input = IO.gets("input?")
        |> String.trim
        |> String.split(" ")
        |> Enum.map(fn part ->
          String.to_integer part
        end)

defmodule Day1_2 do
  def pass(input, state) do

    # pass through the entire list with reduce
    # so we have some kind of state in acc
    result = Enum.reduce(input, state, fn part, acc ->
      case acc do

        # if we already found it, we don't need
        # to keep iterating
        {:ok, res} ->
          {:ok, res}

        # acc grows like this:
        # [0]
        # [0 + first_change | 0]

        # [0 + first_change + second_change |
        # [0 + first_change | 0]], etc
        [current | _rest] ->
          # the head of the list is the current
          # frequency of the device that we'll
          # apply the change to (part)
          new_freq = current + part

          # then we search in the OLD list
          # if we are a member of it
          #
          # if we are, then we are actually the first
          # repeated frequency in the list.
          became_member = Enum.member?(acc, new_freq)

          if became_member do
            # this should make the reduce()
            # call stop creating new frequencies
            # and just returning what it found
            {:ok, new_freq}
          else
            # if we didn't find, we add the new frequency
            # to the list, and keep going.
            [new_freq | acc]
          end
      end
    end)

    case result do
      {:ok, res} -> res

        # if
      _ -> pass(input, result)
    end
  end
end

res = Day1_2.pass(input, [0])

IO.puts "total res: #{inspect res}"