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
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 = 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
118static void *kmp_get_pattern(struct ts_config *conf)
119{
120	struct ts_kmp *kmp = ts_config_priv(conf);
121	return kmp->pattern;
122}
123
124static 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
130static 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
140static int __init init_kmp(void)
141{
142	return textsearch_register(&kmp_ops);
143}
144
145static void __exit exit_kmp(void)
146{
147	textsearch_unregister(&kmp_ops);
148}
149
150MODULE_LICENSE("GPL");
151
152module_init(init_kmp);
153module_exit(exit_kmp);
154