defmodule Understated do
  alias Understated.Transition

  defmacro __using__(_options) do
    quote do
      import Understated.Machine
      import Understated.State
      import Understated.Transition

      Module.register_attribute(__MODULE__, :transitions, accumulate: true)
      Module.register_attribute(__MODULE__, :states, accumulate: true)
      Module.register_attribute(__MODULE__, :current_state, accumulate: false)

      @current_state ""

      @before_compile unquote(__MODULE__)
    end
  end

  defmacro __before_compile__(%{module: module} = _env) do
    transitions = Module.get_attribute(module, :transitions, [])

    transitions_ast =
      transitions
      |> Enum.map(fn {state, to, _on} ->
        quote do
          unquote(Transition.__compile_transitions(state, to))
        end
      end)

    handlers_ast =
      transitions
      |> Enum.map(fn {state, to, on} ->
        quote do
          unquote(Transition.__compile_event_handlers(state, to, on))
        end
      end)

    events_ast =
      transitions
      |> Enum.uniq_by(fn {_state, _to, on} -> on end)
      |> Enum.map(fn {_, _, on} ->
        quote do
          unquote(Transition.__compile_event(on))
        end
      end)

    quote do
      def valid_states() do
        @states
        |> Enum.reverse()
      end

      def valid_transitions() do
        @transitions
      end

      unquote(transitions_ast)
      unquote(handlers_ast)
      unquote(events_ast)

      def before_event(machine, _from, _to), do: machine
      def after_event(machine, _from, _to), do: machine

      def trigger(%__MODULE{} = machine, _to_state) do
        {:error, "Invalid event for current state", machine}
      end

      defp apply_state(%__MODULE__{} = machine, _state) do
        {:error, "Invalid transition", machine}
      end
    end
  end
end