module Exercise_12 where

import Data.List (nub, elemIndex, union, delete)
import Data.Maybe (fromJust, isJust)

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-}
-- Two pretty straightforward implementations.
encrypt :: String -> String
encrypt s = (map ((s!!) . subtract 1) $ primes $ length s) ++ (map snd . filter (not . isPrime . fst) $ zip [1..] s)

decrypt :: String -> String
-- I noticed both the primal and non-primal indices need the number of primes smaller or equal to the respective index.
-- So I factored that out, saving one token; at the price of great unreadability.
decrypt s = [s !! ((if isPrime i then fromJust . elemIndex i else (subtract . subtract (i - 1) $ length $ primes i) . length) $ primes $ length s) | i <- [1..length s]]
{-TTEW-}