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/ctype.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         const int icase = conf->flags & TS_IGNORECASE;
  52
  53         for (;;) {
  54                 text_len = conf->get_next_block(consumed, &text, conf, state);
  55
  56                 if (unlikely(text_len == 0))
  57                         break;
  58
  59                 for (i = 0; i < text_len; i++) {
  60                         while (q > 0 && kmp->pattern[q]
  61                             != (icase ? toupper(text[i]) : text[i]))
  62                                 q = kmp->prefix_tbl[q - 1];
  63                         if (kmp->pattern[q]
  64                             == (icase ? toupper(text[i]) : text[i]))
  65                                 q++;
  66                         if (unlikely(q == kmp->pattern_len)) {
  67                                 state->offset = consumed + i + 1;
  68                                 return state->offset - kmp->pattern_len;
  69                         }
  70                 }
  71
  72                 consumed += text_len;
  73         }
  74
  75         return UINT_MAX;
  76 }
  77
  78 static inline void compute_prefix_tbl(const u8 *pattern, unsigned int len,
  79                                       unsigned int *prefix_tbl, int flags)
  80 {
  81         unsigned int k, q;
  82         const u8 icase = flags & TS_IGNORECASE;
  83
  84         for (k = 0, q = 1; q < len; q++) {
  85                 while (k > 0 && (icase ? toupper(pattern[k]) : pattern[k])
  86                     != (icase ? toupper(pattern[q]) : pattern[q]))
  87                         k = prefix_tbl[k-1];
  88                 if ((icase ? toupper(pattern[k]) : pattern[k])
  89                     == (icase ? toupper(pattern[q]) : pattern[q]))
  90                         k++;
  91                 prefix_tbl[q] = k;
  92         }
  93 }
  94
  95 static struct ts_config *kmp_init(const void *pattern, unsigned int len,
  96                                   gfp_t gfp_mask, int flags)
  97 {
  98         struct ts_config *conf;
  99         struct ts_kmp *kmp;
 100         int i;
 101         unsigned int prefix_tbl_len = len * sizeof(unsigned int);
 102         size_t priv_size = sizeof(*kmp) + len + prefix_tbl_len;
 103
 104         conf = alloc_ts_config(priv_size, gfp_mask);
 105         if (IS_ERR(conf))
 106                 return conf;
 107
 108         conf->flags = flags;
 109         kmp = ts_config_priv(conf);
 110         kmp->pattern_len = len;
 111         compute_prefix_tbl(pattern, len, kmp->prefix_tbl, flags);
 112         kmp->pattern = (u8 *) kmp->prefix_tbl + prefix_tbl_len;
 113         if (flags & TS_IGNORECASE)
 114                 for (i = 0; i < len; i++)
 115                         kmp->pattern[i] = toupper(((u8 *)pattern)[i]);
 116         else
 117                 memcpy(kmp->pattern, pattern, len);
 118
 119         return conf;
 120 }
 121
 122 static void *kmp_get_pattern(struct ts_config *conf)
 123 {
 124         struct ts_kmp *kmp = ts_config_priv(conf);
 125         return kmp->pattern;
 126 }
 127
 128 static unsigned int kmp_get_pattern_len(struct ts_config *conf)
 129 {
 130         struct ts_kmp *kmp = ts_config_priv(conf);
 131         return kmp->pattern_len;
 132 }
 133
 134 static struct ts_ops kmp_ops = {
 135         .name             = "kmp",
 136         .find             = kmp_find,
 137         .init             = kmp_init,
 138         .get_pattern      = kmp_get_pattern,
 139         .get_pattern_len  = kmp_get_pattern_len,
 140         .owner            = THIS_MODULE,
 141         .list             = LIST_HEAD_INIT(kmp_ops.list)
 142 };
 143
 144 static int __init init_kmp(void)
 145 {
 146         return textsearch_register(&kmp_ops);
 147 }
 148
 149 static void __exit exit_kmp(void)
 150 {
 151         textsearch_unregister(&kmp_ops);
 152 }
 153
 154 MODULE_LICENSE("GPL");
 155
 156 module_init(init_kmp);
 157 module_exit(exit_kmp);