module Exercise_12 where

import Data.List

{-H12-}
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 s = [s !! (p-1) | p <- primes n]
            ++
            [s !! (i-1) | i <- enumFromTo 1 n \\ primes n]
            where
              n = length s


decrypt :: String -> String
decrypt s = if length s <= 1
            then s
            else last $ takeWhile (/= s) $ tail $ iterate encrypt s

{-TTEW-}