module Euler14

imports
  Prelude

where

main :: () -> I32
main () = do
  i = new (1 :: W32)
  m = new (1 :: W32)
  n = new (1 :: W32)
  times 1000000
    (\i -> do
      l = collatz_len i
      when (l > @m)
        (do
          m <- l
          n <- i
        )
    )
  putW @n
  assert (@n == 837799)
  0

collatz_len n => do
  t = new (1 :: W32)
  for n (\i -> i > 1) collatz (\_ -> inc t)
  @t

collatz n => branch
  is_odd n -> (3 * n) + 1
  | n / 2

is_odd n => (n & 0x1) == 0x1