Factor Language Blog

Here is a Factor implementation of the Levenshtein edit distance algorithm:

! Copyright (C) 2006 Slava Pestov.
! See http://factorcode.org/license.txt for BSD license.
USING: arrays kernel math namespaces sequences test ;
IN: levenshtein

: <matrix> ( m n -- matrix )
    [ drop 0 <array> ] map-with ; inline

: matrix-> nth nth ; inline
: ->matrix nth set-nth ; inline

SYMBOL: d

: ->d ( n i j -- ) d get ->matrix ; inline
: d-> ( i j -- n ) d get matrix-> ; inline

SYMBOL: costs

: init-d ( str1 str2 -- )
    [ length 1+ ] 2apply 2dup >matrix> d set
    [ 0 over ->d ] each
    [ dup 0 ->d ] each ; inline

: compute-costs ( str1 str2 -- )
    >array [
        swap >array [ = 0 1 ? ] map-with
    ] map-with costs set ; inline

: levenshtein-step ( i j -- )
    [ 1+ d-> 1+ ] 2keep
    [ >r 1+ r> d-> 1+ ] 2keep
    [ d-> ] 2keep
    [ costs get matrix-> + min min ] 2keep
    >r 1+ r> 1+ ->d ; inline

: levenshtein-result ( -- n ) d get peek peek ; inline

: levenshtein ( str1 str2 -- n )
    [
        2dup init-d
        2dup compute-costs
        [ length ] 2apply [
            swap [ swap levenshtein-step ] each-with
        ] each-with
        levenshtein-result
    ] with-scope ;

[ 3 ] [ "sitting" "kitten" levenshtein ] unit-test
[ 3 ] [ "kitten" "sitting" levenshtein ] unit-test
[ 1 ] [ "freshpak" "freshpack" levenshtein ] unit-test
[ 1 ] [ "freshpack" "freshpak" levenshtein ] unit-test

I’ve decided not to use it in the online help search code at this stage, since it yields a lot of false positives so I’d need to implement a better ranking system first. So for now I’m just putting it in examples/.