module Exercise_12 where

import System.IO
import Data.List
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-}
a b c = elem c b
encrypt d = map fst $ c `filter` f ++ (not . c) `filter` f
             where f = d `zip` enumFrom 1
                   c = a (primes $ length d) . snd

decrypt c = map fst $ comparing snd `sortBy` (length b `take` c `zip` b ++ length b `drop` c `zip` (filter (not . a b) $ 1 `enumFromTo` length c))
             where b = primes $ length c
{-TTEW-}