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/module.h>
  34 #include <linux/types.h>
  35 #include <linux/string.h>
  36 #include <linux/textsearch.h>
  37
  38 struct ts_kmp
  39 {
  40         u8 *            pattern;
  41         unsigned int    pattern_len;
  42         unsigned int    prefix_tbl[0];
  43 };
  44
  45 static 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
  74 static 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
  88 static 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
 109 static void *kmp_get_pattern(struct ts_config *conf)
 110 {
 111         struct ts_kmp *kmp = ts_config_priv(conf);
 112         return kmp->pattern;
 113 }
 114
 115 static 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
 121 static 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
 131 static int __init init_kmp(void)
 132 {
 133         return textsearch_register(&kmp_ops);
 134 }
 135
 136 static void __exit exit_kmp(void)
 137 {
 138         textsearch_unregister(&kmp_ops);
 139 }
 140
 141 MODULE_LICENSE("GPL");
 142
 143 module_init(init_kmp);
 144 module_exit(exit_kmp);