{-# LANGUAGE OverloadedStrings #-}

module Text.MFKC ( distance ) where

----------------------------------------------------------------------------
-- |
-- Module      :  MKFC
-- Author      :  (C) Andrew Fatkulin
-- License     :  BSD-3-Clause (see the file LICENSE)
--
--
-- Computing the most frequent k characters distance of two strings
----------------------------------------------------------------------------

import Data.List (sortOn)
import Data.Text (Text)
import Data.HashMap.Strict((!), HashMap)
import qualified Data.Text as Tx
import qualified Data.HashMap.Strict as Hm


infixr 1 ?
(?) :: Bool -> a -> a -> a
(?) True  x _ = x
(?) False _ y = y

type MkHash = HashMap Char Int

toHash :: Text -> MkHash
toHash text = Tx.foldl fillMap emptyMap text
    where
        emptyMap = Hm.empty :: MkHash
        fillMap = \acc ch ->
            let value = (Hm.member ch acc) ? (acc ! ch) + 1 $ 1
            in Hm.insert ch value acc

takeMostFrequentK :: Int ->  MkHash -> MkHash
takeMostFrequentK k hash = Hm.fromList . (take k) . reverse . (sortOn snd) . Hm.toList $ hash

calcSimilarity :: (Int, Int, Int) -> Int
calcSimilarity (sim, v1, v2) = v1 == v2 ? (sim + v2) $ (sim + v1 + v2)

similarity :: MkHash -> MkHash -> Int
similarity h1 h2 = Hm.foldlWithKey' calc 0 h2 
    where
        calc acc k v = (Hm.member k h1)
            ? calcSimilarity (acc, v, ( h1 ! k ))
            $ acc

distance :: Int -> Int -> Text -> Text -> Int
distance k max text text2 = max - (similarity hash1 hash2)
    where
        hash1 = ((takeMostFrequentK k) . toHash) text
        hash2 = ((takeMostFrequentK k) . toHash) text2