Fuzzy string comparison.

1 files changed, 186 insertions, 0 deletions

diff --git a/lib/fstrcmp.c b/lib/fstrcmp.c
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+/* 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, 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., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+
+
+   Derived from GNU diff 2.7, analyze.c et al.
+
+   The basic idea is to consider two vectors as similar if, when
+   transforming the first vector into the second vector through a
+   sequence of edits (inserts and deletes of one element each),
+   this sequence is short - or equivalently, if the ordered list
+   of elements 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 'find_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.  */
+
+#include <config.h>
+
+/* Specification.  */
+#include "fstrcmp.h"
+
+#include <string.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <limits.h>
+
+#include "lock.h"
+#include "tls.h"
+#include "minmax.h"
+#include "xalloc.h"
+
+#ifndef uintptr_t
+# define uintptr_t unsigned long
+#endif
+
+
+#define ELEMENT char
+#define EQUAL(x,y) ((x) == (y))
+#define OFFSET int
+#define EXTRA_CONTEXT_FIELDS \
+  /* The number of elements inserted or deleted. */ \
+  int xvec_edit_count; \
+  int yvec_edit_count;
+#define NOTE_DELETE(ctxt, xoff) ctxt->xvec_edit_count++
+#define NOTE_INSERT(ctxt, yoff) ctxt->yvec_edit_count++
+/* We don't need USE_HEURISTIC, since it is unlikely in typical uses of
+   fstrcmp().  */
+#include "diffseq.h"
+
+
+/* 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 xvec_length;
+  int yvec_length;
+  int i;
+
+  size_t fdiag_len;
+  int *buffer;
+  size_t bufmax;
+
+  /* set the info for each string.  */
+  ctxt.xvec = string1;
+  xvec_length = strlen (string1);
+  ctxt.yvec = string2;
+  yvec_length = strlen (string2);
+
+  /* short-circuit obvious comparisons */
+  if (xvec_length == 0 && yvec_length == 0)
+    return 1.0;
+  if (xvec_length == 0 || yvec_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 = xvec_length + yvec_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 = xvec_length + yvec_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 *) xnmalloc (bufmax, 2 * sizeof (int));
+      gl_tls_set (buffer_key, buffer);
+      gl_tls_set (bufmax_key, (void *) (uintptr_t) bufmax);
+    }
+  ctxt.fdiag = buffer + yvec_length + 1;
+  ctxt.bdiag = ctxt.fdiag + fdiag_len;
+
+  /* Now do the main comparison algorithm */
+  ctxt.xvec_edit_count = 0;
+  ctxt.yvec_edit_count = 0;
+  compareseq (0, xvec_length, 0, yvec_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) (xvec_length + yvec_length
+		    - ctxt.yvec_edit_count - ctxt.xvec_edit_count)
+	  / (xvec_length + yvec_length));
+}