145 lines
3.8 KiB
C
145 lines
3.8 KiB
C
/*
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* lib/ts_kmp.c Knuth-Morris-Pratt text search implementation
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*
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* This program is free software; you can redistribute it and/or
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* modify it under the terms of the GNU General Public License
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* as published by the Free Software Foundation; either version
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* 2 of the License, or (at your option) any later version.
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*
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* Authors: Thomas Graf <tgraf@suug.ch>
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*
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* ==========================================================================
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*
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* Implements a linear-time string-matching algorithm due to Knuth,
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* Morris, and Pratt [1]. Their algorithm avoids the explicit
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* computation of the transition function DELTA altogether. Its
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* matching time is O(n), for n being length(text), using just an
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* auxiliary function PI[1..m], for m being length(pattern),
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* precomputed from the pattern in time O(m). The array PI allows
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* the transition function DELTA to be computed efficiently
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* "on the fly" as needed. Roughly speaking, for any state
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* "q" = 0,1,...,m and any character "a" in SIGMA, the value
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* PI["q"] contains the information that is independent of "a" and
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* is needed to compute DELTA("q", "a") [2]. Since the array PI
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* has only m entries, whereas DELTA has O(m|SIGMA|) entries, we
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* save a factor of |SIGMA| in the preprocessing time by computing
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* PI rather than DELTA.
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*
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* [1] Cormen, Leiserson, Rivest, Stein
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* Introdcution to Algorithms, 2nd Edition, MIT Press
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* [2] See finite automation theory
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*/
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#include <linux/module.h>
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#include <linux/types.h>
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#include <linux/string.h>
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#include <linux/textsearch.h>
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struct ts_kmp
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{
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u8 * pattern;
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unsigned int pattern_len;
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unsigned int prefix_tbl[0];
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};
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static unsigned int kmp_find(struct ts_config *conf, struct ts_state *state)
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{
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struct ts_kmp *kmp = ts_config_priv(conf);
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unsigned int i, q = 0, text_len, consumed = state->offset;
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const u8 *text;
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for (;;) {
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text_len = conf->get_next_block(consumed, &text, conf, state);
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if (unlikely(text_len == 0))
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break;
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for (i = 0; i < text_len; i++) {
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while (q > 0 && kmp->pattern[q] != text[i])
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q = kmp->prefix_tbl[q - 1];
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if (kmp->pattern[q] == text[i])
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q++;
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if (unlikely(q == kmp->pattern_len)) {
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state->offset = consumed + i + 1;
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return state->offset - kmp->pattern_len;
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}
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}
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consumed += text_len;
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}
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return UINT_MAX;
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}
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static inline void compute_prefix_tbl(const u8 *pattern, unsigned int len,
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unsigned int *prefix_tbl)
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{
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unsigned int k, q;
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for (k = 0, q = 1; q < len; q++) {
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while (k > 0 && pattern[k] != pattern[q])
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k = prefix_tbl[k-1];
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if (pattern[k] == pattern[q])
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k++;
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prefix_tbl[q] = k;
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}
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}
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static struct ts_config *kmp_init(const void *pattern, unsigned int len,
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gfp_t gfp_mask)
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{
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struct ts_config *conf;
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struct ts_kmp *kmp;
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unsigned int prefix_tbl_len = len * sizeof(unsigned int);
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size_t priv_size = sizeof(*kmp) + len + prefix_tbl_len;
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conf = alloc_ts_config(priv_size, gfp_mask);
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if (IS_ERR(conf))
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return conf;
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kmp = ts_config_priv(conf);
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kmp->pattern_len = len;
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compute_prefix_tbl(pattern, len, kmp->prefix_tbl);
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kmp->pattern = (u8 *) kmp->prefix_tbl + prefix_tbl_len;
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memcpy(kmp->pattern, pattern, len);
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return conf;
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}
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static void *kmp_get_pattern(struct ts_config *conf)
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{
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struct ts_kmp *kmp = ts_config_priv(conf);
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return kmp->pattern;
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}
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static unsigned int kmp_get_pattern_len(struct ts_config *conf)
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{
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struct ts_kmp *kmp = ts_config_priv(conf);
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return kmp->pattern_len;
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}
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static struct ts_ops kmp_ops = {
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.name = "kmp",
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.find = kmp_find,
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.init = kmp_init,
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.get_pattern = kmp_get_pattern,
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.get_pattern_len = kmp_get_pattern_len,
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.owner = THIS_MODULE,
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.list = LIST_HEAD_INIT(kmp_ops.list)
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};
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static int __init init_kmp(void)
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{
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return textsearch_register(&kmp_ops);
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}
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static void __exit exit_kmp(void)
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{
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textsearch_unregister(&kmp_ops);
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}
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MODULE_LICENSE("GPL");
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module_init(init_kmp);
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module_exit(exit_kmp);
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