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Linux/lib/ts_kmp.c

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  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_LICENSE("GPL");
151 
152 module_init(init_kmp);
153 module_exit(exit_kmp);
154 

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