* Choose the longer suffix. Return the first byte of the right | |
194 | half, rather than the last byte of the left half. */ | |
195 | if (max_suffix_rev + 1 < max_suffix + 1) | |
196 | return max_suffix + 1; | |
197 | *period = p; | |
198 | return max_suffix_rev + 1; | |
199 | } | |
200 | ||
201 | /* Return the first location of non-empty NEEDLE within HAYSTACK, or | |
202 | NULL. HAYSTACK_LEN is the minimum known length of HAYSTACK. This | |
203 | method is optimized for NEEDLE_LEN < LONG_NEEDLE_THRESHOLD. | |
204 | Performance is guaranteed to be linear, with an initialization cost | |
205 | of 2 * NEEDLE_LEN comparisons. | |
206 | ||
207 | If AVAILABLE does not modify HAYSTACK_LEN (as in memmem), then at | |
208 | most 2 * HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching. | |
209 | If AVAILABLE modifies HAYSTACK_LEN (as in strstr), then at most 3 * | |
210 | HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching. */ | |
211 | static RETURN_TYPE | |
212 | two_way_short_needle (const unsigned char *haystack, size_t haystack_len, | |
f434ba03 | 213 | const unsigned char *needle, size_t needle_len) |
f6ea5628 DJ |
214 | { |
215 | size_t i; /* Index into current byte of NEEDLE. */ | |
216 | size_t j; /* Index into current window of HAYSTACK. */ | |
217 | size_t period; /* The period of the right half of needle. */ | |
218 | size_t suffix; /* The index of the right half of needle. */ | |
219 | ||
220 | /* Factor the needle into two halves, such that the left half is | |
221 | smaller than the global period, and the right half is | |
222 | periodic (with a period as large as NEEDLE_LEN - suffix). */ | |
223 | suffix = critical_factorization (needle, needle_len, &period); | |
224 | ||
225 | /* Perform the search. Each iteration compares the right half | |
226 | first. */ | |
227 | if (CMP_FUNC (needle, needle + period, suffix) == 0) | |
228 | { | |
229 | /* Entire needle is periodic; a mismatch can only advance by the | |
f434ba03 PA |
230 | period, so use memory to avoid rescanning known occurrences |
231 | of the period. */ | |
f6ea5628 DJ |
232 | size_t memory = 0; |
233 | j = 0; | |
234 | while (AVAILABLE (haystack, haystack_len, j, needle_len)) | |
f434ba03 PA |
235 | { |
236 | /* Scan for matches in right half. */ | |
237 | i = MAX (suffix, memory); | |
238 | while (i < needle_len && (CANON_ELEMENT (needle[i]) | |
239 | == CANON_ELEMENT (haystack[i + j]))) | |
240 | ++i; | |
241 | if (needle_len <= i) | |
242 | { | |
243 | /* Scan for matches in left half. */ | |
244 | i = suffix - 1; | |
245 | while (memory < i + 1 && (CANON_ELEMENT (needle[i]) | |
246 | == CANON_ELEMENT (haystack[i + j]))) | |
247 | --i; | |
248 | if (i + 1 < memory + 1) | |
249 | return (RETURN_TYPE) (haystack + j); | |
250 | /* No match, so remember how many repetitions of period | |
251 | on the right half were scanned. */ | |
252 | j += period; | |
253 | memory = needle_len - period; | |
254 | } | |
255 | else | |
256 | { | |
257 | j += i - suffix + 1; | |
258 | memory = 0; | |
259 | } | |
260 | } | |
f6ea5628 DJ |
261 | } |
262 | else | |
263 | { | |
264 | /* The two halves of needle are distinct; no extra memory is | |
f434ba03 | 265 | required, and any mismatch results in a maximal shift. */ |
f6ea5628 DJ |
266 | period = MAX (suffix, needle_len - suffix) + 1; |
267 | j = 0; | |
268 | while (AVAILABLE (haystack, haystack_len, j, needle_len)) | |
f434ba03 PA |
269 | { |
270 | /* Scan for matches in right half. */ | |
271 | i = suffix; | |
272 | while (i < needle_len && (CANON_ELEMENT (needle[i]) | |
273 | == CANON_ELEMENT (haystack[i + j]))) | |
274 | ++i; | |
275 | if (needle_len <= i) | |
276 | { | |
277 | /* Scan for matches in left half. */ | |
278 | i = suffix - 1; | |
279 | while (i != SIZE_MAX && (CANON_ELEMENT (needle[i]) | |
280 | == CANON_ELEMENT (haystack[i + j]))) | |
281 | --i; | |
282 | if (i == SIZE_MAX) | |
283 | return (RETURN_TYPE) (haystack + j); | |
284 | j += period; | |
285 | } | |
286 | else | |
287 | j += i - suffix + 1; | |
288 | } | |
f6ea5628 DJ |
289 | } |
290 | return NULL; | |
291 | } | |
292 | ||
293 | /* Return the first location of non-empty NEEDLE within HAYSTACK, or | |
294 | NULL. HAYSTACK_LEN is the minimum known length of HAYSTACK. This | |
295 | method is optimized for LONG_NEEDLE_THRESHOLD <= NEEDLE_LEN. | |
296 | Performance is guaranteed to be linear, with an initialization cost | |
297 | of 3 * NEEDLE_LEN + (1 << CHAR_BIT) operations. | |
298 | ||
299 | If AVAILABLE does not modify HAYSTACK_LEN (as in memmem), then at | |
300 | most 2 * HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching, | |
301 | and sublinear performance O(HAYSTACK_LEN / NEEDLE_LEN) is possible. | |
302 | If AVAILABLE modifies HAYSTACK_LEN (as in strstr), then at most 3 * | |
303 | HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching, and | |
304 | sublinear performance is not possible. */ | |
305 | static RETURN_TYPE | |
306 | two_way_long_needle (const unsigned char *haystack, size_t haystack_len, | |
f434ba03 | 307 | const unsigned char *needle, size_t needle_len) |
f6ea5628 DJ |
308 | { |
309 | size_t i; /* Index into current byte of NEEDLE. */ | |
310 | size_t j; /* Index into current window of HAYSTACK. */ | |
311 | size_t period; /* The period of the right half of needle. */ | |
312 | size_t suffix; /* The index of the right half of needle. */ | |
313 | size_t shift_table[1U << CHAR_BIT]; /* See below. */ | |
314 | ||
315 | /* Factor the needle into two halves, such that the left half is | |
316 | smaller than the global period, and the right half is | |
317 | periodic (with a period as large as NEEDLE_LEN - suffix). */ | |
318 | suffix = critical_factorization (needle, needle_len, &period); | |
319 | ||
320 | /* Populate shift_table. For each possible byte value c, | |
321 | shift_table[c] is the distance from the last occurrence of c to | |
322 | the end of NEEDLE, or NEEDLE_LEN if c is absent from the NEEDLE. | |
323 | shift_table[NEEDLE[NEEDLE_LEN - 1]] contains the only 0. */ | |
324 | for (i = 0; i < 1U << CHAR_BIT; i++) | |
325 | shift_table[i] = needle_len; | |
326 | for (i = 0; i < needle_len; i++) | |
327 | shift_table[CANON_ELEMENT (needle[i])] = needle_len - i - 1; | |
328 | ||
329 | /* Perform the search. Each iteration compares the right half | |
330 | first. */ | |
331 | if (CMP_FUNC (needle, needle + period, suffix) == 0) | |
332 | { | |
333 | /* Entire needle is periodic; a mismatch can only advance by the | |
f434ba03 PA |
334 | period, so use memory to avoid rescanning known occurrences |
335 | of the period. */ | |
f6ea5628 DJ |
336 | size_t memory = 0; |
337 | size_t shift; | |
338 | j = 0; | |
339 | while (AVAILABLE (haystack, haystack_len, j, needle_len)) | |
f434ba03 PA |
340 | { |
341 | /* Check the last byte first; if it does not match, then | |
342 | shift to the next possible match location. */ | |
343 | shift = shift_table[CANON_ELEMENT (haystack[j + needle_len - 1])]; | |
344 | if (0 < shift) | |
345 | { | |
346 | if (memory && shift < period) | |
347 | { | |
348 | /* Since needle is periodic, but the last period has | |
349 | a byte out of place, there can be no match until | |
350 | after the mismatch. */ | |
351 | shift = needle_len - period; | |
352 | memory = 0; | |
353 | } | |
354 | j += shift; | |
355 | continue; | |
356 | } | |
357 | /* Scan for matches in right half. The last byte has | |
358 | already been matched, by virtue of the shift table. */ | |
359 | i = MAX (suffix, memory); | |
360 | while (i < needle_len - 1 && (CANON_ELEMENT (needle[i]) | |
361 | == CANON_ELEMENT (haystack[i + j]))) | |
362 | ++i; | |
363 | if (needle_len - 1 <= i) | |
364 | { | |
365 | /* Scan for matches in left half. */ | |
366 | i = suffix - 1; | |
367 | while (memory < i + 1 && (CANON_ELEMENT (needle[i]) | |
368 | == CANON_ELEMENT (haystack[i + j]))) | |
369 | --i; | |
370 | if (i + 1 < memory + 1) | |
371 | return (RETURN_TYPE) (haystack + j); | |
372 | /* No match, so remember how many repetitions of period | |
373 | on the right half were scanned. */ | |
374 | j += period; | |
375 | memory = needle_len - period; | |
376 | } | |
377 | else | |
378 | { | |
379 | j += i - suffix + 1; | |
380 | memory = 0; | |
381 | } | |
382 | } | |
f6ea5628 DJ |
383 | } |
384 | else | |
385 | { | |
386 | /* The two halves of needle are distinct; no extra memory is | |
f434ba03 | 387 | required, and any mismatch results in a maximal shift. */ |
f6ea5628 DJ |
388 | size_t shift; |
389 | period = MAX (suffix, needle_len - suffix) + 1; | |
390 | j = 0; | |
391 | while (AVAILABLE (haystack, haystack_len, j, needle_len)) | |
f434ba03 PA |
392 | { |
393 | /* Check the last byte first; if it does not match, then | |
394 | shift to the next possible match location. */ | |
395 | shift = shift_table[CANON_ELEMENT (haystack[j + needle_len - 1])]; | |
396 | if (0 < shift) | |
397 | { | |
398 | j += shift; | |
399 | continue; | |
400 | } | |
401 | /* Scan for matches in right half. The last byte has | |
402 | already been matched, by virtue of the shift table. */ | |
403 | i = suffix; | |
404 | while (i < needle_len - 1 && (CANON_ELEMENT (needle[i]) | |
405 | == CANON_ELEMENT (haystack[i + j]))) | |
406 | ++i; | |
407 | if (needle_len - 1 <= i) | |
408 | { | |
409 | /* Scan for matches in left half. */ | |
410 | i = suffix - 1; | |
411 | while (i != SIZE_MAX && (CANON_ELEMENT (needle[i]) | |
412 | == CANON_ELEMENT (haystack[i + j]))) | |
413 | --i; | |
414 | if (i == SIZE_MAX) | |
415 | return (RETURN_TYPE) (haystack + j); | |
416 | j += period; | |
417 | } | |
418 | else | |
419 | j += i - suffix + 1; | |
420 | } | |
f6ea5628 DJ |
421 | } |
422 | return NULL; | |
423 | } | |
424 | ||
425 | #undef AVAILABLE | |
426 | #undef CANON_ELEMENT | |
427 | #undef CMP_FUNC | |
428 | #undef MAX | |
429 | #undef RETURN_TYPE |