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Diffstat (limited to 'lib/ts_kmp.c')
| -rw-r--r-- | lib/ts_kmp.c | 145 |
1 files changed, 145 insertions, 0 deletions
diff --git a/lib/ts_kmp.c b/lib/ts_kmp.c new file mode 100644 index 000000000000..73266b975585 --- /dev/null +++ b/lib/ts_kmp.c @@ -0,0 +1,145 @@ +/* + * 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/config.h> +#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, + int 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); |