defmodule Membrane.RTP.SSRCRouter do
  @moduledoc """
  A filter separating RTP packets from different SSRCs into different outpts.

  When receiving a new SSRC, it creates a new pad and notifies its parent (`t:new_stream_notification_t/0`) that should link
  the new output pad.
  """

  use Membrane.Filter

  alias Membrane.RTP

  def_input_pad :input, demand_unit: :buffers, caps: RTP, availability: :on_request

  def_output_pad :output, caps: RTP, availability: :on_request

  defmodule State do
    @moduledoc false
    use Bunch.Access

    alias Membrane.RTP

    @type t() :: %__MODULE__{
            pads: %{RTP.ssrc_t() => [input_pad :: Pad.ref_t()]},
            linking_buffers: %{RTP.ssrc_t() => [Membrane.Buffer.t()]}
          }

    defstruct pads: %{},
              linking_buffers: %{}
  end

  @typedoc """
  Notification sent when an RTP packet with new SSRC arrives and new output pad should be linked
  """
  @type new_stream_notification_t ::
          {:new_rtp_stream, RTP.ssrc_t(), RTP.payload_type_t()}

  @impl true
  def handle_init(_opts), do: {:ok, %State{}}

  @impl true
  def handle_pad_added(Pad.ref(:output, ssrc) = pad, _ctx, state) do
    {buffered_actions, state} = pop_in(state, [:linking_buffers, ssrc])
    {{:ok, [caps: {pad, %RTP{}}] ++ Enum.reverse(buffered_actions)}, state}
  end

  @impl true
  def handle_pad_added(Pad.ref(:input, _id), _ctx, state) do
    {:ok, state}
  end

  @impl true
  def handle_pad_removed(Pad.ref(:input, _) = pad, _ctx, state) do
    new_pads =
      state.pads
      |> Enum.filter(fn {_ssrc, p} -> p != pad end)
      |> Enum.into(%{})

    {:ok, %State{state | pads: new_pads}}
  end

  @impl true
  def handle_prepared_to_playing(ctx, state) do
    actions =
      ctx.pads
      |> Enum.filter(fn {_pad_ref, pad_data} -> pad_data.direction == :input end)
      |> Enum.map(fn {pad_ref, _pad_data} -> {:demand, {pad_ref, 1}} end)

    {{:ok, actions}, state}
  end

  @impl true
  def handle_caps(Pad.ref(:input, _ref), _caps, _ctx, state) do
    {:ok, state}
  end

  @impl true
  def handle_demand(Pad.ref(:output, ssrc), _size, _unit, ctx, state) do
    input_pad = state.pads[ssrc]
    {{:ok, demand: {input_pad, &(&1 + ctx.incoming_demand)}}, state}
  end

  @impl true
  def handle_process(Pad.ref(:input, _id) = pad, buffer, _ctx, state) do
    %{ssrc: ssrc, payload_type: payload_type} = buffer.metadata.rtp
    {new_stream_actions, state} = maybe_handle_new_stream(pad, ssrc, payload_type, state)
    {actions, state} = maybe_add_to_linking_buffer(:buffer, buffer, ssrc, state)
    {{:ok, new_stream_actions ++ actions}, state}
  end

  @impl true
  def handle_event(Pad.ref(:input, _id), event, _ctx, state) do
    {actions, state} =
      Enum.flat_map_reduce(state.pads, state, fn {ssrc, _input}, state ->
        maybe_add_to_linking_buffer(:event, event, ssrc, state)
      end)

    {{:ok, actions}, state}
  end

  @impl true
  def handle_event(pad, event, ctx, state) do
    super(pad, event, ctx, state)
  end

  defp maybe_handle_new_stream(pad, ssrc, payload_type, state) do
    if Map.has_key?(state.pads, ssrc) do
      {[], state}
    else
      state = state |> put_in([:pads, ssrc], pad) |> put_in([:linking_buffers, ssrc], [])
      {[notify: {:new_rtp_stream, ssrc, payload_type}], state}
    end
  end

  defp maybe_add_to_linking_buffer(type, value, ssrc, state) do
    action = {type, {Pad.ref(:output, ssrc), value}}

    if waiting_for_linking?(ssrc, state) do
      state = update_in(state, [:linking_buffers, ssrc], &[action | &1])
      actions = if type == :buffer, do: [demand: {state.pads[ssrc], &(&1 + 1)}], else: []
      {actions, state}
    else
      {[action], state}
    end
  end

  defp waiting_for_linking?(ssrc, %State{linking_buffers: lb}), do: Map.has_key?(lb, ssrc)
end