module Exercise_12 where

import Data.Bifoldable (biconcat)
import Data.List
    ( (\\)
    , concat
    , find
    , intersect
    , nub
    , partition
    , permutations
    , sortOn
    , union
    )
import Data.Maybe (fromJust)
import Data.Ord (comparing)

{-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]

{-MCCOMMENT
Hello dear MC! I have another, shorter solution, that does not pass the test, as it is O(2^n).
If you happen to only test very short strings while running the tests for the competition, here
is my brute-force solution:

decrypt :: String -> String
decrypt s = fromJust . find ((== s) . encrypt) $ permutations s
-}
{-WETT-}
encrypt :: String -> String
encrypt = map snd . biconcat . partition (isPrime . fst) . zip [1 ..]

decrypt :: String -> String
decrypt s = map fst . sortOn snd . zip s $ primes (length s) `union` enumFrom 1

{-TTEW-}