linux/lib/ts_kmp.c
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   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
  39struct ts_kmp
  40{
  41        u8 *            pattern;
  42        unsigned int    pattern_len;
  43        unsigned int    prefix_tbl[0];
  44};
  45
  46static 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
  78static 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
  95static 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
 122static void *kmp_get_pattern(struct ts_config *conf)
 123{
 124        struct ts_kmp *kmp = ts_config_priv(conf);
 125        return kmp->pattern;
 126}
 127
 128static 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
 134static 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
 144static int __init init_kmp(void)
 145{
 146        return textsearch_register(&kmp_ops);
 147}
 148
 149static void __exit exit_kmp(void)
 150{
 151        textsearch_unregister(&kmp_ops);
 152}
 153
 154MODULE_LICENSE("GPL");
 155
 156module_init(init_kmp);
 157module_exit(exit_kmp);
 158
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