FreeBSD/Linux Kernel Cross Reference
sys/lib/ts_kmp.c

     /*
      * lib/ts_kmp.c         Knuth-Morris-Pratt text search implementation
      *
      *              This program is free software; you can redistribute it and/or
      *              modify it under the terms of the GNU General Public License
      *              as published by the Free Software Foundation; either version
      *              2 of the License, or (at your option) any later version.
      *
      * Authors:     Thomas Graf <tgraf@suug.ch>
     *
     * ==========================================================================
     * 
     *   Implements a linear-time string-matching algorithm due to Knuth,
     *   Morris, and Pratt [1]. Their algorithm avoids the explicit
     *   computation of the transition function DELTA altogether. Its
     *   matching time is O(n), for n being length(text), using just an
     *   auxiliary function PI[1..m], for m being length(pattern),
     *   precomputed from the pattern in time O(m). The array PI allows
     *   the transition function DELTA to be computed efficiently
     *   "on the fly" as needed. Roughly speaking, for any state
     *   "q" = 0,1,...,m and any character "a" in SIGMA, the value
     *   PI["q"] contains the information that is independent of "a" and
     *   is needed to compute DELTA("q", "a") [2]. Since the array PI
     *   has only m entries, whereas DELTA has O(m|SIGMA|) entries, we
     *   save a factor of |SIGMA| in the preprocessing time by computing
     *   PI rather than DELTA.
     *
     *   [1] Cormen, Leiserson, Rivest, Stein
     *       Introdcution to Algorithms, 2nd Edition, MIT Press
     *   [2] See finite automation theory
     */
    
    #include <linux/module.h>
    #include <linux/types.h>
    #include <linux/string.h>
    #include <linux/ctype.h>
    #include <linux/textsearch.h>
    
    struct ts_kmp
    {
            u8 *            pattern;
            unsigned int    pattern_len;
            unsigned int    prefix_tbl[0];
    };
    
    static unsigned int kmp_find(struct ts_config *conf, struct ts_state *state)
    {
            struct ts_kmp *kmp = ts_config_priv(conf);
            unsigned int i, q = 0, text_len, consumed = state->offset;
            const u8 *text;
            const int icase = conf->flags & TS_IGNORECASE;
    
            for (;;) {
                    text_len = conf->get_next_block(consumed, &text, conf, state);
    
                    if (unlikely(text_len == 0))
                            break;
    
                    for (i = 0; i < text_len; i++) {
                            while (q > 0 && kmp->pattern[q]
                                != (icase ? toupper(text[i]) : text[i]))
                                    q = kmp->prefix_tbl[q - 1];
                            if (kmp->pattern[q]
                                == (icase ? toupper(text[i]) : text[i]))
                                    q++;
                            if (unlikely(q == kmp->pattern_len)) {
                                    state->offset = consumed + i + 1;
                                    return state->offset - kmp->pattern_len;
                            }
                    }
    
                    consumed += text_len;
            }
    
            return UINT_MAX;
    }
    
    static inline void compute_prefix_tbl(const u8 *pattern, unsigned int len,
                                          unsigned int *prefix_tbl, int flags)
    {
            unsigned int k, q;
            const u8 icase = flags & TS_IGNORECASE;
    
            for (k = 0, q = 1; q < len; q++) {
                    while (k > 0 && (icase ? toupper(pattern[k]) : pattern[k])
                        != (icase ? toupper(pattern[q]) : pattern[q]))
                            k = prefix_tbl[k-1];
                    if ((icase ? toupper(pattern[k]) : pattern[k])
                        == (icase ? toupper(pattern[q]) : pattern[q]))
                            k++;
                    prefix_tbl[q] = k;
            }
    }
    
    static struct ts_config *kmp_init(const void *pattern, unsigned int len,
                                      gfp_t gfp_mask, int flags)
    {
            struct ts_config *conf;
            struct ts_kmp *kmp;
           int i;
           unsigned int prefix_tbl_len = len * sizeof(unsigned int);
           size_t priv_size = sizeof(*kmp) + len + prefix_tbl_len;
   
           conf = alloc_ts_config(priv_size, gfp_mask);
           if (IS_ERR(conf))
                   return conf;
   
           conf->flags = flags;
           kmp = ts_config_priv(conf);
           kmp->pattern_len = len;
           compute_prefix_tbl(pattern, len, kmp->prefix_tbl, flags);
           kmp->pattern = (u8 *) kmp->prefix_tbl + prefix_tbl_len;
           if (flags & TS_IGNORECASE)
                   for (i = 0; i < len; i++)
                           kmp->pattern[i] = toupper(((u8 *)pattern)[i]);
           else
                   memcpy(kmp->pattern, pattern, len);
   
           return conf;
   }
   
   static void *kmp_get_pattern(struct ts_config *conf)
   {
           struct ts_kmp *kmp = ts_config_priv(conf);
           return kmp->pattern;
   }
   
   static unsigned int kmp_get_pattern_len(struct ts_config *conf)
   {
           struct ts_kmp *kmp = ts_config_priv(conf);
           return kmp->pattern_len;
   }
   
   static struct ts_ops kmp_ops = {
           .name             = "kmp",
           .find             = kmp_find,
           .init             = kmp_init,
           .get_pattern      = kmp_get_pattern,
           .get_pattern_len  = kmp_get_pattern_len,
           .owner            = THIS_MODULE,
           .list             = LIST_HEAD_INIT(kmp_ops.list)
   };
   
   static int __init init_kmp(void)
   {
           return textsearch_register(&kmp_ops);
   }
   
   static void __exit exit_kmp(void)
   {
           textsearch_unregister(&kmp_ops);
   }
   
   MODULE_LICENSE("GPL");
   
   module_init(init_kmp);
   module_exit(exit_kmp);

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