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/fuzzystrmatch/README.fuzzystrmatch

@@ -0,0 +1,121 @@
+/*
+ * 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
+