Per this discussion, here's a patch to implement both levenshtein() and

metaphone() in a contrib. There seem to be a fair number of different
approaches to both of these algorithms. I used the simplest case for
levenshtein which has a cost  of 1 for any character insertion, deletion, or
substitution. For metaphone, I adapted the same code from CPAN that the PHP
folks did.

A couple of questions:
1. Does it make sense to fold the soundex contrib together with this one?

2. I was debating trying to add multibyte support to levenshtein (it would
make no sense at all for metaphone), but a quick search through the contrib
directory found no hits on the word MULTIBYTE. Should worry about adding
multibyte support to levenshtein()?

Joe Conway

contrib/README

@@ -55,6 +55,10 @@ fulltextindex -
 	Full text indexing using triggers
 	by Maarten Boekhold <maartenb@dutepp0.et.tudelft.nl>
 
+fuzzystrmatch -
+	Levenshtein and Metaphone fuzzy string matching
+	by Joe Conway <joseph.conway@home.com>
+
 intarray -
 	Index support for arrays of int4, using GiST
 	by Teodor Sigaev <teodor@stack.net> and Oleg Bartunov

contrib/fuzzystrmatch/Makefile

+subdir = contrib/fuzzystrmatch
+top_builddir = ../..
+include $(top_builddir)/src/Makefile.global
+
+# override libdir to install shlib in contrib not main directory
+libdir := $(libdir)/contrib
+
+# shared library parameters
+NAME= fuzzystrmatch
+SO_MAJOR_VERSION= 0
+SO_MINOR_VERSION= 1
+
+override CPPFLAGS := -I$(srcdir)/src/include $(CPPFLAGS)
+
+OBJS= fuzzystrmatch.o
+
+all: all-lib $(NAME).sql
+
+# Shared library stuff
+include $(top_srcdir)/src/Makefile.shlib
+
+
+$(NAME).sql: $(NAME).sql.in
+	sed -e 's:MODULE_PATHNAME:$(libdir)/$(shlib):g' < $< > $@
+
+install: all installdirs install-lib
+
+installdirs:
+	$(mkinstalldirs) $(DESTDIR)$(libdir)
+
+uninstall: uninstall-lib
+
+clean distclean maintainer-clean: clean-lib
+	rm -f $(OBJS) $(NAME).sql
+
+depend dep:
+	$(CC) -MM $(CFLAGS) *.c >depend
+
+ifeq (depend,$(wildcard depend))
+include depend
+endif

contrib/fuzzystrmatch/README.fuzzystrmatch

+/*
+ * fuzzystrmatch.c
+ *
+ * Functions for "fuzzy" comparison of strings
+ *
+ * Copyright (c) Joseph Conway <joseph.conway@home.com>, 2001;
+ *
+ * levenshtein()
+ * -------------
+ * Written based on a description of the algorithm by Michael Gilleland
+ * found at http://www.merriampark.com/ld.htm
+ * Also looked at levenshtein.c in the PHP 4.0.6 distribution for
+ * inspiration.
+ *
+ * metaphone()
+ * -----------
+ * Modified for PostgreSQL by Joe Conway.
+ * Based on CPAN's "Text-Metaphone-1.96" by Michael G Schwern <schwern@pobox.com>
+ * Code slightly modified for use as PostgreSQL function (palloc, elog, etc).
+ * Metaphone was originally created by Lawrence Philips and presented in article
+ * in "Computer Language" December 1990 issue.
+ *
+ * Permission to use, copy, modify, and distribute this software and its
+ * documentation for any purpose, without fee, and without a written agreement
+ * is hereby granted, provided that the above copyright notice and this
+ * paragraph and the following two paragraphs appear in all copies.
+ * 
+ * IN NO EVENT SHALL THE AUTHORS OR DISTRIBUTORS BE LIABLE TO ANY PARTY FOR
+ * DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES, INCLUDING
+ * LOST PROFITS, ARISING OUT OF THE USE OF THIS SOFTWARE AND ITS
+ * DOCUMENTATION, EVEN IF THE AUTHOR OR DISTRIBUTORS HAVE BEEN ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ * 
+ * THE AUTHORS AND DISTRIBUTORS SPECIFICALLY DISCLAIM ANY WARRANTIES,
+ * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY
+ * AND FITNESS FOR A PARTICULAR PURPOSE.  THE SOFTWARE PROVIDED HEREUNDER IS
+ * ON AN "AS IS" BASIS, AND THE AUTHOR AND DISTRIBUTORS HAS NO OBLIGATIONS TO
+ * PROVIDE MAINTENANCE, SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.
+ *
+ */
+
+
+Version 0.1 (3 August, 2001):
+  Functions to calculate the degree to which two strings match in a "fuzzy" way
+  Tested under Linux (Red Hat 6.2 and 7.0) and PostgreSQL 7.2devel
+
+Release Notes:
+
+  Version 0.1
+    - initial release    
+
+Installation:
+  Place these files in a directory called 'fuzzystrmatch' under 'contrib' in the PostgreSQL source tree. Then run:
+
+    make
+    make install
+
+  You can use fuzzystrmatch.sql to create the functions in your database of choice, e.g.
+
+    psql -U postgres template1 < fuzzystrmatch.sql
+
+  installs following functions into database template1:
+
+     levenshtein() - calculates the levenshtein distance between two strings
+     metaphone() - calculates the metaphone code of an input string
+
+Documentation
+==================================================================
+Name
+
+levenshtein -- calculates the levenshtein distance between two strings
+
+Synopsis
+
+levenshtein(text source, text target)
+
+Inputs
+
+  source
+    any text string, 255 characters max, NOT NULL
+
+  target
+    any text string, 255 characters max, NOT NULL
+
+Outputs
+
+  Returns int
+
+Example usage
+
+  select levenshtein('GUMBO','GAMBOL');
+
+==================================================================
+Name
+
+metaphone -- calculates the metaphone code of an input string
+
+Synopsis
+
+metaphone(text source, int max_output_length)
+
+Inputs
+
+  source
+    any text string, 255 characters max, NOT NULL
+
+  max_output_length
+    maximum length of the output metaphone code; if longer, the output
+    is truncated to this length
+
+Outputs
+
+  Returns text
+
+Example usage
+
+  select metaphone('GUMBO',4);
+
+==================================================================
+-- Joe Conway
+

contrib/fuzzystrmatch/fuzzystrmatch.h

+/*
+ * fuzzystrmatch.h
+ *
+ * Functions for "fuzzy" comparison of strings
+ *
+ * Copyright (c) Joseph Conway <joseph.conway@home.com>, 2001;
+ *
+ * levenshtein()
+ * -------------
+ * Written based on a description of the algorithm by Michael Gilleland
+ * found at http://www.merriampark.com/ld.htm
+ * Also looked at levenshtein.c in the PHP 4.0.6 distribution for
+ * inspiration.
+ *
+ * metaphone()
+ * -----------
+ * Modified for PostgreSQL by Joe Conway.
+ * Based on CPAN's "Text-Metaphone-1.96" by Michael G Schwern <schwern@pobox.com>
+ * Code slightly modified for use as PostgreSQL function (palloc, elog, etc).
+ * Metaphone was originally created by Lawrence Philips and presented in article
+ * in "Computer Language" December 1990 issue.
+ *
+ * Permission to use, copy, modify, and distribute this software and its
+ * documentation for any purpose, without fee, and without a written agreement
+ * is hereby granted, provided that the above copyright notice and this
+ * paragraph and the following two paragraphs appear in all copies.
+ * 
+ * IN NO EVENT SHALL THE AUTHORS OR DISTRIBUTORS BE LIABLE TO ANY PARTY FOR
+ * DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES, INCLUDING
+ * LOST PROFITS, ARISING OUT OF THE USE OF THIS SOFTWARE AND ITS
+ * DOCUMENTATION, EVEN IF THE AUTHOR OR DISTRIBUTORS HAVE BEEN ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ * 
+ * THE AUTHORS AND DISTRIBUTORS SPECIFICALLY DISCLAIM ANY WARRANTIES,
+ * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY
+ * AND FITNESS FOR A PARTICULAR PURPOSE.  THE SOFTWARE PROVIDED HEREUNDER IS
+ * ON AN "AS IS" BASIS, AND THE AUTHOR AND DISTRIBUTORS HAS NO OBLIGATIONS TO
+ * PROVIDE MAINTENANCE, SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.
+ *
+ */
+
+#ifndef FUZZYSTRMATCH_H
+#define FUZZYSTRMATCH_H
+
+#include <stdio.h>
+#include <string.h>
+#include <ctype.h>
+
+#include "postgres.h"
+#include "fmgr.h"
+#include "utils/builtins.h"
+
+
+#define MAX_LEVENSHTEIN_STRLEN		255
+#define MAX_METAPHONE_STRLEN		255
+
+typedef struct dynmatrix
+{
+	int		   value;
+} dynmat;
+
+
+/*
+ * External declarations
+ */
+extern Datum levenshtein(PG_FUNCTION_ARGS);
+extern Datum metaphone(PG_FUNCTION_ARGS);
+
+/*
+ * Internal declarations
+ */
+
+
+
+#define STRLEN(p) strlen(p)
+#define CHAREQ(p1, p2) (*(p1) == *(p2))
+#define NextChar(p) ((p)++)
+
+
+/* 
+ * Original code by Michael G Schwern starts here.
+ * Code slightly modified for use as PostgreSQL
+ * function (combined *.h into here).
+ *------------------------------------------------------------------*/
+
+/**************************************************************************
+	metaphone -- Breaks english phrases down into their phonemes.
+	
+	Input
+		word			-- 	An english word to be phonized
+		max_phonemes 	-- 	How many phonemes to calculate.  If 0, then it
+							will phonize the entire phrase.
+		phoned_word  	-- 	The final phonized word.  (We'll allocate the
+							memory.)
+	Output
+		error	--	A simple error flag, returns TRUE or FALSE
+		
+	NOTES:  ALL non-alpha characters are ignored, this includes whitespace,
+	although non-alpha characters will break up phonemes.
+****************************************************************************/
+
+
+/**************************************************************************
+	my constants -- constants I like
+	
+	Probably redundant.
+	
+***************************************************************************/
+
+#define META_ERROR			FALSE
+#define META_SUCCESS		TRUE
+#define META_FAILURE		FALSE
+
+
+/*  I add modifications to the traditional metaphone algorithm that you
+	might find in books.  Define this if you want metaphone to behave
+	traditionally */
+#undef USE_TRADITIONAL_METAPHONE
+
+/* Special encodings */
+#define  SH 	'X'
+#define  TH		'0'
+
+char Lookahead(char * word, int how_far);
+int _metaphone (
+	/* IN */
+	char * 	word,
+	int 	max_phonemes,
+	/* OUT */
+	char **	phoned_word
+);
+
+/* Metachar.h ... little bits about characters for metaphone */
+
+
+/*-- Character encoding array & accessing macros --*/
+/* Stolen directly out of the book... */
+char _codes[26] = {
+	1,16,4,16,9,2,4,16,9,2,0,2,2,2,1,4,0,2,4,4,1,0,0,0,8,0
+/*  a  b c  d e f g  h i j k l m n o p q r s t u v w x y z */
+};
+
+
+#define ENCODE(c) (isalpha(c) ? _codes[((toupper(c)) - 'A')] : 0)
+
+#define isvowel(c)	(ENCODE(c) & 1)		/* AEIOU */
+
+/* These letters are passed through unchanged */
+#define NOCHANGE(c)	(ENCODE(c) & 2) 	/* FJMNR */
+
+/* These form dipthongs when preceding H */
+#define AFFECTH(c)	(ENCODE(c) & 4) 	/* CGPST */
+
+/* These make C and G soft */
+#define MAKESOFT(c)	(ENCODE(c) & 8) 	/* EIY */
+
+/* These prevent GH from becoming F */
+#define NOGHTOF(c)	(ENCODE(c) & 16) 	/* BDH */
+
+
+#endif	 /* FUZZYSTRMATCH_H */

contrib/fuzzystrmatch/fuzzystrmatch.sql.in

+CREATE FUNCTION levenshtein (text,text) RETURNS int
+  AS 'MODULE_PATHNAME','levenshtein' LANGUAGE 'c' with (iscachable, isstrict);
+
+CREATE FUNCTION metaphone (text,int) RETURNS text
+  AS 'MODULE_PATHNAME','metaphone' LANGUAGE 'c' with (iscachable, isstrict);