module Exercise_12 where

import Data.List (nub, elemIndex, (\\), sortBy)
import Data.Maybe
import Data.Ord

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' id ++ s' not
  where
    s' p = [s!!(i-1) | i <- [1..length s], p $ isPrime i]

decrypt :: String -> String
decrypt s = map fst $ sortBy (comparing snd) $ zip s $ p id ++ p not
  where p f = filter (f . isPrime) [1..length s]
{-TTEW-}