LXR linux/lib/ts

   1/*
   2 * lib/ts_kmp.c         Knuth-Morris-Pratt text search implementation
   3 *
   4 *              This program is free software; you can redistribute it and/or
   5 *              modify it under the terms of the GNU General Public License
   6 *              as published by the Free Software Foundation; either version
   7 *              2 of the License, or (at your option) any later version.
   8 *
   9 * Authors:     Thomas Graf <tgraf@suug.ch>
  10 *
  11 * ==========================================================================
  12 * 
  13 *   Implements a linear-time string-matching algorithm due to Knuth,
  14 *   Morris, and Pratt [1]. Their algorithm avoids the explicit
  15 *   computation of the transition function DELTA altogether. Its
  16 *   matching time is O(n), for n being length(text), using just an
  17 *   auxiliary function PI[1..m], for m being length(pattern),
  18 *   precomputed from the pattern in time O(m). The array PI allows
  19 *   the transition function DELTA to be computed efficiently
  20 *   "on the fly" as needed. Roughly speaking, for any state
  21 *   "q" = 0,1,...,m and any character "a" in SIGMA, the value
  22 *   PI["q"] contains the information that is independent of "a" and
  23 *   is needed to compute DELTA("q", "a") [2]. Since the array PI
  24 *   has only m entries, whereas DELTA has O(m|SIGMA|) entries, we
  25 *   save a factor of |SIGMA| in the preprocessing time by computing
  26 *   PI rather than DELTA.
  27 *
  28 *   [1] Cormen, Leiserson, Rivest, Stein
  29 *       Introdcution to Algorithms, 2nd Edition, MIT Press
  30 *   [2] See finite automation theory
  31 */
  32
  33#include <linux/module.h>
  34#include <linux/types.h>
  35#include <linux/string.h>
  36#include <linux/textsearch.h>
  37
  38struct ts_kmp
  39{
  40        u8 *            pattern;
  41        unsigned int    pattern_len;
  42        unsigned int    prefix_tbl[0];
  43};
  44
  45static unsigned int kmp_find(struct ts_config *conf, struct ts_state *state)
  46{
  47        struct ts_kmp *kmp = ts_config_priv(conf);
  48        unsigned int i, q = 0, text_len, consumed = state->offset;
  49        const u8 *text;
  50
  51        for (;;) {
  52                text_len = conf->get_next_block(consumed, &text, conf, state);
  53
  54                if (unlikely(text_len == 0))
  55                        break;
  56
  57                for (i = 0; i < text_len; i++) {
  58                        while (q > 0 && kmp->pattern[q] != text[i])
  59                                q = kmp->prefix_tbl[q - 1];
  60                        if (kmp->pattern[q] == text[i])
  61                                q++;
  62                        if (unlikely(q == kmp->pattern_len)) {
  63                                state->offset = consumed + i + 1;
  64                                return state->offset - kmp->pattern_len;
  65                        }
  66                }
  67
  68                consumed += text_len;
  69        }
  70
  71        return UINT_MAX;
  72}
  73
  74static inline void compute_prefix_tbl(const u8 *pattern, unsigned int len,
  75                                      unsigned int *prefix_tbl)
  76{
  77        unsigned int k, q;
  78
  79        for (k = 0, q = 1; q < len; q++) {
  80                while (k > 0 && pattern[k] != pattern[q])
  81                        k = prefix_tbl[k-1];
  82                if (pattern[k] == pattern[q])
  83                        k++;
  84                prefix_tbl[q] = k;
  85        }
  86}
  87
  88static struct ts_config *kmp_init(const void *pattern, unsigned int len,
  89                                  gfp_t gfp_mask)
  90{
  91        struct ts_config *conf;
  92        struct ts_kmp *kmp;
  93        unsigned int prefix_tbl_len = len * sizeof(unsigned int);
  94        size_t priv_size = sizeof(*kmp) + len + prefix_tbl_len;
  95
  96        conf = alloc_ts_config(priv_size, gfp_mask);
  97        if (IS_ERR(conf))
  98                return conf;
  99
 100        kmp = ts_config_priv(conf);
 101        kmp->pattern_len = len;
 102        compute_prefix_tbl(pattern, len, kmp->prefix_tbl);
 103        kmp->pattern = (u8 *) kmp->prefix_tbl + prefix_tbl_len;
 104        memcpy(kmp->pattern, pattern, len);
 105
 106        return conf;
 107}
 108
 109static void *kmp_get_pattern(struct ts_config *conf)
 110{
 111        struct ts_kmp *kmp = ts_config_priv(conf);
 112        return kmp->pattern;
 113}
 114
 115static unsigned int kmp_get_pattern_len(struct ts_config *conf)
 116{
 117        struct ts_kmp *kmp = ts_config_priv(conf);
 118        return kmp->pattern_len;
 119}
 120
 121static struct ts_ops kmp_ops = {
 122        .name             = "kmp",
 123        .find             = kmp_find,
 124        .init             = kmp_init,
 125        .get_pattern      = kmp_get_pattern,
 126        .get_pattern_len  = kmp_get_pattern_len,
 127        .owner            = THIS_MODULE,
 128        .list             = LIST_HEAD_INIT(kmp_ops.list)
 129};
 130
 131static int __init init_kmp(void)
 132{
 133        return textsearch_register(&kmp_ops);
 134}
 135
 136static void __exit exit_kmp(void)
 137{
 138        textsearch_unregister(&kmp_ops);
 139}
 140
 141MODULE_LICENSE("GPL");
 142
 143module_init(init_kmp);
 144module_exit(exit_kmp);
 145