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/config.h>
#include <linux/module.h>
#include <linux/types.h>
#include <linux/string.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;

	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] != text[i])
				q = kmp->prefix_tbl[q - 1];
			if (kmp->pattern[q] == 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)
{
	unsigned int k, q;

	for (k = 0, q = 1; q < len; q++) {
		while (k > 0 && pattern[k] != pattern[q])
			k = prefix_tbl[k-1];
		if (pattern[k] == pattern[q])
			k++;
		prefix_tbl[q] = k;
	}
}

static struct ts_config *kmp_init(const void *pattern, unsigned int len,
				  int gfp_mask)
{
	struct ts_config *conf;
	struct ts_kmp *kmp;
	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;

	kmp = ts_config_priv(conf);
	kmp->pattern_len = len;
	compute_prefix_tbl(pattern, len, kmp->prefix_tbl);
	kmp->pattern = (u8 *) kmp->prefix_tbl + prefix_tbl_len;
	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);