Day1.ex

defmodule Day1 do
  defp _position(str) do
    str
    |> Stream.unfold(&String.next_codepoint/1)
    |> Stream.map(&do_decode/1)
    |> Stream.with_index
    |> Stream.transform(0, &do_floor/2)
    |> Stream.map(&IO.inspect(&1))
    |> Enum.to_list
    |> List.last
  end

  defp do_decode("("), do: 1
  defp do_decode(")"), do: -1
  defp do_decode(_), do: 0

  defp do_floor({x,index}, acc) when acc + x != -1 do
    floor = acc + x
    position = index + 1
    IO.puts "position: #{position}, floor: #{floor}"
    {[{position, floor}], floor}
  end

  defp do_floor({x,index}, acc) do
    floor = acc + x
    position = index + 1
    IO.puts "Found the floor -1 at position: #{position}"
    {:halt, position}
  end
end

Output:

iex(44)> Day1.position("())")
position: 1, floor: 1
{1, 1}
position: 2, floor: 0
{2, 0}
Found the floor -1 at position: 3
{2, 0}

The output from the Day1.position("())") call returns {2,0} but really I want to get the position value of 3