require Integer

defmodule Euler do
  def collatz(num) when is_integer(num), do: do_collatz([num])

  defp next_collatz(num) when Integer.is_even(num), do: floor(num / 2)
  defp next_collatz(num), do: 3 * num + 1

  defp do_collatz([last_num | _] = lst) when last_num == 1, do: lst
  defp do_collatz([last_num | _] = lst), do: do_collatz([next_collatz(last_num) | lst])
end

IO.puts 1..1_000_000 |> Enum.max_by(&Enum.count(Euler.collatz(&1)))