module Exercise_12 where

import           Data.List
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 = map snd . flip h (isPrime . fst) . zip [1 ..]
h xs f = ((++) `on` flip filter xs) f (not . f)

decrypt :: String -> String
decrypt s =
  map fst $ sortOn snd $ zip s $ flip h isPrime $ enumFromTo 1 $ length s

{-MCCOMMENT
Hello MC! You also asked for a cipher, so....
primesString = map pred . primes . length
f s = uncurry (\a b -> reverse a ++ b) $ splitAt (length $ primesString s) s
g op op2 s = f $ foldl op s $ op2 $ primesString s
encrypt = g prep id
decrypt = f . g reconstruct reverse . f
prep s i = s !! i : take i s ++ drop (succ i) s
reconstruct (s : ss) i = take i ss ++ s : drop i ss
-}

{-TTEW-}