module Exercise_12 where

import Data.List (nub, partition, elemIndex, splitAt)
import Data.Char (ord, chr)

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 = map snd . uncurry (++) . partition (isPrime . fst) . zip [1..] 

decrypt :: String -> String
decrypt cipher = [cipher !! if isPrime i then lp i -1 else i - lp i + lp n - 1 | i <- [1..n]]
    where   n = length cipher
            lp = length . primes
{-TTEW-}