module Exercise_12 where

import Data.List (nub,sortBy)
import Data.Ord (comparing)
import System.IO

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 (s!!) $ strPrimes s ++ strNonPrimes s

strNonPrimes s = filter (`notElem` strPrimes s) [0..length s-1]
strPrimes = map pred . primes . length

decrypt :: String -> String
decrypt s = map fst $ sortBy (comparing snd) (f take ps ++ f drop (strNonPrimes s))
  where
    f x = zip $ x (length ps) s
    ps = strPrimes s
{-TTEW-}