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+;;; soundex.el -- implement Soundex algorithm
+
+;; Copyright (C) 1993 Free Software Foundation, Inc.
+
+;; Author: Christian Plaunt <chris@bliss.berkeley.edu>
+;; Maintainer: FSF 
+;; Keywords: matching
+;; Created: Sat May 15 14:48:18 1993
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs 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.
+
+;; GNU Emacs 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 GNU Emacs; see the file COPYING.  If not, write to
+;; the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.
+
+;;; Commentary:
+
+;; The Soundex algorithm maps English words into representations of
+;; how they sound.  Words with vaguely similar sound map to the same string.
+
+;;; Code: 
+
+(defun soundex (word)
+  "Return a Soundex key for WORD.
+Implemented as described in:
+Knuth, Donald E. \"The Art of Computer Programming, Vol. 3: Sorting
+and Searching\", Addison-Wesley (1973), pp. 391-392."
+  (let* ((word (upcase word)) (length (length word))
+	 (code (cdr (assq (aref word 0) soundex-alist)))
+	 (key (substring word 0 1)) (index 1) (prev-code code))
+    ;; once we have a four char key, were done
+    (while (and (> 4 (length key)) (< index length))
+      ;; look up the code for each letter in word at index
+      (setq code (cdr (assq (aref word index) soundex-alist))
+	    index (1+ index)
+	    ;; append code to key unless the same codes belong to
+	    ;; adjacent letters in the original string
+	    key (concat key (if (or (null code) (string= code prev-code))
+				()
+			      code))
+	    prev-code code))
+    ;; return a key that is 4 chars long and padded by "0"s if needed
+    (if (> 4 (length key))
+	(substring (concat key "000") 0 4)
+      key)))
+
+(defvar soundex-alist
+  '((?A . nil) (?E . nil) (?H . nil) (?I . nil) (?O . nil) (?U . nil)
+    (?W . nil) (?Y . nil) (?B . "1") (?F . "1") (?P . "1") (?V . "1")
+    (?C . "2") (?G . "2") (?J . "2") (?K . "2") (?Q . "2") (?S . "2")
+    (?X . "2") (?Z . "2") (?D . "3") (?T . "3") (?L . "4") (?M . "5")
+    (?N . "5") (?R . "6"))
+  "Alist of chars-to-key-code for building Soundex keys.")
+
+;(defvar soundex-test
+;  '("Euler" "Gauss" "Hilbert" "Knuth" "Lloyd" "Lukasiewicz"
+;    "Ellery" "Ghosh" "Heilbronn" "Kant" "Ladd" "Lissajous")
+;  "\n  Knuth's names to demonstrate the Soundex algorithm.")
+;
+;(mapcar 'soundex soundex-test)
+;("E460" "G200" "H416" "K530" "L300" "L222"
+; "E460" "G200" "H416" "K530" "L300" "L222")
+
+;; soundex.el ends here