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 <tgraf@suug.ch>
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
35 struct ts_kmp
36 {
37 u8 * pattern;
38 unsigned int pattern_len;
39 unsigned int prefix_tbl[];
40 };
41
42 static 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
74 static 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
91 static 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 = len * sizeof(unsigned int);
98 size_t priv_size = sizeof(*kmp) + len + prefix_tbl_len;
99
100 conf = alloc_ts_config(priv_size, gfp_mask);
101 if (IS_ERR(conf))
102 return conf;
103
104 conf->flags = flags;
105 kmp = ts_config_priv(conf);
106 kmp->pattern_len = len;
107 compute_prefix_tbl(pattern, len, kmp->prefix_tbl, flags);
108 kmp->pattern = (u8 *) kmp->prefix_tbl + prefix_tbl_len;
109 if (flags & TS_IGNORECASE)
110 for (i = 0; i < len; i++)
111 kmp->pattern[i] = toupper(((u8 *)pattern)[i]);
112 else
113 memcpy(kmp->pattern, pattern, len);
114
115 return conf;
116 }
117
118 static void *kmp_get_pattern(struct ts_config *conf)
119 {
120 struct ts_kmp *kmp = ts_config_priv(conf);
121 return kmp->pattern;
122 }
123
124 static unsigned int kmp_get_pattern_len(struct ts_config *conf)
125 {
126 struct ts_kmp *kmp = ts_config_priv(conf);
127 return kmp->pattern_len;
128 }
129
130 static struct ts_ops kmp_ops = {
131 .name = "kmp",
132 .find = kmp_find,
133 .init = kmp_init,
134 .get_pattern = kmp_get_pattern,
135 .get_pattern_len = kmp_get_pattern_len,
136 .owner = THIS_MODULE,
137 .list = LIST_HEAD_INIT(kmp_ops.list)
138 };
139
140 static int __init init_kmp(void)
141 {
142 return textsearch_register(&kmp_ops);
143 }
144
145 static void __exit exit_kmp(void)
146 {
147 textsearch_unregister(&kmp_ops);
148 }
149
150 MODULE_DESCRIPTION("Knuth-Morris-Pratt text search implementation");
151 MODULE_LICENSE("GPL");
152
153 module_init(init_kmp);
154 module_exit(exit_kmp);
155
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