lib/ts_kmp.c

   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/config.h>
  34 #include <linux/module.h>
  35 #include <linux/types.h>
  36 #include <linux/string.h>
  37 #include <linux/textsearch.h>
  38
  39 struct ts_kmp
  40 {
  41         u8 *            pattern;
  42         unsigned int    pattern_len;
  43         unsigned int    prefix_tbl[0];
  44 };
  45
  46 static unsigned int kmp_find(struct ts_config *conf, struct ts_state *state)
  47 {
  48         struct ts_kmp *kmp = ts_config_priv(conf);
  49         unsigned int i, q = 0, text_len, consumed = state->offset;
  50         const u8 *text;
  51
  52         for (;;) {
  53                 text_len = conf->get_next_block(consumed, &text, conf, state);
  54
  55                 if (unlikely(text_len == 0))
  56                         break;
  57
  58                 for (i = 0; i < text_len; i++) {
  59                         while (q > 0 && kmp->pattern[q] != text[i])
  60                                 q = kmp->prefix_tbl[q - 1];
  61                         if (kmp->pattern[q] == text[i])
  62                                 q++;
  63                         if (unlikely(q == kmp->pattern_len)) {
  64                                 state->offset = consumed + i + 1;
  65                                 return state->offset - kmp->pattern_len;
  66                         }
  67                 }
  68
  69                 consumed += text_len;
  70         }
  71
  72         return UINT_MAX;
  73 }
  74
  75 static inline void compute_prefix_tbl(const u8 *pattern, unsigned int len,
  76                                       unsigned int *prefix_tbl)
  77 {
  78         unsigned int k, q;
  79
  80         for (k = 0, q = 1; q < len; q++) {
  81                 while (k > 0 && pattern[k] != pattern[q])
  82                         k = prefix_tbl[k-1];
  83                 if (pattern[k] == pattern[q])
  84                         k++;
  85                 prefix_tbl[q] = k;
  86         }
  87 }
  88
  89 static struct ts_config *kmp_init(const void *pattern, unsigned int len,
  90                                   gfp_t gfp_mask)
  91 {
  92         struct ts_config *conf;
  93         struct ts_kmp *kmp;
  94         unsigned int prefix_tbl_len = len * sizeof(unsigned int);
  95         size_t priv_size = sizeof(*kmp) + len + prefix_tbl_len;
  96
  97         conf = alloc_ts_config(priv_size, gfp_mask);
  98         if (IS_ERR(conf))
  99                 return conf;
 100
 101         kmp = ts_config_priv(conf);
 102         kmp->pattern_len = len;
 103         compute_prefix_tbl(pattern, len, kmp->prefix_tbl);
 104         kmp->pattern = (u8 *) kmp->prefix_tbl + prefix_tbl_len;
 105         memcpy(kmp->pattern, pattern, len);
 106
 107         return conf;
 108 }
 109
 110 static void *kmp_get_pattern(struct ts_config *conf)
 111 {
 112         struct ts_kmp *kmp = ts_config_priv(conf);
 113         return kmp->pattern;
 114 }
 115
 116 static unsigned int kmp_get_pattern_len(struct ts_config *conf)
 117 {
 118         struct ts_kmp *kmp = ts_config_priv(conf);
 119         return kmp->pattern_len;
 120 }
 121
 122 static struct ts_ops kmp_ops = {
 123         .name             = "kmp",
 124         .find             = kmp_find,
 125         .init             = kmp_init,
 126         .get_pattern      = kmp_get_pattern,
 127         .get_pattern_len  = kmp_get_pattern_len,
 128         .owner            = THIS_MODULE,
 129         .list             = LIST_HEAD_INIT(kmp_ops.list)
 130 };
 131
 132 static int __init init_kmp(void)
 133 {
 134         return textsearch_register(&kmp_ops);
 135 }
 136
 137 static void __exit exit_kmp(void)
 138 {
 139         textsearch_unregister(&kmp_ops);
 140 }
 141
 142 MODULE_LICENSE("GPL");
 143
 144 module_init(init_kmp);
 145 module_exit(exit_kmp);