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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 <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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