defmodule X3m.System.Router do
  @moduledoc """
  Registers system wide services.

  Each `service/2` macro registers system-wide service and function with
  documentation in module that `uses` this module.

  `servicep/2` is considered as private service and is not introduced to other nodes
  in the cluster.

  Service functions invoke function of the same name of specified module.
  If result of that invocation is `{:reply, %X3m.System.Message{}}`, 
  it sends message to `message.reply_to` pid.

  If result of invocation is `:noreply`, nothing is sent to that pid.

  In any case function returns `:ok`.

  ## Examples

  ### Defining router

      defmodule MyRouter do
        use X3m.System.Router

        @servicedoc false
        service :create_user, MessageHandler

        @servicedoc \"""
        overridden!
        \"""
        service :get_user, MessageHandler

        service :edit_user, MessageHandler

        servicep :private_service, MessageHandler
      end


  ### Getting registered services (public, private, or by default all)

      iex> MyRouter.registered_services()
      [create_user: 1, get_user: 1, edit_user: 1, private_service: 1]

      iex> MyRouter.registered_services(:public)
      [create_user: 1, get_user: 1, edit_user: 1]

  ### Invoking service as a function

      iex> :create_user |>
      ...>   X3m.System.Message.new() |>
      ...>   MyRouter.create_user()
      :ok
  """
  require Logger
  alias X3m.System.Message

  defmacro service(service_name, message_handler, f) do
    quote do
      case(@servicedoc) do
        nil ->
          @doc """
          Accepts `#{unquote(service_name)}` service call, routing it's `message` to 
          `#{unquote(message_handler)}.#{unquote(f)}/1`.

          If result of that invocation is `{:reply, %X3m.System.Message{}}`, 
          it sends message to `message.reply_to` pid.

          If result of invocation is `:noreply`, nothing is sent to that pid.

          In any case function returns `:ok`.

          ## Example:

              iex> #{inspect(unquote(service_name))} |>
              ...>   X3m.System.Message.new() |>
              ...>   #{__MODULE__}.#{unquote(service_name)}()
              :ok
          """

        other ->
          @doc other
      end

      @x3m_service [{unquote(service_name), 1}]
      @spec unquote(service_name)(Message.t()) :: :ok
      def unquote(service_name)(%Message{service_name: unquote(service_name)} = message) do
        Logger.metadata(message.logger_metadata)

        X3m.System.Instrumenter.execute(:service_request_received, %{}, %{
          service: unquote(service_name)
        })

        message
        |> choose_node()
        |> _invoke(unquote(message_handler), unquote(f), message)
      end

      @servicedoc nil
    end
  end

  defmacro service(service_name, message_handler) do
    quote do
      service(unquote(service_name), unquote(message_handler), unquote(service_name))
    end
  end

  defmacro servicep(service_name, message_handler, f) do
    quote do
      case(@servicedoc) do
        nil ->
          @doc """
          This service is not shared with other nodes!

          Accepts `#{unquote(service_name)}` service call, routing it's `message` to 
          `#{unquote(message_handler)}.#{unquote(f)}/1`.

          If result of that invocation is `{:reply, %X3m.System.Message{}}`, 
          it sends message to `message.reply_to` pid.

          If result of invocation is `:noreply`, nothing is sent to that pid.

          In any case function returns `:ok`.

          ## Example:

              iex> #{inspect(unquote(service_name))} |>
              ...>   X3m.System.Message.new() |>
              ...>   #{__MODULE__}.#{unquote(service_name)}()
              :ok
          """

        other ->
          @doc other
      end

      @x3m_servicep [{unquote(service_name), 1}]
      @spec unquote(service_name)(Message.t()) :: :ok
      def unquote(service_name)(%Message{service_name: unquote(service_name)} = message) do
        Logger.metadata(message.logger_metadata)

        X3m.System.Instrumenter.execute(:service_request_received, %{}, %{
          service: unquote(service_name)
        })

        message
        |> choose_node()
        |> _invoke(unquote(message_handler), unquote(f), message)
      end

      @servicedoc nil
    end
  end

  defmacro servicep(service_name, message_handler) do
    quote do
      servicep(unquote(service_name), unquote(message_handler), unquote(service_name))
    end
  end

  defmacro __using__(_opts) do
    quote do
      alias X3m.System.Router
      require Router
      import Router

      Module.register_attribute(
        __MODULE__,
        :x3m_service,
        accumulate: true,
        persist: true
      )

      Module.register_attribute(
        __MODULE__,
        :x3m_servicep,
        accumulate: true,
        persist: true
      )

      @servicedoc nil

      @doc !"""
           Returns list of public, private or all service functions with their arrity.
           """
      @spec registered_services(:public | :private | :all) :: [{:atom, non_neg_integer}]
      def registered_services(visibility \\ :all)

      def registered_services(:public) do
        __MODULE__.__info__(:attributes)
        |> Keyword.get_values(:x3m_service)
        |> List.flatten()
      end

      def registered_services(:private) do
        __MODULE__.__info__(:attributes)
        |> Keyword.get_values(:x3m_servicep)
        |> List.flatten()
      end

      def registered_services(:all),
        do: registered_services(:private) ++ registered_services(:public)

      @doc !"""
           Sends internal event for each service to be registered in runtime.
           """
      @spec register_services :: :ok
      def register_services do
        public_services =
          registered_services(:public)
          |> Enum.map(fn {service, _arrity} -> {service, __MODULE__} end)
          |> Enum.into(%{})

        private_services =
          registered_services(:private)
          |> Enum.map(fn {service, _arrity} -> {service, __MODULE__} end)
          |> Enum.into(%{})

        X3m.System.Instrumenter.execute(:register_local_services, %{}, %{
          public: public_services,
          private: private_services
        })

        :ok
      end

      @doc false
      @spec _invoke(:local | node(), atom, atom, Message.t()) :: :ok
      def _invoke(node, message_handler, f, message)

      def _invoke(:local, message_handler, f, message) do
        Logger.metadata(message.logger_metadata)
        mono_start = System.monotonic_time()

        X3m.System.Instrumenter.execute(
          :invoking_service,
          %{start: DateTime.utc_now(), mono_start: mono_start},
          %{
            node: Node.self(),
            service: message.service_name
          }
        )

        case apply(message_handler, f, [message]) do
          {:reply, %Message{} = message} ->
            message =
              case message do
                %Message{dry_run: :verbose} = msg ->
                  %Message{msg | request: nil}

                %Message{} = msg ->
                  %Message{msg | request: nil, events: []}
              end

            send(message.reply_to, message)

            X3m.System.Instrumenter.execute(
              :service_responded,
              %{
                time: DateTime.utc_now(),
                duration: X3m.System.Instrumenter.duration(mono_start)
              },
              %{message: message}
            )

            :ok

          :noreply ->
            :ok
        end
      end

      def _invoke(node, message_handler, f, message) do
        true =
          :rpc.cast(node, __MODULE__, :_invoke, [
            :local,
            message_handler,
            f,
            message
          ])

        :ok
      end

      @doc !"""
           Choose node on which MFA will be applied.

           This is optional callback. By default it will return `:local`,
           meaning that `sys_msg` will be handled by local module.

           It can be overriden like:

           ```
           def choose_node(%X3m.System.Message{}) do
             [:jobs_1@my_comp_name, :local] |> Enum.random()
           end
           ```
           """
      @spec choose_node(Message.t()) :: :local | node()
      def choose_node(_sys_msg),
        do: :local

      defoverridable choose_node: 1
    end
  end
end