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
|
/* Find near-matches for strings.
Copyright (C) 2015-2016 Free Software Foundation, Inc.
This file is part of GCC.
GCC 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.
GCC 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 GCC; see the file COPYING3. If not see
<http://www.gnu.org/licenses/>. */
#include "config.h"
#include "system.h"
#include "coretypes.h"
#include "tm.h"
#include "tree.h"
#include "spellcheck.h"
/* The Levenshtein distance is an "edit-distance": the minimal
number of one-character insertions, removals or substitutions
that are needed to change one string into another.
This implementation uses the Wagner-Fischer algorithm. */
edit_distance_t
levenshtein_distance (const char *s, int len_s,
const char *t, int len_t)
{
const bool debug = false;
if (debug)
{
printf ("s: \"%s\" (len_s=%i)\n", s, len_s);
printf ("t: \"%s\" (len_t=%i)\n", t, len_t);
}
if (len_s == 0)
return len_t;
if (len_t == 0)
return len_s;
/* We effectively build a matrix where each (i, j) contains the
Levenshtein distance between the prefix strings s[0:j]
and t[0:i].
Rather than actually build an (len_t + 1) * (len_s + 1) matrix,
we simply keep track of the last row, v0 and a new row, v1,
which avoids an (len_t + 1) * (len_s + 1) allocation and memory accesses
in favor of two (len_s + 1) allocations. These could potentially be
statically-allocated if we impose a maximum length on the
strings of interest. */
edit_distance_t *v0 = new edit_distance_t[len_s + 1];
edit_distance_t *v1 = new edit_distance_t[len_s + 1];
/* The first row is for the case of an empty target string, which
we can reach by deleting every character in the source string. */
for (int i = 0; i < len_s + 1; i++)
v0[i] = i;
/* Build successive rows. */
for (int i = 0; i < len_t; i++)
{
if (debug)
{
printf ("i:%i v0 = ", i);
for (int j = 0; j < len_s + 1; j++)
printf ("%i ", v0[j]);
printf ("\n");
}
/* The initial column is for the case of an empty source string; we
can reach prefixes of the target string of length i
by inserting i characters. */
v1[0] = i + 1;
/* Build the rest of the row by considering neighbours to
the north, west and northwest. */
for (int j = 0; j < len_s; j++)
{
edit_distance_t cost = (s[j] == t[i] ? 0 : 1);
edit_distance_t deletion = v1[j] + 1;
edit_distance_t insertion = v0[j + 1] + 1;
edit_distance_t substitution = v0[j] + cost;
edit_distance_t cheapest = MIN (deletion, insertion);
cheapest = MIN (cheapest, substitution);
v1[j + 1] = cheapest;
}
/* Prepare to move on to next row. */
for (int j = 0; j < len_s + 1; j++)
v0[j] = v1[j];
}
if (debug)
{
printf ("final v1 = ");
for (int j = 0; j < len_s + 1; j++)
printf ("%i ", v1[j]);
printf ("\n");
}
edit_distance_t result = v1[len_s];
delete[] v0;
delete[] v1;
return result;
}
/* Calculate Levenshtein distance between two nil-terminated strings. */
edit_distance_t
levenshtein_distance (const char *s, const char *t)
{
return levenshtein_distance (s, strlen (s), t, strlen (t));
}
|
