module Exercise_12 where

import Data.List --(nub)
import Control.Arrow

{-H12-}
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]

-- Shorter Version BELOW
{-WETT-}
encrypt :: String -> String
decrypt :: String -> String

f = not . isPrime
encrypt = map snd . sortOn (f . fst) . zip (enumFrom 1)
decrypt = map snd . sort <<< zip =<< sortOn f . enumFromTo 1 . length
{-TTEW-}

{-MCCOMMENT
Not sure if we are allowed to change the signature (this would be shorter -> 34t):

encrypt :: Ord a => [a] -> [a]
decrypt :: Ord a => [a] -> [a]
encrypt = map snd . sortOn (not . isPrime . fst) . zip (enumFrom 1)
decrypt = map snd . sort <<< zip =<< encrypt . enumFromTo 1 . length
-}

encryptShort :: Ord a => [a] -> [a]
decryptShort :: Ord a => [a] -> [a]
encryptShort = map snd . sortOn (not . isPrime . fst) . zip (enumFrom 1)
decryptShort = map snd . sort <<< zip =<< encryptShort . enumFromTo 1 . length