module Exercise_12 where

import Data.List
import Data.Maybe
import Data.Function

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 str = let ps = primes $ length str
                  (l, r) = partition (isPrime . fst) $ zip [1..] str in
                      map snd $ l ++ r

decrypt :: String -> String
decrypt str = let ps = primes $ length str
                  l = zip ps str
                  index = [a|a <- [1..(length str)], a `notElem` ps]
                  r = zip index (drop (length l) str)
                  str' = sortOn fst (l ++ r) in
                      map snd str'
{-TTEW-}