module Exercise_12 where

import Data.List

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 as =
  let ps = primes $ length as
  in  [ as !! (i - 1) | i <- ps ++ ([1 .. length as] \\ ps) ]

decrypt :: String -> String
decrypt as =
  let ps = primes $ length as
  in  [ c
      | (_, c) <- sortOn
        fst
        (zip [ i - 1 | i <- ps ++ ([1 .. length as] \\ ps) ] as)
      ]
{-TTEW-}