module Exercise_12 where

import Data.List
import Data.Bifoldable
import Control.Arrow

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 = map snd <<< biconcat <<< partition (isPrime .fst)  <<< zip (enumFrom 1)
decrypt :: String -> String
decrypt = map snd <<< sortOn fst <<< zip =<< biconcat . partition isPrime . enumFromTo 1 . length

{-TTEW-}

{-MCCOMMENT
decrypt = head . ap (filter . (.encrypt) . isPrefixOf) permutations
why am i wasting my life with this?
i have to learn for exams
-}