module Exercise_12 where

import           Data.List
import           Data.Monoid
import           Data.Function
import           Data.Char
import           Control.Arrow
import           Control.Monad
import           Control.Applicative

isPrime :: Int -> Bool
isPrime n | n <= 1    = False
          | otherwise = primeAux (n - 1)
  where
    primeAux 1 = True
    primeAux i = n `mod` i /= 0 && primeAux (i - 1)

-- returns all primes up to the passed number
primes :: Int -> [Int]
primes n = [ i | i <- [2 .. n], isPrime i ]

{-WETT-}
encrypt :: String -> String
encrypt = map snd . uncurry mappend . partition (isPrime . fst) . zip [1 ..]

decrypt :: String -> String
decrypt =
    map fst
        .   sortOn snd
        <<< zip          -- some magic with precedence
        <*> encrypt
        .   map chr
        .   enumFromTo 1 -- either of the enc/dec functions must be strict
        .   length       -- else this won't work

{- This one apparently works, but the usage of unsafe methode is terrible -}
-- import           Unsafe.Coerce
-- decrypt :: String -> String
-- decrypt = map fst . sortOn snd <<< zip <*> encrypt . unsafeCoerce . enumFromTo 1 . length

{- if the type signature could be generalized, 
-- this can be even shorter by remove mapping to Char -}
-- encrypt :: [a] -> [a]
-- encrypt = map snd . uncurry mappend . partition (isPrime . fst) . zip [1 ..]
--
-- dec :: [a] -> [a]
-- decrypt = map fst . sortOn snd <<< zip <*> encrypt . enumFromTo 1 . length
{-TTEW-}