1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
|
/* Substring search in a NUL terminated string of UNIT elements,
using the Knuth-Morris-Pratt algorithm.
Copyright (C) 2005-2020 Free Software Foundation, Inc.
Written by Bruno Haible <bruno@clisp.org>, 2005.
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 3, or (at your option)
any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, see <https://www.gnu.org/licenses/>. */
/* Before including this file, you need to define:
UNIT The element type of the needle and haystack.
CANON_ELEMENT(c) A macro that canonicalizes an element right after
it has been fetched from needle or haystack.
The argument is of type UNIT; the result must be
of type UNIT as well. */
/* Knuth-Morris-Pratt algorithm.
See https://en.wikipedia.org/wiki/Knuth-Morris-Pratt_algorithm
HAYSTACK is the NUL terminated string in which to search for.
NEEDLE is the string to search for in HAYSTACK, consisting of NEEDLE_LEN
units.
Return a boolean indicating success:
Return true and set *RESULTP if the search was completed.
Return false if it was aborted because not enough memory was available. */
static bool
knuth_morris_pratt (const UNIT *haystack,
const UNIT *needle, size_t needle_len,
const UNIT **resultp)
{
size_t m = needle_len;
/* Allocate the table. */
size_t *table = (size_t *) nmalloca (m, sizeof (size_t));
if (table == NULL)
return false;
/* Fill the table.
For 0 < i < m:
0 < table[i] <= i is defined such that
forall 0 < x < table[i]: needle[x..i-1] != needle[0..i-1-x],
and table[i] is as large as possible with this property.
This implies:
1) For 0 < i < m:
If table[i] < i,
needle[table[i]..i-1] = needle[0..i-1-table[i]].
2) For 0 < i < m:
rhaystack[0..i-1] == needle[0..i-1]
and exists h, i <= h < m: rhaystack[h] != needle[h]
implies
forall 0 <= x < table[i]: rhaystack[x..x+m-1] != needle[0..m-1].
table[0] remains uninitialized. */
{
size_t i, j;
/* i = 1: Nothing to verify for x = 0. */
table[1] = 1;
j = 0;
for (i = 2; i < m; i++)
{
/* Here: j = i-1 - table[i-1].
The inequality needle[x..i-1] != needle[0..i-1-x] is known to hold
for x < table[i-1], by induction.
Furthermore, if j>0: needle[i-1-j..i-2] = needle[0..j-1]. */
UNIT b = CANON_ELEMENT (needle[i - 1]);
for (;;)
{
/* Invariants: The inequality needle[x..i-1] != needle[0..i-1-x]
is known to hold for x < i-1-j.
Furthermore, if j>0: needle[i-1-j..i-2] = needle[0..j-1]. */
if (b == CANON_ELEMENT (needle[j]))
{
/* Set table[i] := i-1-j. */
table[i] = i - ++j;
break;
}
/* The inequality needle[x..i-1] != needle[0..i-1-x] also holds
for x = i-1-j, because
needle[i-1] != needle[j] = needle[i-1-x]. */
if (j == 0)
{
/* The inequality holds for all possible x. */
table[i] = i;
break;
}
/* The inequality needle[x..i-1] != needle[0..i-1-x] also holds
for i-1-j < x < i-1-j+table[j], because for these x:
needle[x..i-2]
= needle[x-(i-1-j)..j-1]
!= needle[0..j-1-(x-(i-1-j))] (by definition of table[j])
= needle[0..i-2-x],
hence needle[x..i-1] != needle[0..i-1-x].
Furthermore
needle[i-1-j+table[j]..i-2]
= needle[table[j]..j-1]
= needle[0..j-1-table[j]] (by definition of table[j]). */
j = j - table[j];
}
/* Here: j = i - table[i]. */
}
}
/* Search, using the table to accelerate the processing. */
{
size_t j;
const UNIT *rhaystack;
const UNIT *phaystack;
*resultp = NULL;
j = 0;
rhaystack = haystack;
phaystack = haystack;
/* Invariant: phaystack = rhaystack + j. */
while (*phaystack != 0)
if (CANON_ELEMENT (needle[j]) == CANON_ELEMENT (*phaystack))
{
j++;
phaystack++;
if (j == m)
{
/* The entire needle has been found. */
*resultp = rhaystack;
break;
}
}
else if (j > 0)
{
/* Found a match of needle[0..j-1], mismatch at needle[j]. */
rhaystack += table[j];
j -= table[j];
}
else
{
/* Found a mismatch at needle[0] already. */
rhaystack++;
phaystack++;
}
}
freea (table);
return true;
}
#undef CANON_ELEMENT
|
