ts_kmp.c

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/*
 * 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 <[email protected]>
 *
 * ==========================================================================
 * 
 *   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);