xref: /openbmc/linux/lib/ts_kmp.c (revision d5cb9783536a41df9f9cba5b0a1d78047ed787f7)
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/config.h>
34 #include <linux/module.h>
35 #include <linux/types.h>
36 #include <linux/string.h>
37 #include <linux/textsearch.h>
38 
39 struct ts_kmp
40 {
41 	u8 *		pattern;
42 	unsigned int	pattern_len;
43 	unsigned int 	prefix_tbl[0];
44 };
45 
46 static 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 
52 	for (;;) {
53 		text_len = conf->get_next_block(consumed, &text, conf, state);
54 
55 		if (unlikely(text_len == 0))
56 			break;
57 
58 		for (i = 0; i < text_len; i++) {
59 			while (q > 0 && kmp->pattern[q] != text[i])
60 				q = kmp->prefix_tbl[q - 1];
61 			if (kmp->pattern[q] == text[i])
62 				q++;
63 			if (unlikely(q == kmp->pattern_len)) {
64 				state->offset = consumed + i + 1;
65 				return state->offset - kmp->pattern_len;
66 			}
67 		}
68 
69 		consumed += text_len;
70 	}
71 
72 	return UINT_MAX;
73 }
74 
75 static inline void compute_prefix_tbl(const u8 *pattern, unsigned int len,
76 				      unsigned int *prefix_tbl)
77 {
78 	unsigned int k, q;
79 
80 	for (k = 0, q = 1; q < len; q++) {
81 		while (k > 0 && pattern[k] != pattern[q])
82 			k = prefix_tbl[k-1];
83 		if (pattern[k] == pattern[q])
84 			k++;
85 		prefix_tbl[q] = k;
86 	}
87 }
88 
89 static struct ts_config *kmp_init(const void *pattern, unsigned int len,
90 				  gfp_t gfp_mask)
91 {
92 	struct ts_config *conf;
93 	struct ts_kmp *kmp;
94 	unsigned int prefix_tbl_len = len * sizeof(unsigned int);
95 	size_t priv_size = sizeof(*kmp) + len + prefix_tbl_len;
96 
97 	conf = alloc_ts_config(priv_size, gfp_mask);
98 	if (IS_ERR(conf))
99 		return conf;
100 
101 	kmp = ts_config_priv(conf);
102 	kmp->pattern_len = len;
103 	compute_prefix_tbl(pattern, len, kmp->prefix_tbl);
104 	kmp->pattern = (u8 *) kmp->prefix_tbl + prefix_tbl_len;
105 	memcpy(kmp->pattern, pattern, len);
106 
107 	return conf;
108 }
109 
110 static void *kmp_get_pattern(struct ts_config *conf)
111 {
112 	struct ts_kmp *kmp = ts_config_priv(conf);
113 	return kmp->pattern;
114 }
115 
116 static unsigned int kmp_get_pattern_len(struct ts_config *conf)
117 {
118 	struct ts_kmp *kmp = ts_config_priv(conf);
119 	return kmp->pattern_len;
120 }
121 
122 static struct ts_ops kmp_ops = {
123 	.name		  = "kmp",
124 	.find		  = kmp_find,
125 	.init		  = kmp_init,
126 	.get_pattern	  = kmp_get_pattern,
127 	.get_pattern_len  = kmp_get_pattern_len,
128 	.owner		  = THIS_MODULE,
129 	.list		  = LIST_HEAD_INIT(kmp_ops.list)
130 };
131 
132 static int __init init_kmp(void)
133 {
134 	return textsearch_register(&kmp_ops);
135 }
136 
137 static void __exit exit_kmp(void)
138 {
139 	textsearch_unregister(&kmp_ops);
140 }
141 
142 MODULE_LICENSE("GPL");
143 
144 module_init(init_kmp);
145 module_exit(exit_kmp);
146