module Exercise_12 where

import Data.List (nub, partition, permutations)
import Data.Bifoldable
import Control.Arrow

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 = zip (enumFrom 1) >>> biconcat . partition (isPrime . fst) >>> map snd

decrypt :: String -> String
decrypt s = until ((s ==) . encrypt) encrypt s
{-TTEW-}