module Exercise_12 where

import System.IO
import qualified Data.Map as Map

import Data.List (nub, partition, sortBy)
import Data.Functor ((<$>))
import Data.Ord (comparing)
import Data.Bifoldable (biconcat)


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-}
-- Move prime numbers to front by splitting number sequence accordingly
-- [a] -> [a]
ordering = map snd . biconcat . partition fst . zip (isPrime <$> enumFrom 1)

encrypt :: String -> String
encrypt = ordering

-- Retrieve ordering in terms of indices and sort chars back accordingly
-- Example : [(2,'x'), (3,'a'), (1,'e'), (4,'m')] -> [(1,'e'), (2,'x'), (3, 'a'), (4,'m')]
decrypt :: String -> String
decrypt ws = fst <$> (sortBy (comparing snd) . zip ws . ordering . enumFromTo 1 . length) ws
{-TTEW-}