1--
2-- Copyright (c) 2013-2019 Nicola Bonelli <nicola@pfq.io>
3--
4-- This program is free software; you can redistribute it and/or modify
5-- it under the terms of the GNU General Public License as published by
6-- the Free Software Foundation; either version 2 of the License, or
7-- (at your option) any later version.
8--
9-- This program is distributed in the hope that it will be useful,
10-- but WITHOUT ANY WARRANTY; without even the implied warranty of
11-- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
12-- GNU General Public License for more details.
13--
14-- You should have received a copy of the GNU General Public License
15-- along with this program; if not, write to the Free Software
16-- Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
17--
18
19
20module CGrep.Distance (distance, (~==)) where
21
22-- from http://www.haskell.org/haskellwiki/Edit_distance
23--
24
25distance :: Eq a => [a] -> [a] -> Int
26distance a b
27    = last (if lab == 0 then mainDiag
28        else if lab > 0 then lowers !! (lab - 1)
29         else {- < 0 -} uppers !! (-1 - lab))
30    where mainDiag = oneDiag a b (head uppers) (-1 : head lowers)
31          uppers = eachDiag a b (mainDiag : uppers) -- upper diagonals
32          lowers = eachDiag b a (mainDiag : lowers) -- lower diagonals
33          eachDiag _a [] _diags = []
34          eachDiag a' (_bch:bs) (lastDiag:diags) = oneDiag a' bs nextDiag lastDiag : eachDiag a' bs diags
35              where nextDiag = head (tail diags)
36          eachDiag _ _ [] = undefined -- the original implementation does not cover this case...
37          oneDiag a' b' diagAbove diagBelow = thisdiag
38              where doDiag [] _b _nw _n _w = []
39                    doDiag _a [] _nw _n _w = []
40                    doDiag (ach:as) (bch:bs) nw n w = me : doDiag as bs me (tail n) (tail w)
41                        where me = if ach == bch then nw else 1 + min3 (head w) nw (head n)
42                    firstelt = 1 + head diagBelow
43                    thisdiag = firstelt : doDiag a' b' firstelt diagAbove (tail diagBelow)
44          lab = length a - length b
45          min3 x y z = if x < y then x else min y z
46
47
48(~==) :: String -> String -> Bool
49a ~== b |  len < 5  = dist < 3
50        | otherwise = dist < (len * 40 `div` 100)
51    where len  = fromIntegral (length a `min` length b)
52          dist = distance a b
53
54