module Exercise_12 where

import Data.List (nub, delete)

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 = map (\p -> s!!(p-1)) (ps ++ foldr delete [1..n] ps) where
  n = length s
  ps = primes n

decrypt :: String -> String
decrypt cs = reverse $ decryptAux n n (length $ primes n) where
  n = length cs
  decryptAux 0 _ _ = "" 
  decryptAux l taken acc
    | isPrime l = cs!!(acc-1):decryptAux (l-1) taken (acc-1)
    | otherwise = cs!!(taken-1):decryptAux (l-1) (taken-1) acc
{-TTEW-}