hs-caesar/src/Caesar.hs

module Caesar where

import Data.Char
import Data.Maybe

alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
wtf = '¿'

downGrade :: String -> String
downGrade = fmap (clip . toUpper)
  where
    clip c | elem (toUpper c) alphabet = toUpper c
    clip ' ' = ' '
    clip _ = wtf

encode :: Int -> String -> String
encode key msg = unwords $ encodeWord key <$> (words $ downGrade msg)

decode :: Int -> String -> String
decode key msg = unwords $ encodeWord k <$> (words $ msg)
  where
    k = l - (key `mod` l)
    l = length alphabet

encodeWord :: Int -> String -> String
encodeWord k = fmap (substitute $ mapping k)

substitute :: [(Char,Char)] -> Char -> Char
substitute m e = fromMaybe wtf $ lookup e m

mapping :: Int -> [(Char,Char)]
mapping k = zip alphabet (alphabet `rotateBy` k)

l `rotateBy` n = take (length l) $ drop n (cycle l)