--
-- Copyright (c) 2013 Bonelli Nicola
--
-- This program 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 of the License, or
-- (at your option) any later version.
--
-- This program 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 this program; if not, write to the Free Software
-- Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
--
module CGrep.Distance (distance, (~==)) where
-- from http://www.haskell.org/haskellwiki/Edit_distance
--
distance :: Eq a => [a] -> [a] -> Int
distance a b
= last (if lab == 0 then mainDiag
else if lab > 0 then lowers !! (lab - 1)
else {- < 0 -} uppers !! (-1 - lab))
where mainDiag = oneDiag a b (head uppers) (-1 : head lowers)
uppers = eachDiag a b (mainDiag : uppers) -- upper diagonals
lowers = eachDiag b a (mainDiag : lowers) -- lower diagonals
eachDiag _a [] _diags = []
eachDiag a' (_bch:bs) (lastDiag:diags) = oneDiag a' bs nextDiag lastDiag : eachDiag a' bs diags
where nextDiag = head (tail diags)
eachDiag _ _ [] = undefined -- the original implementation does not cover this case...
oneDiag a' b' diagAbove diagBelow = thisdiag
where doDiag [] _b _nw _n _w = []
doDiag _a [] _nw _n _w = []
doDiag (ach:as) (bch:bs) nw n w = me : doDiag as bs me (tail n) (tail w)
where me = if ach == bch then nw else 1 + min3 (head w) nw (head n)
firstelt = 1 + head diagBelow
thisdiag = firstelt : doDiag a' b' firstelt diagAbove (tail diagBelow)
lab = length a - length b
min3 x y z = if x < y then x else min y z
(~==) :: String -> String -> Bool
a ~== b | len < 5 = dist < 3
| otherwise = dist < (len * 40 `div` 100)
where len = fromIntegral (length a `min` length b)
dist = distance a b