defmodule Calculator do
  def start do
    spawn(fn -> loop(0) end)
  end

  defp loop(current_value) do
    new_value =
      receive do
        message -> process_message(current_value, message)
      end

    loop(new_value)
  end

  defp process_message(current_value, {:value, caller}) do
    send(caller, {:response, current_value})
    current_value
  end

  defp process_message(current_value, {:add, value}) do
    current_value + value
  end

  defp process_message(current_value, {:sub, value}) do
    current_value - value
  end

  defp process_message(current_value, {:mul, value}) do
    current_value * value
  end

  defp process_message(current_value, {:div, value}) do
    current_value / value
  end
end