module Exercise_12 where

import Data.List (nub, elemIndex)
import Data.Maybe (fromJust)

{-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 !! (k-1) | k <- sc]
             where sc = scheme $ length s

scheme n = primes n ++ [k | k <- [1..n], not (isPrime k)]

decrypt :: String -> String
decrypt s = [s !! (fromJust $ elemIndex k sc) | k <- [1..n] ]
             where sc = scheme $ n
                   n = length s
{-TTEW-}