emacs/lisp/soundex.el

;;; soundex.el --- implement Soundex algorithm -*- lexical-binding: t -*-

;; Copyright (C) 1993, 2001-2021 Free Software Foundation, Inc.

;; Author: Christian Plaunt <chris@bliss.berkeley.edu>
;; Maintainer: emacs-devel@gnu.org
;; 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 3 of the License, 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.  If not, see <https://www.gnu.org/licenses/>.

;;; Commentary:

;; The Soundex algorithm maps English words into representations of
;; how they sound.  Words with vaguely similar sound map to the same string.

;;; Code:

(defconst soundex-alist
  '((?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.")

(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, we're 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)))

(provide 'soundex)

;;; soundex.el ends here