import Data.List (maximumBy)
import Data.Ord (comparing)

-------------------- HAILSTONE SEQUENCE ------------------

collatz :: Int -> Int
collatz n
  | even n = n `div` 2
  | otherwise = 1 + 3 * n

hailstone :: Int -> [Int]
hailstone = takeWhile (1 /=) . iterate collatz

longestChain :: Int
longestChain =
  fst $
    maximumBy (comparing snd) $
      (,) <*> (length . hailstone) <$> [1 .. 100000]

--------------------------- TEST -------------------------
main :: IO ()
main =
  mapM_
    putStrLn
    [ "Collatz sequence for 27: ",
      (show . hailstone) 27,
      "The number " <> show longestChain,
      "has the longest hailstone sequence",
      "for any number less then 100000. ",
      "The sequence has length: "
        <> (show . length . hailstone $ longestChain)
    ]