Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!rutgers!ucsd!ucbvax!decwrl!sun!pitstop!sundc!seismo!uunet!mcvax!tuvie!tuhold!thom
From: thom@tuhold (Thom Fruehwirth)
Newsgroups: comp.lang.prolog
Subject: Re: Soundex revisited
Message-ID: <1190@tuhold>
Date: 1 Sep 88 11:52:11 GMT
Organization: Institut f. Angewandte Informatik, TU Vienna
Lines: 40

In a recent article ok@quintus.uucp (Richard A. O'Keefe) writes:  
> Since the specification of the Soundex function requires that 
> duplicate characters be discarded, it is possible to construct
> strings of arbitrary length _all_ of which must be inspected
> even if only 3 codes are produced.  (Stick "sss.......sss" in
> front of any word you like.)
But what sense does this make ?

> Fruehwirth is quite right that I didn't take my "implementation"
> version as far as I could have.  Here's my "optimised" version: the
> soundex_atoms/2 predicate and char_to_code/2 table didn't change.

I pleased to see that Richard O'Keefes code follows my initial 
suggestions on how to transform soundex(-like) specifications.
Only one little transformation is still missing: That of transforming
zeros/2 away. It's a minor change, but it avoids going the roundabout
way of counting the number of character-codes produced so far:

%   soundex_chars(+Letters, +PreviousCode, +FillInCode, -Digits)
%   	in our case FillInCode = "000"

soundex_chars([], _, Zeros, Zeros).
soundex_chars([Char|Chars], Prev, Zeros, Codes) :-
   char_to_code(Char, Code),
   (   Code =:= Prev ->        % Discard duplicate characters
            soundex_chars(Chars, Code, Zeros, Codes)
   ;   Code =:= 0 ->           % Ignore vowels
            soundex_chars(Chars, Code, Zeros, Codes)
   ;   Zeros = [Zero] ->       % Stop when 3 digits have been done
            Codes = [Code]
   ;   Zeros = [Zero|Zeros1],  % otherwise, convert 1 more character
            Codes = [Code|Codes1],
            soundex_chars(Chars, Code, Zeros1, Codes1)
   ).

Isn't it more beautiful this way ?

thom fruehwirth

PS: Effiency is about the same, results depend on the test cases used.