Mercurial > hg > octave-lyh > gnulib-hg
changeset 7424:e4bf7f5e0140
Fuzzy string comparison.
Taken from GNU gettext.
author | Bruno Haible <bruno@clisp.org> |
---|---|
date | Sat, 07 Oct 2006 15:19:14 +0000 |
parents | 2381239541ff |
children | 4d508d4f6533 |
files | lib/fstrcmp.c |
diffstat | 1 files changed, 670 insertions(+), 0 deletions(-) [+] |
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new file mode 100644 --- /dev/null +++ b/lib/fstrcmp.c @@ -0,0 +1,670 @@ +/* Functions to make fuzzy comparisons between strings + Copyright (C) 1988-1989, 1992-1993, 1995, 2001-2003, 2006 Free Software Foundation, Inc. + + 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 2 of the License, 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, write to the Free Software + Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. + + + Derived from GNU diff 2.7, analyze.c et al. + + The basic idea is to consider two strings as similar if, when + transforming the first string into the second string through a + sequence of edits (inserts and deletes of one character each), + this sequence is short - or equivalently, if the ordered list + of characters that are untouched by these edits is long. For a + good introduction to the subject, read about the "Levenshtein + distance" in Wikipedia. + + The basic algorithm is described in: + "An O(ND) Difference Algorithm and its Variations", Eugene Myers, + Algorithmica Vol. 1 No. 2, 1986, pp. 251-266; + see especially section 4.2, which describes the variation used below. + + The basic algorithm was independently discovered as described in: + "Algorithms for Approximate String Matching", E. Ukkonen, + Information and Control Vol. 64, 1985, pp. 100-118. + + Unless the 'minimal' flag is set, this code uses the TOO_EXPENSIVE + heuristic, by Paul Eggert, to limit the cost to O(N**1.5 log N) + at the price of producing suboptimal output for large inputs with + many differences. + + Modified to work on strings rather than files + by Peter Miller <pmiller@agso.gov.au>, October 1995 */ + +#include <config.h> + +/* Specification. */ +#include "fstrcmp.h" + +#include <string.h> +#include <stdio.h> +#include <stdlib.h> +#include <limits.h> + +#include "lock.h" +#include "tls.h" +#include "xalloc.h" + +#ifndef uintptr_t +# define uintptr_t unsigned long +#endif + + +/* + * Context of comparison operation. + */ +struct context +{ + /* + * Data on one input string being compared. + */ + struct string_data + { + /* The string to be compared. */ + const char *data; + + /* The length of the string to be compared. */ + int data_length; + + /* The number of characters inserted or deleted. */ + int edit_count; + } + string[2]; + + #ifdef MINUS_H_FLAG + + /* This corresponds to the diff -H flag. With this heuristic, for + strings with a constant small density of changes, the algorithm is + linear in the strings size. This is unlikely in typical uses of + fstrcmp, and so is usually compiled out. Besides, there is no + interface to set it true. */ + int heuristic; + + #endif + + /* Vector, indexed by diagonal, containing 1 + the X coordinate of the + point furthest along the given diagonal in the forward search of the + edit matrix. */ + int *fdiag; + + /* Vector, indexed by diagonal, containing the X coordinate of the point + furthest along the given diagonal in the backward search of the edit + matrix. */ + int *bdiag; + + /* Edit scripts longer than this are too expensive to compute. */ + int too_expensive; + + /* Snakes bigger than this are considered `big'. */ + #define SNAKE_LIMIT 20 +}; + +struct partition +{ + /* Midpoints of this partition. */ + int xmid, ymid; + + /* Nonzero if low half will be analyzed minimally. */ + int lo_minimal; + + /* Likewise for high half. */ + int hi_minimal; +}; + + +/* NAME + diag - find diagonal path + + SYNOPSIS + int diag(int xoff, int xlim, int yoff, int ylim, int minimal, + struct partition *part, struct context *ctxt); + + DESCRIPTION + Find the midpoint of the shortest edit script for a specified + portion of the two strings. + + Scan from the beginnings of the strings, and simultaneously from + the ends, doing a breadth-first search through the space of + edit-sequence. When the two searches meet, we have found the + midpoint of the shortest edit sequence. + + If MINIMAL is nonzero, find the minimal edit script regardless + of expense. Otherwise, if the search is too expensive, use + heuristics to stop the search and report a suboptimal answer. + + RETURNS + Set PART->(XMID,YMID) to the midpoint (XMID,YMID). The diagonal + number XMID - YMID equals the number of inserted characters + minus the number of deleted characters (counting only characters + before the midpoint). Return the approximate edit cost; this is + the total number of characters inserted or deleted (counting + only characters before the midpoint), unless a heuristic is used + to terminate the search prematurely. + + Set PART->LEFT_MINIMAL to nonzero iff the minimal edit script + for the left half of the partition is known; similarly for + PART->RIGHT_MINIMAL. + + CAVEAT + This function assumes that the first characters of the specified + portions of the two strings do not match, and likewise that the + last characters do not match. The caller must trim matching + characters from the beginning and end of the portions it is + going to specify. + + If we return the "wrong" partitions, the worst this can do is + cause suboptimal diff output. It cannot cause incorrect diff + output. */ + +static int +diag (int xoff, int xlim, int yoff, int ylim, int minimal, + struct partition *part, struct context *ctxt) +{ + int *const fd = ctxt->fdiag; /* Give the compiler a chance. */ + int *const bd = ctxt->bdiag; /* Additional help for the compiler. */ + const char *const xv = ctxt->string[0].data; /* Still more help for the compiler. */ + const char *const yv = ctxt->string[1].data; /* And more and more . . . */ + const int dmin = xoff - ylim; /* Minimum valid diagonal. */ + const int dmax = xlim - yoff; /* Maximum valid diagonal. */ + const int fmid = xoff - yoff; /* Center diagonal of top-down search. */ + const int bmid = xlim - ylim; /* Center diagonal of bottom-up search. */ + int fmin = fmid; + int fmax = fmid; /* Limits of top-down search. */ + int bmin = bmid; + int bmax = bmid; /* Limits of bottom-up search. */ + int c; /* Cost. */ + int odd = (fmid - bmid) & 1; + + /* + * True if southeast corner is on an odd diagonal with respect + * to the northwest. + */ + fd[fmid] = xoff; + bd[bmid] = xlim; + for (c = 1;; ++c) + { + int d; /* Active diagonal. */ + int big_snake; + + big_snake = 0; + /* Extend the top-down search by an edit step in each diagonal. */ + if (fmin > dmin) + fd[--fmin - 1] = -1; + else + ++fmin; + if (fmax < dmax) + fd[++fmax + 1] = -1; + else + --fmax; + for (d = fmax; d >= fmin; d -= 2) + { + int x; + int y; + int oldx; + int tlo; + int thi; + + tlo = fd[d - 1], + thi = fd[d + 1]; + + if (tlo >= thi) + x = tlo + 1; + else + x = thi; + oldx = x; + y = x - d; + while (x < xlim && y < ylim && xv[x] == yv[y]) + { + ++x; + ++y; + } + if (x - oldx > SNAKE_LIMIT) + big_snake = 1; + fd[d] = x; + if (odd && bmin <= d && d <= bmax && bd[d] <= x) + { + part->xmid = x; + part->ymid = y; + part->lo_minimal = part->hi_minimal = 1; + return 2 * c - 1; + } + } + /* Similarly extend the bottom-up search. */ + if (bmin > dmin) + bd[--bmin - 1] = INT_MAX; + else + ++bmin; + if (bmax < dmax) + bd[++bmax + 1] = INT_MAX; + else + --bmax; + for (d = bmax; d >= bmin; d -= 2) + { + int x; + int y; + int oldx; + int tlo; + int thi; + + tlo = bd[d - 1], + thi = bd[d + 1]; + if (tlo < thi) + x = tlo; + else + x = thi - 1; + oldx = x; + y = x - d; + while (x > xoff && y > yoff && xv[x - 1] == yv[y - 1]) + { + --x; + --y; + } + if (oldx - x > SNAKE_LIMIT) + big_snake = 1; + bd[d] = x; + if (!odd && fmin <= d && d <= fmax && x <= fd[d]) + { + part->xmid = x; + part->ymid = y; + part->lo_minimal = part->hi_minimal = 1; + return 2 * c; + } + } + + if (minimal) + continue; + +#ifdef MINUS_H_FLAG + /* Heuristic: check occasionally for a diagonal that has made lots + of progress compared with the edit distance. If we have any + such, find the one that has made the most progress and return + it as if it had succeeded. + + With this heuristic, for strings with a constant small density + of changes, the algorithm is linear in the strings size. */ + if (c > 200 && big_snake && ctxt->heuristic) + { + int best; + + best = 0; + for (d = fmax; d >= fmin; d -= 2) + { + int dd; + int x; + int y; + int v; + + dd = d - fmid; + x = fd[d]; + y = x - d; + v = (x - xoff) * 2 - dd; + + if (v > 12 * (c + (dd < 0 ? -dd : dd))) + { + if + ( + v > best + && + xoff + SNAKE_LIMIT <= x + && + x < xlim + && + yoff + SNAKE_LIMIT <= y + && + y < ylim + ) + { + /* We have a good enough best diagonal; now insist + that it end with a significant snake. */ + int k; + + for (k = 1; xv[x - k] == yv[y - k]; k++) + { + if (k == SNAKE_LIMIT) + { + best = v; + part->xmid = x; + part->ymid = y; + break; + } + } + } + } + } + if (best > 0) + { + part->lo_minimal = 1; + part->hi_minimal = 0; + return 2 * c - 1; + } + best = 0; + for (d = bmax; d >= bmin; d -= 2) + { + int dd; + int x; + int y; + int v; + + dd = d - bmid; + x = bd[d]; + y = x - d; + v = (xlim - x) * 2 + dd; + + if (v > 12 * (c + (dd < 0 ? -dd : dd))) + { + if (v > best && xoff < x && x <= xlim - SNAKE_LIMIT && + yoff < y && y <= ylim - SNAKE_LIMIT) + { + /* We have a good enough best diagonal; now insist + that it end with a significant snake. */ + int k; + + for (k = 0; xv[x + k] == yv[y + k]; k++) + { + if (k == SNAKE_LIMIT - 1) + { + best = v; + part->xmid = x; + part->ymid = y; + break; + } + } + } + } + } + if (best > 0) + { + part->lo_minimal = 0; + part->hi_minimal = 1; + return 2 * c - 1; + } + } +#endif /* MINUS_H_FLAG */ + + /* Heuristic: if we've gone well beyond the call of duty, give up + and report halfway between our best results so far. */ + if (c >= ctxt->too_expensive) + { + int fxybest; + int fxbest; + int bxybest; + int bxbest; + + /* Pacify `gcc -Wall'. */ + fxbest = 0; + bxbest = 0; + + /* Find forward diagonal that maximizes X + Y. */ + fxybest = -1; + for (d = fmax; d >= fmin; d -= 2) + { + int x; + int y; + + x = fd[d] < xlim ? fd[d] : xlim; + y = x - d; + + if (ylim < y) + { + x = ylim + d; + y = ylim; + } + if (fxybest < x + y) + { + fxybest = x + y; + fxbest = x; + } + } + /* Find backward diagonal that minimizes X + Y. */ + bxybest = INT_MAX; + for (d = bmax; d >= bmin; d -= 2) + { + int x; + int y; + + x = xoff > bd[d] ? xoff : bd[d]; + y = x - d; + + if (y < yoff) + { + x = yoff + d; + y = yoff; + } + if (x + y < bxybest) + { + bxybest = x + y; + bxbest = x; + } + } + /* Use the better of the two diagonals. */ + if ((xlim + ylim) - bxybest < fxybest - (xoff + yoff)) + { + part->xmid = fxbest; + part->ymid = fxybest - fxbest; + part->lo_minimal = 1; + part->hi_minimal = 0; + } + else + { + part->xmid = bxbest; + part->ymid = bxybest - bxbest; + part->lo_minimal = 0; + part->hi_minimal = 1; + } + return 2 * c - 1; + } + } +} + + +/* NAME + compareseq - find edit sequence + + SYNOPSIS + void compareseq(int xoff, int xlim, int yoff, int ylim, int minimal, + struct context *ctxt); + + DESCRIPTION + Compare in detail contiguous subsequences of the two strings + which are known, as a whole, to match each other. + + The subsequence of string 0 is [XOFF, XLIM) and likewise for + string 1. + + Note that XLIM, YLIM are exclusive bounds. All character + numbers are origin-0. + + If MINIMAL is nonzero, find a minimal difference no matter how + expensive it is. */ + +static void +compareseq (int xoff, int xlim, int yoff, int ylim, int minimal, + struct context *ctxt) +{ + const char *const xv = ctxt->string[0].data; /* Help the compiler. */ + const char *const yv = ctxt->string[1].data; + + /* Slide down the bottom initial diagonal. */ + while (xoff < xlim && yoff < ylim && xv[xoff] == yv[yoff]) + { + ++xoff; + ++yoff; + } + + /* Slide up the top initial diagonal. */ + while (xlim > xoff && ylim > yoff && xv[xlim - 1] == yv[ylim - 1]) + { + --xlim; + --ylim; + } + + /* Handle simple cases. */ + if (xoff == xlim) + { + while (yoff < ylim) + { + ctxt->string[1].edit_count++; + ++yoff; + } + } + else if (yoff == ylim) + { + while (xoff < xlim) + { + ctxt->string[0].edit_count++; + ++xoff; + } + } + else + { + int c; + struct partition part; + + /* Find a point of correspondence in the middle of the strings. */ + c = diag (xoff, xlim, yoff, ylim, minimal, &part, ctxt); + if (c == 1) + { +#if 0 + /* This should be impossible, because it implies that one of + the two subsequences is empty, and that case was handled + above without calling `diag'. Let's verify that this is + true. */ + abort (); +#else + /* The two subsequences differ by a single insert or delete; + record it and we are done. */ + if (part.xmid - part.ymid < xoff - yoff) + ctxt->string[1].edit_count++; + else + ctxt->string[0].edit_count++; +#endif + } + else + { + /* Use the partitions to split this problem into subproblems. */ + compareseq (xoff, part.xmid, yoff, part.ymid, part.lo_minimal, ctxt); + compareseq (part.xmid, xlim, part.ymid, ylim, part.hi_minimal, ctxt); + } + } +} + + +/* Because fstrcmp is typically called multiple times, attempt to minimize + the number of memory allocations performed. Thus, let a call reuse the + memory already allocated by the previous call, if it is sufficient. + To make it multithread-safe, without need for a lock that protects the + already allocated memory, store the allocated memory per thread. Free + it only when the thread exits. */ + +static gl_tls_key_t buffer_key; /* TLS key for a 'int *' */ +static gl_tls_key_t bufmax_key; /* TLS key for a 'size_t' */ + +static void +keys_init (void) +{ + gl_tls_key_init (buffer_key, free); + gl_tls_key_init (bufmax_key, NULL); + /* The per-thread initial values are NULL and 0, respectively. */ +} + +/* Ensure that keys_init is called once only. */ +gl_once_define(static, keys_init_once); + + +/* NAME + fstrcmp - fuzzy string compare + + SYNOPSIS + double fstrcmp(const char *, const char *); + + DESCRIPTION + The fstrcmp function may be used to compare two string for + similarity. It is very useful in reducing "cascade" or + "secondary" errors in compilers or other situations where + symbol tables occur. + + RETURNS + double; 0 if the strings are entirly dissimilar, 1 if the + strings are identical, and a number in between if they are + similar. */ + +double +fstrcmp (const char *string1, const char *string2) +{ + struct context ctxt; + int i; + + size_t fdiag_len; + int *buffer; + size_t bufmax; + + /* set the info for each string. */ + ctxt.string[0].data = string1; + ctxt.string[0].data_length = strlen (string1); + ctxt.string[1].data = string2; + ctxt.string[1].data_length = strlen (string2); + + /* short-circuit obvious comparisons */ + if (ctxt.string[0].data_length == 0 && ctxt.string[1].data_length == 0) + return 1.0; + if (ctxt.string[0].data_length == 0 || ctxt.string[1].data_length == 0) + return 0.0; + + /* Set TOO_EXPENSIVE to be approximate square root of input size, + bounded below by 256. */ + ctxt.too_expensive = 1; + for (i = ctxt.string[0].data_length + ctxt.string[1].data_length; + i != 0; + i >>= 2) + ctxt.too_expensive <<= 1; + if (ctxt.too_expensive < 256) + ctxt.too_expensive = 256; + + /* Allocate memory for fdiag and bdiag from a thread-local pool. */ + fdiag_len = ctxt.string[0].data_length + ctxt.string[1].data_length + 3; + gl_once (keys_init_once, keys_init); + buffer = (int *) gl_tls_get (buffer_key); + bufmax = (size_t) (uintptr_t) gl_tls_get (bufmax_key); + if (fdiag_len > bufmax) + { + /* Need more memory. */ + bufmax = 2 * bufmax; + if (fdiag_len > bufmax) + bufmax = fdiag_len; + /* Calling xrealloc would be a waste: buffer's contents does not need + to be preserved. */ + if (buffer != NULL) + free (buffer); + buffer = (int *) xmalloc (bufmax * (2 * sizeof (int))); + gl_tls_set (buffer_key, buffer); + gl_tls_set (bufmax_key, (void *) (uintptr_t) bufmax); + } + ctxt.fdiag = buffer + ctxt.string[1].data_length + 1; + ctxt.bdiag = ctxt.fdiag + fdiag_len; + + /* Now do the main comparison algorithm */ + ctxt.string[0].edit_count = 0; + ctxt.string[1].edit_count = 0; + compareseq (0, ctxt.string[0].data_length, 0, ctxt.string[1].data_length, 0, + &ctxt); + + /* The result is + ((number of chars in common) / (average length of the strings)). + This is admittedly biased towards finding that the strings are + similar, however it does produce meaningful results. */ + return ((double) (ctxt.string[0].data_length + ctxt.string[1].data_length + - ctxt.string[1].edit_count - ctxt.string[0].edit_count) + / (ctxt.string[0].data_length + ctxt.string[1].data_length)); +}