1 /* Functions to make fuzzy comparisons between strings
2 Copyright (C) 1988-1989, 1992-1993, 1995, 2001-2003, 2006, 2008
3 Free Software Foundation, Inc.
5 This program is free software: you can redistribute it and/or modify
6 it under the terms of the GNU General Public License as published by
7 the Free Software Foundation; either version 3 of the License, or
8 (at your option) any later version.
10 This program is distributed in the hope that it will be useful,
11 but WITHOUT ANY WARRANTY; without even the implied warranty of
12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
13 GNU General Public License for more details.
15 You should have received a copy of the GNU General Public License
16 along with this program. If not, see <http://www.gnu.org/licenses/>.
19 Derived from GNU diff 2.7, analyze.c et al.
21 The basic idea is to consider two vectors as similar if, when
22 transforming the first vector into the second vector through a
23 sequence of edits (inserts and deletes of one element each),
24 this sequence is short - or equivalently, if the ordered list
25 of elements that are untouched by these edits is long. For a
26 good introduction to the subject, read about the "Levenshtein
27 distance" in Wikipedia.
29 The basic algorithm is described in:
30 "An O(ND) Difference Algorithm and its Variations", Eugene Myers,
31 Algorithmica Vol. 1 No. 2, 1986, pp. 251-266;
32 see especially section 4.2, which describes the variation used below.
34 The basic algorithm was independently discovered as described in:
35 "Algorithms for Approximate String Matching", E. Ukkonen,
36 Information and Control Vol. 64, 1985, pp. 100-118.
38 Unless the 'find_minimal' flag is set, this code uses the TOO_EXPENSIVE
39 heuristic, by Paul Eggert, to limit the cost to O(N**1.5 log N)
40 at the price of producing suboptimal output for large inputs with
54 #include "glthread/lock.h"
55 #include "glthread/tls.h"
60 # define uintptr_t unsigned long
65 #define EQUAL(x,y) ((x) == (y))
67 #define EXTRA_CONTEXT_FIELDS \
68 /* The number of edits beyond which the computation can be aborted. */ \
69 int edit_count_limit; \
70 /* The number of edits (= number of elements inserted, plus the number of \
71 elements deleted), temporarily minus edit_count_limit. */ \
73 #define NOTE_DELETE(ctxt, xoff) ctxt->edit_count++
74 #define NOTE_INSERT(ctxt, yoff) ctxt->edit_count++
75 #define EARLY_ABORT(ctxt) ctxt->edit_count > 0
76 /* We don't need USE_HEURISTIC, since it is unlikely in typical uses of
81 /* Because fstrcmp is typically called multiple times, attempt to minimize
82 the number of memory allocations performed. Thus, let a call reuse the
83 memory already allocated by the previous call, if it is sufficient.
84 To make it multithread-safe, without need for a lock that protects the
85 already allocated memory, store the allocated memory per thread. Free
86 it only when the thread exits. */
88 static gl_tls_key_t buffer_key; /* TLS key for a 'int *' */
89 static gl_tls_key_t bufmax_key; /* TLS key for a 'size_t' */
94 gl_tls_key_init (buffer_key, free);
95 gl_tls_key_init (bufmax_key, NULL);
96 /* The per-thread initial values are NULL and 0, respectively. */
99 /* Ensure that keys_init is called once only. */
100 gl_once_define(static, keys_init_once)
104 fstrcmp_bounded (const char *string1, const char *string2, double lower_bound)
107 int xvec_length = strlen (string1);
108 int yvec_length = strlen (string2);
115 /* short-circuit obvious comparisons */
116 if (xvec_length == 0 || yvec_length == 0)
117 return (xvec_length == 0 && yvec_length == 0 ? 1.0 : 0.0);
121 /* Compute a quick upper bound.
122 Each edit is an insertion or deletion of an element, hence modifies
123 the length of the sequence by at most 1.
124 Therefore, when starting from a sequence X and ending at a sequence Y,
125 with N edits, | yvec_length - xvec_length | <= N. (Proof by
127 So, at the end, we will have
128 edit_count >= | xvec_length - yvec_length |.
131 = (xvec_length + yvec_length - edit_count)
132 / (xvec_length + yvec_length)
133 <= (xvec_length + yvec_length - | yvec_length - xvec_length |)
134 / (xvec_length + yvec_length)
135 = 2 * min (xvec_length, yvec_length) / (xvec_length + yvec_length).
137 volatile double upper_bound =
138 (double) (2 * MIN (xvec_length, yvec_length))
139 / (xvec_length + yvec_length);
141 if (upper_bound < lower_bound)
142 /* Return an arbitrary value < LOWER_BOUND. */
146 /* set the info for each string. */
150 /* Set TOO_EXPENSIVE to be approximate square root of input size,
151 bounded below by 256. */
152 ctxt.too_expensive = 1;
153 for (i = xvec_length + yvec_length;
156 ctxt.too_expensive <<= 1;
157 if (ctxt.too_expensive < 256)
158 ctxt.too_expensive = 256;
160 /* Allocate memory for fdiag and bdiag from a thread-local pool. */
161 fdiag_len = xvec_length + yvec_length + 3;
162 gl_once (keys_init_once, keys_init);
163 buffer = (int *) gl_tls_get (buffer_key);
164 bufmax = (size_t) (uintptr_t) gl_tls_get (bufmax_key);
165 if (fdiag_len > bufmax)
167 /* Need more memory. */
169 if (fdiag_len > bufmax)
171 /* Calling xrealloc would be a waste: buffer's contents does not need
175 buffer = (int *) xnmalloc (bufmax, 2 * sizeof (int));
176 gl_tls_set (buffer_key, buffer);
177 gl_tls_set (bufmax_key, (void *) (uintptr_t) bufmax);
179 ctxt.fdiag = buffer + yvec_length + 1;
180 ctxt.bdiag = ctxt.fdiag + fdiag_len;
182 /* The edit_count is only ever increased. The computation can be aborted
184 (xvec_length + yvec_length - edit_count) / (xvec_length + yvec_length)
187 edit_count > (xvec_length + yvec_length) * (1 - lower_bound)
189 edit_count > floor((xvec_length + yvec_length) * (1 - lower_bound)).
190 We need to add an epsilon inside the floor(...) argument, to neutralize
192 ctxt.edit_count_limit =
194 ? (int) ((xvec_length + yvec_length) * (1.0 - lower_bound + 0.000001))
197 /* Now do the main comparison algorithm */
198 ctxt.edit_count = - ctxt.edit_count_limit;
199 if (compareseq (0, xvec_length, 0, yvec_length, 0, &ctxt))
200 /* The edit_count passed the limit. Hence the result would be
201 < lower_bound. We can return any value < lower_bound instead. */
203 ctxt.edit_count += ctxt.edit_count_limit;
206 ((number of chars in common) / (average length of the strings)).
208 = xvec_length - (number of calls to NOTE_DELETE)
209 = yvec_length - (number of calls to NOTE_INSERT)
210 = 1/2 * (xvec_length + yvec_length - (number of edits)).
211 This is admittedly biased towards finding that the strings are
212 similar, however it does produce meaningful results. */
213 return ((double) (xvec_length + yvec_length - ctxt.edit_count)
214 / (xvec_length + yvec_length));