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;;; soundex.el --- implement Soundex algorithm -*- lexical-binding: t -*-
;; Copyright (C) 1993, 2001-2019 Free Software Foundation, Inc.
;; Author: Christian Plaunt <chris@bliss.berkeley.edu>
;; 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
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