1/*
2 * lib/ts_kmp.c		Knuth-Morris-Pratt text search implementation
3 *
4 *		This program is free software; you can redistribute it and/or
5 *		modify it under the terms of the GNU General Public License
6 *		as published by the Free Software Foundation; either version
7 *		2 of the License, or (at your option) any later version.
8 *
9 * Authors:	Thomas Graf <tgraf@suug.ch>
10 *
11 * ==========================================================================
12 *
13 *   Implements a linear-time string-matching algorithm due to Knuth,
14 *   Morris, and Pratt [1]. Their algorithm avoids the explicit
15 *   computation of the transition function DELTA altogether. Its
16 *   matching time is O(n), for n being length(text), using just an
17 *   auxiliary function PI[1..m], for m being length(pattern),
18 *   precomputed from the pattern in time O(m). The array PI allows
19 *   the transition function DELTA to be computed efficiently
20 *   "on the fly" as needed. Roughly speaking, for any state
21 *   "q" = 0,1,...,m and any character "a" in SIGMA, the value
22 *   PI["q"] contains the information that is independent of "a" and
23 *   is needed to compute DELTA("q", "a") [2]. Since the array PI
24 *   has only m entries, whereas DELTA has O(m|SIGMA|) entries, we
25 *   save a factor of |SIGMA| in the preprocessing time by computing
26 *   PI rather than DELTA.
27 *
28 *   [1] Cormen, Leiserson, Rivest, Stein
29 *       Introdcution to Algorithms, 2nd Edition, MIT Press
30 *   [2] See finite automation theory
31 */
32
33#include <linux/module.h>
34#include <linux/types.h>
35#include <linux/string.h>
36#include <linux/ctype.h>
37#include <linux/textsearch.h>
38
39struct ts_kmp
40{
41	u8 *		pattern;
42	unsigned int	pattern_len;
43	unsigned int 	prefix_tbl[0];
44};
45
46static unsigned int kmp_find(struct ts_config *conf, struct ts_state *state)
47{
48	struct ts_kmp *kmp = ts_config_priv(conf);
49	unsigned int i, q = 0, text_len, consumed = state->offset;
50	const u8 *text;
51	const int icase = conf->flags & TS_IGNORECASE;
52
53	for (;;) {
54		text_len = conf->get_next_block(consumed, &text, conf, state);
55
56		if (unlikely(text_len == 0))
57			break;
58
59		for (i = 0; i < text_len; i++) {
60			while (q > 0 && kmp->pattern[q]
61			    != (icase ? toupper(text[i]) : text[i]))
62				q = kmp->prefix_tbl[q - 1];
63			if (kmp->pattern[q]
64			    == (icase ? toupper(text[i]) : text[i]))
65				q++;
66			if (unlikely(q == kmp->pattern_len)) {
67				state->offset = consumed + i + 1;
68				return state->offset - kmp->pattern_len;
69			}
70		}
71
72		consumed += text_len;
73	}
74
75	return UINT_MAX;
76}
77
78static inline void compute_prefix_tbl(const u8 *pattern, unsigned int len,
79				      unsigned int *prefix_tbl, int flags)
80{
81	unsigned int k, q;
82	const u8 icase = flags & TS_IGNORECASE;
83
84	for (k = 0, q = 1; q < len; q++) {
85		while (k > 0 && (icase ? toupper(pattern[k]) : pattern[k])
86		    != (icase ? toupper(pattern[q]) : pattern[q]))
87			k = prefix_tbl[k-1];
88		if ((icase ? toupper(pattern[k]) : pattern[k])
89		    == (icase ? toupper(pattern[q]) : pattern[q]))
90			k++;
91		prefix_tbl[q] = k;
92	}
93}
94
95static struct ts_config *kmp_init(const void *pattern, unsigned int len,
96				  gfp_t gfp_mask, int flags)
97{
98	struct ts_config *conf;
99	struct ts_kmp *kmp;
100	int i;
101	unsigned int prefix_tbl_len = len * sizeof(unsigned int);
102	size_t priv_size = sizeof(*kmp) + len + prefix_tbl_len;
103
104	conf = alloc_ts_config(priv_size, gfp_mask);
105	if (IS_ERR(conf))
106		return conf;
107
108	conf->flags = flags;
109	kmp = ts_config_priv(conf);
110	kmp->pattern_len = len;
111	compute_prefix_tbl(pattern, len, kmp->prefix_tbl, flags);
112	kmp->pattern = (u8 *) kmp->prefix_tbl + prefix_tbl_len;
113	if (flags & TS_IGNORECASE)
114		for (i = 0; i < len; i++)
115			kmp->pattern[i] = toupper(((u8 *)pattern)[i]);
116	else
117		memcpy(kmp->pattern, pattern, len);
118
119	return conf;
120}
121
122static void *kmp_get_pattern(struct ts_config *conf)
123{
124	struct ts_kmp *kmp = ts_config_priv(conf);
125	return kmp->pattern;
126}
127
128static unsigned int kmp_get_pattern_len(struct ts_config *conf)
129{
130	struct ts_kmp *kmp = ts_config_priv(conf);
131	return kmp->pattern_len;
132}
133
134static struct ts_ops kmp_ops = {
135	.name		  = "kmp",
136	.find		  = kmp_find,
137	.init		  = kmp_init,
138	.get_pattern	  = kmp_get_pattern,
139	.get_pattern_len  = kmp_get_pattern_len,
140	.owner		  = THIS_MODULE,
141	.list		  = LIST_HEAD_INIT(kmp_ops.list)
142};
143
144static int __init init_kmp(void)
145{
146	return textsearch_register(&kmp_ops);
147}
148
149static void __exit exit_kmp(void)
150{
151	textsearch_unregister(&kmp_ops);
152}
153
154MODULE_LICENSE("GPL");
155
156module_init(init_kmp);
157module_exit(exit_kmp);
158