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