xref: /openbmc/linux/lib/ts_kmp.c (revision e868d61272caa648214046a096e5a6bfc068dc8c)
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/textsearch.h>
37 
38 struct ts_kmp
39 {
40 	u8 *		pattern;
41 	unsigned int	pattern_len;
42 	unsigned int 	prefix_tbl[0];
43 };
44 
45 static unsigned int kmp_find(struct ts_config *conf, struct ts_state *state)
46 {
47 	struct ts_kmp *kmp = ts_config_priv(conf);
48 	unsigned int i, q = 0, text_len, consumed = state->offset;
49 	const u8 *text;
50 
51 	for (;;) {
52 		text_len = conf->get_next_block(consumed, &text, conf, state);
53 
54 		if (unlikely(text_len == 0))
55 			break;
56 
57 		for (i = 0; i < text_len; i++) {
58 			while (q > 0 && kmp->pattern[q] != text[i])
59 				q = kmp->prefix_tbl[q - 1];
60 			if (kmp->pattern[q] == text[i])
61 				q++;
62 			if (unlikely(q == kmp->pattern_len)) {
63 				state->offset = consumed + i + 1;
64 				return state->offset - kmp->pattern_len;
65 			}
66 		}
67 
68 		consumed += text_len;
69 	}
70 
71 	return UINT_MAX;
72 }
73 
74 static inline void compute_prefix_tbl(const u8 *pattern, unsigned int len,
75 				      unsigned int *prefix_tbl)
76 {
77 	unsigned int k, q;
78 
79 	for (k = 0, q = 1; q < len; q++) {
80 		while (k > 0 && pattern[k] != pattern[q])
81 			k = prefix_tbl[k-1];
82 		if (pattern[k] == pattern[q])
83 			k++;
84 		prefix_tbl[q] = k;
85 	}
86 }
87 
88 static struct ts_config *kmp_init(const void *pattern, unsigned int len,
89 				  gfp_t gfp_mask)
90 {
91 	struct ts_config *conf;
92 	struct ts_kmp *kmp;
93 	unsigned int prefix_tbl_len = len * sizeof(unsigned int);
94 	size_t priv_size = sizeof(*kmp) + len + prefix_tbl_len;
95 
96 	conf = alloc_ts_config(priv_size, gfp_mask);
97 	if (IS_ERR(conf))
98 		return conf;
99 
100 	kmp = ts_config_priv(conf);
101 	kmp->pattern_len = len;
102 	compute_prefix_tbl(pattern, len, kmp->prefix_tbl);
103 	kmp->pattern = (u8 *) kmp->prefix_tbl + prefix_tbl_len;
104 	memcpy(kmp->pattern, pattern, len);
105 
106 	return conf;
107 }
108 
109 static void *kmp_get_pattern(struct ts_config *conf)
110 {
111 	struct ts_kmp *kmp = ts_config_priv(conf);
112 	return kmp->pattern;
113 }
114 
115 static unsigned int kmp_get_pattern_len(struct ts_config *conf)
116 {
117 	struct ts_kmp *kmp = ts_config_priv(conf);
118 	return kmp->pattern_len;
119 }
120 
121 static struct ts_ops kmp_ops = {
122 	.name		  = "kmp",
123 	.find		  = kmp_find,
124 	.init		  = kmp_init,
125 	.get_pattern	  = kmp_get_pattern,
126 	.get_pattern_len  = kmp_get_pattern_len,
127 	.owner		  = THIS_MODULE,
128 	.list		  = LIST_HEAD_INIT(kmp_ops.list)
129 };
130 
131 static int __init init_kmp(void)
132 {
133 	return textsearch_register(&kmp_ops);
134 }
135 
136 static void __exit exit_kmp(void)
137 {
138 	textsearch_unregister(&kmp_ops);
139 }
140 
141 MODULE_LICENSE("GPL");
142 
143 module_init(init_kmp);
144 module_exit(exit_kmp);
145