{-# LANGUAGE BangPatterns #-}
module Main (main) where

import Data.Bits

main :: IO ()
main = putStrLn $ "longest: " ++ show longest ++ " (" ++ show terms ++ ")"
       where IP longest terms = euler14

data IntPair = IP {-# UNPACK #-} !Int {-# UNPACK #-} !Int

euler14 :: IntPair
euler14 = go 1 0 0
    where
      go :: Int -> Int -> Int -> IntPair
      go i !longest !terms | i <= 1000000 = while i 1 longest terms
                           | otherwise    = IP longest terms
        where
            while :: Int -> Int -> Int -> Int -> IntPair
            while 1 !_ !longest !terms = go (i+1) longest terms
            while j thisTerms longest terms =
                let thisTerms' = thisTerms + 1
                    IP terms' longest' = if thisTerms > terms
                                            then IP thisTerms i
                                            else IP terms longest
                    j' = if j<0 then 1 else 
                                            if j .&. 1 == 0
                                               then j `quot` 2
                                               else 3 * j + 1
                in while j' thisTerms' longest' terms'