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/*

 * lib/ts_bm.c          Boyer-Moore 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:     Pablo Neira Ayuso <pablo@eurodev.net>

 *

 * ==========================================================================

 *

 *   Implements Boyer-Moore string matching algorithm:

 *

 *   [1] A Fast String Searching Algorithm, R.S. Boyer and Moore.

 *       Communications of the Association for Computing Machinery,

 *       20(10), 1977, pp. 762-772.

 *       http://www.cs.utexas.edu/users/moore/publications/fstrpos.pdf

 *

 *   [2] Handbook of Exact String Matching Algorithms, Thierry Lecroq, 2004

 *       http://www-igm.univ-mlv.fr/~lecroq/string/string.pdf

 *

 *   Note: Since Boyer-Moore (BM) performs searches for matchings from right

 *   to left, it's still possible that a matching could be spread over

 *   multiple blocks, in that case this algorithm won't find any coincidence.

 *

 *   If you're willing to ensure that such thing won't ever happen, use the

 *   Knuth-Pratt-Morris (KMP) implementation instead. In conclusion, choose

 *   the proper string search algorithm depending on your setting.

 *

 *   Say you're using the textsearch infrastructure for filtering, NIDS or

 *   any similar security focused purpose, then go KMP. Otherwise, if you

 *   really care about performance, say you're classifying packets to apply

 *   Quality of Service (QoS) policies, and you don't mind about possible

 *   matchings spread over multiple fragments, then go BM.

 */

#include <linux/kernel.h>

#include <linux/module.h>

#include <linux/types.h>

#include <linux/string.h>

#include <linux/textsearch.h>

/* Alphabet size, use ASCII */

#define ASIZE 256

#if 0

#define DEBUGP printk

#else

#define DEBUGP(args, format...)

#endif

struct ts_bm

{

        u8 *            pattern;

        unsigned int    patlen;

        unsigned int    bad_shift[ASIZE];

        unsigned int    good_shift[0];

};

static unsigned int bm_find(struct ts_config *conf, struct ts_state *state)

{

        struct ts_bm *bm = ts_config_priv(conf);

        unsigned int i, text_len, consumed = state->offset;

        const u8 *text;

        int shift = bm->patlen, bs;

        for (;;) {

                text_len = conf->get_next_block(consumed, &text, conf, state);

                if (unlikely(text_len == 0))

                        break;

                while (shift < text_len) {

                        DEBUGP("Searching in position %d (%c)\n",

                                shift, text[shift]);

                        for (i = 0; i < bm->patlen; i++)

                             if (text[shift-i] != bm->pattern[bm->patlen-1-i])

                                     goto next;

                        /* London calling... */

                        DEBUGP("found!\n");

                        return consumed += (shift-(bm->patlen-1));

next:                   bs = bm->bad_shift[text[shift-i]];

                        /* Now jumping to... */

                        shift = max_t(int, shift-i+bs, shift+bm->good_shift[i]);

                }

                consumed += text_len;

        }

        return UINT_MAX;

}

static int subpattern(u8 *pattern, int i, int j, int g)

{

        int x = i+g-1, y = j+g-1, ret = 0;

        while(pattern[x--] == pattern[y--]) {

                if (y < 0) {

                        ret = 1;

                        break;

                }

                if (--g == 0) {

                        ret = pattern[i-1] != pattern[j-1];

                        break;

                }

        }

        return ret;

}

static void compute_prefix_tbl(struct ts_bm *bm)

{

        int i, j, g;

        for (i = 0; i < ASIZE; i++)

                bm->bad_shift[i] = bm->patlen;

        for (i = 0; i < bm->patlen - 1; i++)

                bm->bad_shift[bm->pattern[i]] = bm->patlen - 1 - i;

        /* Compute the good shift array, used to match reocurrences

         * of a subpattern */

        bm->good_shift[0] = 1;

        for (i = 1; i < bm->patlen; i++)

                bm->good_shift[i] = bm->patlen;

        for (i = bm->patlen-1, g = 1; i > 0; g++, i--) {

                for (j = i-1; j >= 1-g ; j--)

                        if (subpattern(bm->pattern, i, j, g)) {

                                bm->good_shift[g] = bm->patlen-j-g;

                                break;

                        }

        }

}

static struct ts_config *bm_init(const void *pattern, unsigned int len,

                                 gfp_t gfp_mask)

{

        struct ts_config *conf;

        struct ts_bm *bm;

        unsigned int prefix_tbl_len = len * sizeof(unsigned int);

        size_t priv_size = sizeof(*bm) + len + prefix_tbl_len;

        conf = alloc_ts_config(priv_size, gfp_mask);

        if (IS_ERR(conf))

                return conf;

        bm = ts_config_priv(conf);

        bm->patlen = len;

        bm->pattern = (u8 *) bm->good_shift + prefix_tbl_len;

        memcpy(bm->pattern, pattern, len);

        compute_prefix_tbl(bm);

        return conf;

}

static void *bm_get_pattern(struct ts_config *conf)

{

        struct ts_bm *bm = ts_config_priv(conf);

        return bm->pattern;

}

static unsigned int bm_get_pattern_len(struct ts_config *conf)

{

        struct ts_bm *bm = ts_config_priv(conf);

        return bm->patlen;

}

static struct ts_ops bm_ops = {

        .name             = "bm",

        .find             = bm_find,

        .init             = bm_init,

        .get_pattern      = bm_get_pattern,

        .get_pattern_len  = bm_get_pattern_len,

        .owner            = THIS_MODULE,

        .list             = LIST_HEAD_INIT(bm_ops.list)

};

static int __init init_bm(void)

{

        return textsearch_register(&bm_ops);

}

static void __exit exit_bm(void)

{

        textsearch_unregister(&bm_ops);

}

MODULE_LICENSE("GPL");

module_init(init_bm);

module_exit(exit_bm);