the-tree/lib/ts_kmp.c

0001 // SPDX-License-Identifier: GPL-2.0-or-later
0002 /*
0003  * lib/ts_kmp.c     Knuth-Morris-Pratt text search implementation
0004  *
0005  * Authors: Thomas Graf <tgraf@suug.ch>
0006  *
0007  * ==========================================================================
0008  *
0009  *   Implements a linear-time string-matching algorithm due to Knuth,
0010  *   Morris, and Pratt [1]. Their algorithm avoids the explicit
0011  *   computation of the transition function DELTA altogether. Its
0012  *   matching time is O(n), for n being length(text), using just an
0013  *   auxiliary function PI[1..m], for m being length(pattern),
0014  *   precomputed from the pattern in time O(m). The array PI allows
0015  *   the transition function DELTA to be computed efficiently
0016  *   "on the fly" as needed. Roughly speaking, for any state
0017  *   "q" = 0,1,...,m and any character "a" in SIGMA, the value
0018  *   PI["q"] contains the information that is independent of "a" and
0019  *   is needed to compute DELTA("q", "a") [2]. Since the array PI
0020  *   has only m entries, whereas DELTA has O(m|SIGMA|) entries, we
0021  *   save a factor of |SIGMA| in the preprocessing time by computing
0022  *   PI rather than DELTA.
0023  *
0024  *   [1] Cormen, Leiserson, Rivest, Stein
0025  *       Introdcution to Algorithms, 2nd Edition, MIT Press
0026  *   [2] See finite automaton theory
0027  */
0028
0029 #include <linux/module.h>
0030 #include <linux/types.h>
0031 #include <linux/string.h>
0032 #include <linux/ctype.h>
0033 #include <linux/textsearch.h>
0034
0035 struct ts_kmp
0036 {
0037     u8 *        pattern;
0038     unsigned int    pattern_len;
0039     unsigned int    prefix_tbl[];
0040 };
0041
0042 static unsigned int kmp_find(struct ts_config *conf, struct ts_state *state)
0043 {
0044     struct ts_kmp *kmp = ts_config_priv(conf);
0045     unsigned int i, q = 0, text_len, consumed = state->offset;
0046     const u8 *text;
0047     const int icase = conf->flags & TS_IGNORECASE;
0048
0049     for (;;) {
0050         text_len = conf->get_next_block(consumed, &text, conf, state);
0051
0052         if (unlikely(text_len == 0))
0053             break;
0054
0055         for (i = 0; i < text_len; i++) {
0056             while (q > 0 && kmp->pattern[q]
0057                 != (icase ? toupper(text[i]) : text[i]))
0058                 q = kmp->prefix_tbl[q - 1];
0059             if (kmp->pattern[q]
0060                 == (icase ? toupper(text[i]) : text[i]))
0061                 q++;
0062             if (unlikely(q == kmp->pattern_len)) {
0063                 state->offset = consumed + i + 1;
0064                 return state->offset - kmp->pattern_len;
0065             }
0066         }
0067
0068         consumed += text_len;
0069     }
0070
0071     return UINT_MAX;
0072 }
0073
0074 static inline void compute_prefix_tbl(const u8 *pattern, unsigned int len,
0075                       unsigned int *prefix_tbl, int flags)
0076 {
0077     unsigned int k, q;
0078     const u8 icase = flags & TS_IGNORECASE;
0079
0080     for (k = 0, q = 1; q < len; q++) {
0081         while (k > 0 && (icase ? toupper(pattern[k]) : pattern[k])
0082             != (icase ? toupper(pattern[q]) : pattern[q]))
0083             k = prefix_tbl[k-1];
0084         if ((icase ? toupper(pattern[k]) : pattern[k])
0085             == (icase ? toupper(pattern[q]) : pattern[q]))
0086             k++;
0087         prefix_tbl[q] = k;
0088     }
0089 }
0090
0091 static struct ts_config *kmp_init(const void *pattern, unsigned int len,
0092                   gfp_t gfp_mask, int flags)
0093 {
0094     struct ts_config *conf;
0095     struct ts_kmp *kmp;
0096     int i;
0097     unsigned int prefix_tbl_len = len * sizeof(unsigned int);
0098     size_t priv_size = sizeof(*kmp) + len + prefix_tbl_len;
0099
0100     conf = alloc_ts_config(priv_size, gfp_mask);
0101     if (IS_ERR(conf))
0102         return conf;
0103
0104     conf->flags = flags;
0105     kmp = ts_config_priv(conf);
0106     kmp->pattern_len = len;
0107     compute_prefix_tbl(pattern, len, kmp->prefix_tbl, flags);
0108     kmp->pattern = (u8 *) kmp->prefix_tbl + prefix_tbl_len;
0109     if (flags & TS_IGNORECASE)
0110         for (i = 0; i < len; i++)
0111             kmp->pattern[i] = toupper(((u8 *)pattern)[i]);
0112     else
0113         memcpy(kmp->pattern, pattern, len);
0114
0115     return conf;
0116 }
0117
0118 static void *kmp_get_pattern(struct ts_config *conf)
0119 {
0120     struct ts_kmp *kmp = ts_config_priv(conf);
0121     return kmp->pattern;
0122 }
0123
0124 static unsigned int kmp_get_pattern_len(struct ts_config *conf)
0125 {
0126     struct ts_kmp *kmp = ts_config_priv(conf);
0127     return kmp->pattern_len;
0128 }
0129
0130 static struct ts_ops kmp_ops = {
0131     .name         = "kmp",
0132     .find         = kmp_find,
0133     .init         = kmp_init,
0134     .get_pattern      = kmp_get_pattern,
0135     .get_pattern_len  = kmp_get_pattern_len,
0136     .owner        = THIS_MODULE,
0137     .list         = LIST_HEAD_INIT(kmp_ops.list)
0138 };
0139
0140 static int __init init_kmp(void)
0141 {
0142     return textsearch_register(&kmp_ops);
0143 }
0144
0145 static void __exit exit_kmp(void)
0146 {
0147     textsearch_unregister(&kmp_ops);
0148 }
0149
0150 MODULE_LICENSE("GPL");
0151
0152 module_init(init_kmp);
0153 module_exit(exit_kmp);