1 /* Functions to make fuzzy comparisons between strings
2 Copyright (C) 1988-1989, 1992-1993, 1995, 2001-2003, 2006, 2008-2020 Free
3 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 <https://www.gnu.org/licenses/>. */
32 #include "glthread/lock.h"
33 #include "glthread/tls.h"
39 #define EQUAL(x,y) ((x) == (y))
40 #define OFFSET ptrdiff_t
41 #define EXTRA_CONTEXT_FIELDS \
42 /* The number of edits beyond which the computation can be aborted. */ \
43 ptrdiff_t edit_count_limit; \
44 /* The number of edits (= number of elements inserted, plus the number of \
45 elements deleted), temporarily minus edit_count_limit. */ \
47 #define NOTE_DELETE(ctxt, xoff) ctxt->edit_count++
48 #define NOTE_INSERT(ctxt, yoff) ctxt->edit_count++
49 #define EARLY_ABORT(ctxt) ctxt->edit_count > 0
50 /* We don't need USE_HEURISTIC, since it is unlikely in typical uses of
55 /* Because fstrcmp is typically called multiple times, attempt to minimize
56 the number of memory allocations performed. Thus, let a call reuse the
57 memory already allocated by the previous call, if it is sufficient.
58 To make it multithread-safe, without need for a lock that protects the
59 already allocated memory, store the allocated memory per thread. Free
60 it only when the thread exits. */
62 static gl_tls_key_t buffer_key
; /* TLS key for a 'ptrdiff_t *' */
63 static gl_tls_key_t bufmax_key
; /* TLS key for a 'uintptr_t' */
68 gl_tls_key_init (buffer_key
, free
);
69 gl_tls_key_init (bufmax_key
, NULL
);
70 /* The per-thread initial values are NULL and 0, respectively. */
73 /* Ensure that keys_init is called once only. */
74 gl_once_define(static, keys_init_once
)
77 fstrcmp_free_resources (void)
81 gl_once (keys_init_once
, keys_init
);
82 buffer
= gl_tls_get (buffer_key
);
85 gl_tls_set (buffer_key
, NULL
);
86 gl_tls_set (bufmax_key
, (void *) (uintptr_t) 0);
92 /* In the code below, branch probabilities were measured by Ralf Wildenhues,
93 by running "msgmerge LL.po coreutils.pot" with msgmerge 0.18 for many
94 values of LL. The probability indicates that the condition evaluates
95 to true; whether that leads to a branch or a non-branch in the code,
96 depends on the compiler's reordering of basic blocks. */
100 fstrcmp_bounded (const char *string1
, const char *string2
, double lower_bound
)
103 size_t xvec_length
= strlen (string1
);
104 size_t yvec_length
= strlen (string2
);
105 size_t length_sum
= xvec_length
+ yvec_length
;
112 /* short-circuit obvious comparisons */
113 if (xvec_length
== 0 || yvec_length
== 0) /* Prob: 1% */
114 return length_sum
== 0;
116 if (! (xvec_length
<= length_sum
117 && length_sum
<= MIN (UINTPTR_MAX
, PTRDIFF_MAX
) - 3))
122 /* Compute a quick upper bound.
123 Each edit is an insertion or deletion of an element, hence modifies
124 the length of the sequence by at most 1.
125 Therefore, when starting from a sequence X and ending at a sequence Y,
126 with N edits, | yvec_length - xvec_length | <= N. (Proof by
128 So, at the end, we will have
129 edit_count >= | xvec_length - yvec_length |.
132 = (xvec_length + yvec_length - edit_count)
133 / (xvec_length + yvec_length)
134 <= (xvec_length + yvec_length - | yvec_length - xvec_length |)
135 / (xvec_length + yvec_length)
136 = 2 * min (xvec_length, yvec_length) / (xvec_length + yvec_length).
138 ptrdiff_t length_min
= MIN (xvec_length
, yvec_length
);
139 volatile double upper_bound
= 2.0 * length_min
/ length_sum
;
141 if (upper_bound
< lower_bound
) /* Prob: 74% */
142 /* Return an arbitrary value < LOWER_BOUND. */
146 /* When X and Y are both small, avoid the overhead of setting up an
147 array of size 256. */
148 if (length_sum
>= 20) /* Prob: 99% */
150 /* Compute a less quick upper bound.
151 Each edit is an insertion or deletion of a character, hence
152 modifies the occurrence count of a character by 1 and leaves the
153 other occurrence counts unchanged.
154 Therefore, when starting from a sequence X and ending at a
155 sequence Y, and denoting the occurrence count of C in X with
156 OCC (X, C), with N edits,
157 sum_C | OCC (X, C) - OCC (Y, C) | <= N.
158 (Proof by induction over N.)
159 So, at the end, we will have
160 edit_count >= sum_C | OCC (X, C) - OCC (Y, C) |,
163 = (xvec_length + yvec_length - edit_count)
164 / (xvec_length + yvec_length)
165 <= (xvec_length + yvec_length - sum_C | OCC(X,C) - OCC(Y,C) |)
166 / (xvec_length + yvec_length).
168 ptrdiff_t occ_diff
[UCHAR_MAX
+ 1]; /* array C -> OCC(X,C) - OCC(Y,C) */
172 /* Determine the occurrence counts in X. */
173 memset (occ_diff
, 0, sizeof (occ_diff
));
174 for (i
= xvec_length
- 1; i
>= 0; i
--)
175 occ_diff
[(unsigned char) string1
[i
]]++;
176 /* Subtract the occurrence counts in Y. */
177 for (i
= yvec_length
- 1; i
>= 0; i
--)
178 occ_diff
[(unsigned char) string2
[i
]]--;
179 /* Sum up the absolute values. */
181 for (i
= 0; i
<= UCHAR_MAX
; i
++)
183 ptrdiff_t d
= occ_diff
[i
];
184 sum
+= (d
>= 0 ? d
: -d
);
188 upper_bound
= 1.0 - dsum
/ length_sum
;
190 if (upper_bound
< lower_bound
) /* Prob: 66% */
191 /* Return an arbitrary value < LOWER_BOUND. */
197 /* set the info for each string. */
201 /* Set TOO_EXPENSIVE to be approximate square root of input size,
202 bounded below by 4096. */
203 ctxt
.too_expensive
= 1;
204 for (i
= xvec_length
+ yvec_length
; i
!= 0; i
>>= 2)
205 ctxt
.too_expensive
<<= 1;
206 if (ctxt
.too_expensive
< 4096)
207 ctxt
.too_expensive
= 4096;
209 /* Allocate memory for fdiag and bdiag from a thread-local pool. */
210 fdiag_len
= length_sum
+ 3;
211 gl_once (keys_init_once
, keys_init
);
212 buffer
= gl_tls_get (buffer_key
);
213 bufmax
= (uintptr_t) gl_tls_get (bufmax_key
);
214 if (fdiag_len
> bufmax
)
216 /* Need more memory. */
218 if (fdiag_len
> bufmax
)
220 /* Calling xrealloc would be a waste: buffer's contents does not need
223 buffer
= xnmalloc (bufmax
, 2 * sizeof *buffer
);
224 gl_tls_set (buffer_key
, buffer
);
225 gl_tls_set (bufmax_key
, (void *) (uintptr_t) bufmax
);
227 ctxt
.fdiag
= buffer
+ yvec_length
+ 1;
228 ctxt
.bdiag
= ctxt
.fdiag
+ fdiag_len
;
230 /* The edit_count is only ever increased. The computation can be aborted
232 (xvec_length + yvec_length - edit_count) / (xvec_length + yvec_length)
235 edit_count > (xvec_length + yvec_length) * (1 - lower_bound)
237 edit_count > floor((xvec_length + yvec_length) * (1 - lower_bound)).
238 We need to add an epsilon inside the floor(...) argument, to neutralize
240 ctxt
.edit_count_limit
=
242 ? (ptrdiff_t) (length_sum
* (1.0 - lower_bound
+ 0.000001))
245 /* Now do the main comparison algorithm */
246 ctxt
.edit_count
= - ctxt
.edit_count_limit
;
247 if (compareseq (0, xvec_length
, 0, yvec_length
, 0, &ctxt
)) /* Prob: 98% */
248 /* The edit_count passed the limit. Hence the result would be
249 < lower_bound. We can return any value < lower_bound instead. */
251 ctxt
.edit_count
+= ctxt
.edit_count_limit
;
254 ((number of chars in common) / (average length of the strings)).
256 = xvec_length - (number of calls to NOTE_DELETE)
257 = yvec_length - (number of calls to NOTE_INSERT)
258 = 1/2 * (xvec_length + yvec_length - (number of edits)).
259 This is admittedly biased towards finding that the strings are
260 similar, however it does produce meaningful results. */
261 return ((double) (xvec_length
+ yvec_length
- ctxt
.edit_count
)
262 / (xvec_length
+ yvec_length
));