lib/ts_kmp.c

   1 // SPDX-License-Identifier: GPL-2.0-or-later
   2 /*
   3  * lib/ts_kmp.c         Knuth-Morris-Pratt text search implementation
   4  *
   5  * Authors:     Thomas Graf <tgraf@suug.ch>
   6  *
   7  * ==========================================================================
   8  *
   9  *   Implements a linear-time string-matching algorithm due to Knuth,
  10  *   Morris, and Pratt [1]. Their algorithm avoids the explicit
  11  *   computation of the transition function DELTA altogether. Its
  12  *   matching time is O(n), for n being length(text), using just an
  13  *   auxiliary function PI[1..m], for m being length(pattern),
  14  *   precomputed from the pattern in time O(m). The array PI allows
  15  *   the transition function DELTA to be computed efficiently
  16  *   "on the fly" as needed. Roughly speaking, for any state
  17  *   "q" = 0,1,...,m and any character "a" in SIGMA, the value
  18  *   PI["q"] contains the information that is independent of "a" and
  19  *   is needed to compute DELTA("q", "a") [2]. Since the array PI
  20  *   has only m entries, whereas DELTA has O(m|SIGMA|) entries, we
  21  *   save a factor of |SIGMA| in the preprocessing time by computing
  22  *   PI rather than DELTA.
  23  *
  24  *   [1] Cormen, Leiserson, Rivest, Stein
  25  *       Introdcution to Algorithms, 2nd Edition, MIT Press
  26  *   [2] See finite automaton theory
  27  */
  28
  29 #include <linux/module.h>
  30 #include <linux/types.h>
  31 #include <linux/string.h>
  32 #include <linux/ctype.h>
  33 #include <linux/textsearch.h>
  34
  35 struct ts_kmp
  36 {
  37         u8 *            pattern;
  38         unsigned int    pattern_len;
  39         unsigned int    prefix_tbl[];
  40 };
  41
  42 static unsigned int kmp_find(struct ts_config *conf, struct ts_state *state)
  43 {
  44         struct ts_kmp *kmp = ts_config_priv(conf);
  45         unsigned int i, q = 0, text_len, consumed = state->offset;
  46         const u8 *text;
  47         const int icase = conf->flags & TS_IGNORECASE;
  48
  49         for (;;) {
  50                 text_len = conf->get_next_block(consumed, &text, conf, state);
  51
  52                 if (unlikely(text_len == 0))
  53                         break;
  54
  55                 for (i = 0; i < text_len; i++) {
  56                         while (q > 0 && kmp->pattern[q]
  57                             != (icase ? toupper(text[i]) : text[i]))
  58                                 q = kmp->prefix_tbl[q - 1];
  59                         if (kmp->pattern[q]
  60                             == (icase ? toupper(text[i]) : 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, int flags)
  76 {
  77         unsigned int k, q;
  78         const u8 icase = flags & TS_IGNORECASE;
  79
  80         for (k = 0, q = 1; q < len; q++) {
  81                 while (k > 0 && (icase ? toupper(pattern[k]) : pattern[k])
  82                     != (icase ? toupper(pattern[q]) : pattern[q]))
  83                         k = prefix_tbl[k-1];
  84                 if ((icase ? toupper(pattern[k]) : pattern[k])
  85                     == (icase ? toupper(pattern[q]) : pattern[q]))
  86                         k++;
  87                 prefix_tbl[q] = k;
  88         }
  89 }
  90
  91 static struct ts_config *kmp_init(const void *pattern, unsigned int len,
  92                                   gfp_t gfp_mask, int flags)
  93 {
  94         struct ts_config *conf;
  95         struct ts_kmp *kmp;
  96         int i;
  97         unsigned int prefix_tbl_len = len * sizeof(unsigned int);
  98         size_t priv_size = sizeof(*kmp) + len + prefix_tbl_len;
  99
 100         conf = alloc_ts_config(priv_size, gfp_mask);
 101         if (IS_ERR(conf))
 102                 return conf;
 103
 104         conf->flags = flags;
 105         kmp = ts_config_priv(conf);
 106         kmp->pattern_len = len;
 107         compute_prefix_tbl(pattern, len, kmp->prefix_tbl, flags);
 108         kmp->pattern = (u8 *) kmp->prefix_tbl + prefix_tbl_len;
 109         if (flags & TS_IGNORECASE)
 110                 for (i = 0; i < len; i++)
 111                         kmp->pattern[i] = toupper(((u8 *)pattern)[i]);
 112         else
 113                 memcpy(kmp->pattern, pattern, len);
 114
 115         return conf;
 116 }
 117
 118 static void *kmp_get_pattern(struct ts_config *conf)
 119 {
 120         struct ts_kmp *kmp = ts_config_priv(conf);
 121         return kmp->pattern;
 122 }
 123
 124 static unsigned int kmp_get_pattern_len(struct ts_config *conf)
 125 {
 126         struct ts_kmp *kmp = ts_config_priv(conf);
 127         return kmp->pattern_len;
 128 }
 129
 130 static struct ts_ops kmp_ops = {
 131         .name             = "kmp",
 132         .find             = kmp_find,
 133         .init             = kmp_init,
 134         .get_pattern      = kmp_get_pattern,
 135         .get_pattern_len  = kmp_get_pattern_len,
 136         .owner            = THIS_MODULE,
 137         .list             = LIST_HEAD_INIT(kmp_ops.list)
 138 };
 139
 140 static int __init init_kmp(void)
 141 {
 142         return textsearch_register(&kmp_ops);
 143 }
 144
 145 static void __exit exit_kmp(void)
 146 {
 147         textsearch_unregister(&kmp_ops);
 148 }
 149
 150 MODULE_LICENSE("GPL");
 151
 152 module_init(init_kmp);
 153 module_exit(exit_kmp);