module Exercise_12 where

import Data.List (nub, (\\), sortBy)
import Data.Ord (comparing)

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 = ns isPrime ++ ns (not . isPrime)
    where ns f = fst $ unzip $ filter (f . snd) $ zip s [1..]

decrypt :: String -> String
decrypt e = fst $ unzip $ sortBy (comparing snd) $ zip e p ++ zip (drop (length p) e) ([1..l] \\ p)
    where l = length e
          p = filter isPrime [1..l]
{-TTEW-}