defmodule Orcha do
  @type message() :: term()
  @type callback() :: {module(), func :: atom(), args :: list()}

  @opaque realm() :: %__MODULE__{
            registry: %{atom() => service_state :: term()}
          }

  @callback handle_request(message(), realm(), state :: term()) ::
              {result :: term(), new_state :: term()}
              | {result :: term(), new_state :: term(), new_realm :: realm()}

  defstruct registry: %{}

  @default_init :initialize

  def initialize(_args \\ []), do: %__MODULE__{}

  @spec register(realm(), callback() | module()) :: realm()
  def register(%__MODULE__{} = realm, {mod, func, args}) do
    state = apply(mod, func, args)
    put_state(mod, state, realm)
  end

  def register(%__MODULE__{} = realm, mod) when is_atom(mod) do
    register(realm, {mod, @default_init, []})
  end

  @spec request({module(), message()}, realm()) ::
          {:ok, result :: term(), realm()} | {:error, reason :: term()}
  def request({mod, msg}, %__MODULE__{} = realm) do
    with {:ok, state} <- Map.fetch(realm.registry, mod) do
      case apply(mod, :handle_request, [msg, realm, state]) do
        {:result, result} ->
          result

        {:result, result, new_state} ->
          new_realm = put_state(mod, new_state, realm)
          {result, new_realm}

        {:result, result, new_state, %__MODULE__{} = new_realm} ->
          new_realm = put_state(mod, new_state, new_realm)
          {result, new_realm}
      end
    else
      :error ->
        raise "Requesting unregistered service: #{inspect(mod)}."
    end
  end

  @spec put_state(module(), state :: term(), realm()) :: realm()
  defp put_state(mod, state, %__MODULE__{} = realm) do
    update_in(realm.registry, &Map.put(&1, mod, state))
  end
end