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