defmodule X3m.System.Dispatcher do
  @moduledoc """
  Sends a `X3m.System.Message` to whichever node offers its service and waits for the
  response.

  This is the client-facing entry point of the messaging layer. You build a message
  with `X3m.System.Message.new/2`, optionally `assign` values onto it, and then call
  `dispatch/2`:

      :open_account
      |> X3m.System.Message.new(raw_request: %{"id" => id})
      |> X3m.System.Dispatcher.dispatch()

  Service discovery is transparent: the dispatcher asks the (internal) service
  registry which nodes provide `message.service_name`. A **local** provider is invoked
  directly; **remote** providers are invoked over `:rpc`. The reply is delivered back
  to the calling process, so `dispatch/2` returns the resolved `X3m.System.Message`
  with its `response` set. See the "Distribution" guide for how nodes are chosen and
  how a service can ask the dispatcher to `:try_another_node`.

  Using the dispatcher does **not** require aggregates or event sourcing — any module
  registered through `X3m.System.Router` can be a dispatch target.
  """
  alias X3m.System.{Message, Response, Instrumenter, ServiceRegistry}

  @doc """
  Returns whether the (discovered) service authorizes `message`.

  Discovery is performed first; if no node offers the service `:service_unavailable`
  is returned. Otherwise authorization is delegated to the providing node's router
  (`X3m.System.Router` `authorize/1`).
  """
  @spec authorized?(Message.t()) :: boolean() | {:service_unavailable, atom}
  def authorized?(%Message{} = message) do
    mono_start = System.monotonic_time()
    message = %{message | invoked_at: DateTime.utc_now(), reply_to: self()}

    Instrumenter.execute(
      :checking_if_service_call_is_authorized,
      %{start: DateTime.utc_now(), mono_start: mono_start},
      %{message: message, caller_node: Node.self()}
    )

    message
    |> discover_service()
    |> case do
      :not_found ->
        :service_unavailable

      # we can ask any node for authorization
      [{node, mod} | _] ->
        _authorized?(node, mod, message)
    end
  end

  @doc """
  Sets `message.dry_run` to `true` if it was (by default) `false`
  and dispatches service call.

  Pay attention if you have some side effects (like persistence of unique values in DB)
  in your command handling. Such validations should be either avoided or
  Aggregate needs to implement `rollback/2` and `commit/2` callbacks.

  If service call is valid, `message.response` will be in `{:ok, aggregate_version}` format,
  otherwise response will have error message as it would have if dispatch was called.
  """
  @spec validate(Message.t()) :: Message.t()
  def validate(%Message{dry_run: false} = message) do
    message
    |> Map.put(:dry_run, true)
    |> dispatch()
  end

  def validate(%Message{} = message),
    do: dispatch(message)

  @doc """
  Discovers a node offering `message.service_name`, invokes the service there and
  returns the resolved `message` with its `response` set.

  A halted message (`halted?: true`) is returned untouched.

  Options:

    * `:timeout` - milliseconds to wait for the service reply (default `5_000`). On
      expiry the response is set to `Response.service_timeout/3`.

  If no node offers the service the response is set to `Response.service_unavailable/1`.
  When several nodes offer it, one is picked at random; a node may reply with
  `{:error, {:try_another_node, reason}}` to make the dispatcher try the next one.
  """
  @spec dispatch(Message.t()) :: Message.t()
  @spec dispatch(Message.t(), opts :: Keyword.t()) :: Message.t()
  def dispatch(%Message{halted?: true} = message), do: message

  def dispatch(%Message{} = message, opts \\ []) do
    timeout = Keyword.get(opts, :timeout, 5_000)
    mono_start = System.monotonic_time()
    message = %{message | invoked_at: DateTime.utc_now(), reply_to: self()}

    Instrumenter.execute(
      :discovering_service,
      %{start: DateTime.utc_now(), mono_start: mono_start},
      %{message: message, caller_node: Node.self()}
    )

    message
    |> discover_service()
    |> case do
      :not_found ->
        Instrumenter.execute(
          :service_not_found,
          %{time: DateTime.utc_now(), duration: Instrumenter.duration(mono_start)},
          %{message: message, caller_node: Node.self()}
        )

        _unavailable(message)

      nodes ->
        _try_on_nodes(nodes, message, fn node, mod, message ->
          _dispatch(node, mod, message, timeout, mono_start)
        end)
    end
  end

  defp _try_on_nodes(nodes, message, fun) when is_list(nodes) do
    {message, _} =
      nodes
      |> Enum.shuffle()
      |> Enum.reduce_while({%Message{} = message, []}, fn
        {node, mod}, {%Message{} = message, nodes} ->
          node
          |> fun.(mod, message)
          |> case do
            %Message{response: {:error, {:try_another_node, reason}}} = message ->
              nodes = [{node, reason} | nodes]
              message = %{message | response: {:error, {:no_nodes_available, nodes}}}
              {:cont, {message, nodes}}

            %Message{} = message ->
              {:halt, {message, nil}}
          end
      end)

    message
  end

  @doc """
  Looks up which nodes offer `message.service_name`.

  Returns `:not_found` when no node provides it, or a list of
  `{:local | node, router_module}` pairs otherwise. Used internally by `dispatch/2`
  and `authorized?/1`; exposed for introspection.
  """
  @spec discover_service(Message.t()) ::
          :not_found
          | [{:local | atom(), router_mod :: module()}]
  def discover_service(%Message{service_name: service}) do
    service
    |> ServiceRegistry.find_nodes_with_service()
    |> case do
      :not_found -> :not_found
      {:local, {mod, _fun}} -> [{:local, mod}]
      {:remote, nodes} -> Enum.into(nodes, [])
    end
  end

  defp _authorized?(:local, mod, %Message{} = message),
    do: apply(mod, :authorized?, [message])

  defp _authorized?(node, mod, %Message{} = message),
    do: :rpc.call(node, mod, :authorized?, [message])

  defp _dispatch(node, mod, %Message{} = message, timeout, mono_start) do
    Instrumenter.execute(
      :service_found,
      %{time: DateTime.utc_now(), duration: Instrumenter.duration(mono_start)},
      %{message: message, caller_node: Node.self(), service_node: node}
    )

    mono_start = System.monotonic_time()

    Instrumenter.execute(
      :invoking_service,
      %{start: DateTime.utc_now(), mono_start: mono_start},
      %{message: message, caller_node: Node.self(), service_node: node}
    )

    message = _dispatch(node, mod, message, timeout)

    Instrumenter.execute(
      :service_responded,
      %{time: DateTime.utc_now(), duration: Instrumenter.duration(mono_start)},
      %{message: message, caller_node: Node.self(), service_node: node}
    )

    message
  end

  defp _dispatch(:local, mod, %Message{} = message, timeout) do
    # if calling function does something with big binaries, they can leak if
    # caller process is long-lived (refc binary leaks)
    X3m.System.TaskSupervisor
    |> Task.Supervisor.start_child(fn ->
      :ok = apply(mod, message.service_name, [message])
    end)

    _wait_for_response(message, timeout)
  end

  defp _dispatch(node, mod, %Message{} = message, timeout) do
    :ok = :rpc.call(node, mod, message.service_name, [message])
    _wait_for_response(message, timeout)
  end

  defp _wait_for_response(%Message{id: message_id} = message, timeout) do
    receive do
      %Message{id: ^message_id} = message -> message
    after
      timeout ->
        response = Response.service_timeout(message.service_name, message.id, timeout)

        Message.return(message, response)
    end
  end

  defp _unavailable(%Message{service_name: service_name} = message) do
    response = Response.service_unavailable(service_name)

    Message.return(message, response)
  end
end