projects/linux·blob master · refs

168 lines · 4.6 KiB

1// SPDX-License-Identifier: GPL-2.0-or-later
2/*
3 * lib/ts_kmp.c Knuth-Morris-Pratt text search implementation
4 *
5 * Authors: Thomas Graf <[email protected]>
6 *
7 * ==========================================================================
8 *
9 * Implements a linear-time string-matching algorithm due to Knuth,
10 * Morris, and Pratt [1]. Their algorithm avoids the explicit
11 * computation of the transition function DELTA altogether. Its
12 * matching time is O(n), for n being length(text), using just an
13 * auxiliary function PI[1..m], for m being length(pattern),
14 * precomputed from the pattern in time O(m). The array PI allows
15 * the transition function DELTA to be computed efficiently
16 * "on the fly" as needed. Roughly speaking, for any state
17 * "q" = 0,1,...,m and any character "a" in SIGMA, the value
18 * PI["q"] contains the information that is independent of "a" and
19 * is needed to compute DELTA("q", "a") [2]. Since the array PI
20 * has only m entries, whereas DELTA has O(m|SIGMA|) entries, we
21 * save a factor of |SIGMA| in the preprocessing time by computing
22 * PI rather than DELTA.
23 *
24 * [1] Cormen, Leiserson, Rivest, Stein
25 * Introdcution to Algorithms, 2nd Edition, MIT Press
26 * [2] See finite automaton theory
27 */
28
29#include <linux/module.h>
30#include <linux/types.h>
31#include <linux/string.h>
32#include <linux/ctype.h>
33#include <linux/textsearch.h>
34
35struct ts_kmp
36{
37 u8 * pattern;
38 unsigned int pattern_len;
39 unsigned int prefix_tbl[];
40};
41
42static unsigned int kmp_find(struct ts_config *conf, struct ts_state *state)
43{
44 struct ts_kmp *kmp = ts_config_priv(conf);
45 unsigned int i, q = 0, text_len, consumed = state->offset;
46 const u8 *text;
47 const int icase = conf->flags & TS_IGNORECASE;
48
49 for (;;) {
50 text_len = conf->get_next_block(consumed, &text, conf, state);
51
52 if (unlikely(text_len == 0))
53 break;
54
55 for (i = 0; i < text_len; i++) {
56 while (q > 0 && kmp->pattern[q]
57 != (icase ? toupper(text[i]) : text[i]))
58 q = kmp->prefix_tbl[q - 1];
59 if (kmp->pattern[q]
60 == (icase ? toupper(text[i]) : 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
74static inline void compute_prefix_tbl(const u8 *pattern, unsigned int len,
75 unsigned int *prefix_tbl, int flags)
76{
77 unsigned int k, q;
78 const u8 icase = flags & TS_IGNORECASE;
79
80 for (k = 0, q = 1; q < len; q++) {
81 while (k > 0 && (icase ? toupper(pattern[k]) : pattern[k])
82 != (icase ? toupper(pattern[q]) : pattern[q]))
83 k = prefix_tbl[k-1];
84 if ((icase ? toupper(pattern[k]) : pattern[k])
85 == (icase ? toupper(pattern[q]) : pattern[q]))
86 k++;
87 prefix_tbl[q] = k;
88 }
89}
90
91static struct ts_config *kmp_init(const void *pattern, unsigned int len,
92 gfp_t gfp_mask, int flags)
93{
94 struct ts_config *conf;
95 struct ts_kmp *kmp;
96 int i;
97 unsigned int prefix_tbl_len;
98 size_t priv_size;
99
100 /* Zero-length patterns would make kmp_find() read beyond kmp->pattern. */
101 if (unlikely(!len))
102 return ERR_PTR(-EINVAL);
103
104 /*
105 * kmp->pattern is stored immediately after the prefix_tbl[] table.
106 * Reject lengths that would wrap while sizing either region.
107 */
108 if (unlikely(check_mul_overflow(len, sizeof(*kmp->prefix_tbl),
109 &prefix_tbl_len) ||
110 check_add_overflow(sizeof(*kmp), (size_t)len, &priv_size) ||
111 check_add_overflow(priv_size, prefix_tbl_len, &priv_size)))
112 return ERR_PTR(-EINVAL);
113
114 conf = alloc_ts_config(priv_size, gfp_mask);
115 if (IS_ERR(conf))
116 return conf;
117
118 conf->flags = flags;
119 kmp = ts_config_priv(conf);
120 kmp->pattern_len = len;
121 compute_prefix_tbl(pattern, len, kmp->prefix_tbl, flags);
122 kmp->pattern = (u8 *) kmp->prefix_tbl + prefix_tbl_len;
123 if (flags & TS_IGNORECASE)
124 for (i = 0; i < len; i++)
125 kmp->pattern[i] = toupper(((u8 *)pattern)[i]);
126 else
127 memcpy(kmp->pattern, pattern, len);
128
129 return conf;
130}
131
132static void *kmp_get_pattern(struct ts_config *conf)
133{
134 struct ts_kmp *kmp = ts_config_priv(conf);
135 return kmp->pattern;
136}
137
138static unsigned int kmp_get_pattern_len(struct ts_config *conf)
139{
140 struct ts_kmp *kmp = ts_config_priv(conf);
141 return kmp->pattern_len;
142}
143
144static struct ts_ops kmp_ops = {
145 .name = "kmp",
146 .find = kmp_find,
147 .init = kmp_init,
148 .get_pattern = kmp_get_pattern,
149 .get_pattern_len = kmp_get_pattern_len,
150 .owner = THIS_MODULE,
151 .list = LIST_HEAD_INIT(kmp_ops.list)
152};
153
154static int __init init_kmp(void)
155{
156 return textsearch_register(&kmp_ops);
157}
158
159static void __exit exit_kmp(void)
160{
161 textsearch_unregister(&kmp_ops);
162}
163
164MODULE_DESCRIPTION("Knuth-Morris-Pratt text search implementation");
165MODULE_LICENSE("GPL");
166
167module_init(init_kmp);
168module_exit(exit_kmp);