defmodule Membrane.HLS.TrimAlign do
  @moduledoc """
  Trims leading buffers so all input pads start at a common synchronization point.

  When H264 pads are present, synchronization is anchored on H264 keyframe cut points.
  The aligner chooses the earliest H264 cut point for which all non-H264 pads have already
  started (`first_ts <= cut_point`). If this is not satisfied, it advances to the next H264
  cut point and retries.

  Without H264 pads, synchronization falls back to the latest first cuttable timestamp among
  all pads.

  H264 pads can only be trimmed at keyframe boundaries and require parser metadata
  (`%Membrane.H264{alignment: :au, nalu_in_metadata?: true}`).
  """

  use Membrane.Filter, flow_control_hints?: false

  alias Membrane.Buffer

  @type cut_strategy :: :any | :h264_keyframe

  def_options(
    max_leading_trim: [
      spec: Membrane.Time.t(),
      default: Membrane.Time.seconds(3),
      description: """
      Maximum amount of leading content that can be trimmed from a single pad.
      """
    ],
    max_queued_buffers: [
      spec: pos_integer(),
      default: 2_000,
      description: """
      Maximum number of buffers allowed in a single pad queue before alignment is established.
      """
    ]
  )

  def_input_pad(:input,
    availability: :on_request,
    flow_control: :auto,
    accepted_format: _any,
    options: [
      cut_strategy: [
        spec: cut_strategy(),
        default: :any,
        description: """
        Defines where trimming can cut:
        * `:any` - cut at any buffer boundary
        * `:h264_keyframe` - cut only on H264 keyframe AUs
        """
      ]
    ]
  )

  def_output_pad(:output,
    availability: :on_request,
    flow_control: :auto,
    accepted_format: _any
  )

  @impl true
  def handle_init(_ctx, opts) do
    {[],
     %{
       pads: %{},
       alignment_reference: nil,
       max_leading_trim: opts.max_leading_trim,
       max_queued_buffers: opts.max_queued_buffers
     }}
  end

  @impl true
  def handle_pad_added({Membrane.Pad, :output, _id}, _ctx, state), do: {[], state}

  def handle_pad_added(pad, ctx, state) do
    pad_state = %{
      queue: :queue.new(),
      cut_strategy: ctx.pad_options[:cut_strategy] || :any,
      cut_candidate: nil,
      first_ts: nil,
      started?: false,
      ended?: false,
      eos_sent?: false
    }

    {[], put_in(state, [:pads, pad], pad_state)}
  end

  @impl true
  def handle_stream_format(pad, format, _ctx, state) do
    pad_state = Map.fetch!(state.pads, pad)
    validate_stream_format!(pad_state.cut_strategy, format, pad)

    {[stream_format: {output_pad(pad), format}], state}
  end

  @impl true
  def handle_buffer(pad, buffer, _ctx, %{alignment_reference: nil} = state) do
    ts = get_buffer_timestamp!(buffer)

    pad_state = Map.fetch!(state.pads, pad)
    updated_queue = :queue.in(buffer, pad_state.queue)

    if :queue.len(updated_queue) > state.max_queued_buffers do
      raise RuntimeError,
            """
            Alignment queue for pad #{inspect(pad)} exceeded max_queued_buffers=#{state.max_queued_buffers}.
            Increase :trim_align_max_queued_buffers or fix upstream timing skew.
            """
    end

    updated_pad_state =
      pad_state
      |> Map.put(:queue, updated_queue)
      |> maybe_set_first_timestamp(ts)
      |> maybe_set_cut_candidate(buffer, ts)

    state = put_in(state, [:pads, pad], updated_pad_state)

    state
    |> maybe_establish_alignment()
    |> maybe_emit_pending_eos()
  end

  def handle_buffer(pad, buffer, _ctx, state) do
    {[buffer: {output_pad(pad), buffer}], state}
  end

  @impl true
  def handle_end_of_stream(pad, _ctx, state) do
    state = update_in(state, [:pads, pad], &Map.put(&1, :ended?, true))

    if is_integer(state.alignment_reference) do
      state = update_in(state, [:pads, pad], &Map.put(&1, :eos_sent?, true))
      {[end_of_stream: output_pad(pad)], state}
    else
      state
      |> maybe_establish_alignment()
      |> maybe_emit_pending_eos()
    end
  end

  defp maybe_establish_alignment(%{alignment_reference: reference} = state)
       when is_integer(reference),
       do: {[], state}

  defp maybe_establish_alignment(state) do
    if state.pads == %{} or
         Enum.any?(state.pads, fn {_pad, data} -> is_nil(data.cut_candidate) end) do
      {[], state}
    else
      case resolve_alignment_reference(state.pads) do
        {:ok, reference} ->
          case build_alignment_actions(state, reference) do
            {:ok, actions, pads, aligned_reference} ->
              {actions, %{state | alignment_reference: aligned_reference, pads: pads}}

            :not_ready ->
              {[], state}

            {:error, reason} ->
              raise RuntimeError, reason
          end

        :not_ready ->
          {[], state}
      end
    end
  end

  defp maybe_emit_pending_eos({actions, %{alignment_reference: reference} = state})
       when is_integer(reference) do
    {eos_actions, pads} =
      Enum.reduce(state.pads, {[], %{}}, fn {pad, pad_state}, {eos_acc, pads_acc} ->
        if pad_state.ended? and not pad_state.eos_sent? do
          {
            [{:end_of_stream, output_pad(pad)} | eos_acc],
            Map.put(pads_acc, pad, %{pad_state | eos_sent?: true})
          }
        else
          {eos_acc, Map.put(pads_acc, pad, pad_state)}
        end
      end)

    {actions ++ Enum.reverse(eos_actions), %{state | pads: pads}}
  end

  defp maybe_emit_pending_eos(result), do: result

  defp resolve_alignment_reference(pads) do
    h264_candidates =
      pads
      |> Map.values()
      |> Enum.filter(&h264_pad?/1)
      |> Enum.map(& &1.cut_candidate)

    case h264_candidates do
      [] ->
        {:ok, latest_cut_candidate(pads)}

      _h264 ->
        h264_candidates
        |> Enum.min()
        |> resolve_h264_reference(pads)
    end
  end

  defp resolve_h264_reference(reference, pads) do
    with {:ok, h264_reference} <- next_common_h264_cut_point(pads, reference) do
      case latest_non_h264_start_after(pads, h264_reference) do
        nil -> {:ok, h264_reference}
        later_start -> resolve_h264_reference(later_start, pads)
      end
    end
  end

  defp next_common_h264_cut_point(pads, reference) do
    case next_h264_cut_points(pads, reference) do
      :not_ready ->
        :not_ready

      {:ok, []} ->
        {:ok, reference}

      {:ok, cut_points} ->
        next_reference = Enum.max(cut_points)

        if next_reference == reference do
          {:ok, reference}
        else
          next_common_h264_cut_point(pads, next_reference)
        end
    end
  end

  defp next_h264_cut_points(pads, reference) do
    Enum.reduce_while(pads, {:ok, []}, fn {_pad, pad_state}, {:ok, acc} ->
      if h264_pad?(pad_state) do
        case next_h264_cut_point(pad_state, reference) do
          :not_ready -> {:halt, :not_ready}
          ts -> {:cont, {:ok, [ts | acc]}}
        end
      else
        {:cont, {:ok, acc}}
      end
    end)
  end

  defp next_h264_cut_point(pad_state, reference) do
    buffers = :queue.to_list(pad_state.queue)

    case take_from_reference(buffers, pad_state.cut_strategy, reference) do
      :not_ready -> :not_ready
      %{first_forward_ts: ts} -> ts
    end
  end

  defp latest_non_h264_start_after(pads, reference) do
    pads
    |> Map.values()
    |> Enum.reject(&h264_pad?/1)
    |> Enum.map(& &1.first_ts)
    |> Enum.filter(&(is_integer(&1) and &1 > reference))
    |> Enum.max(fn -> nil end)
  end

  defp latest_cut_candidate(pads) do
    pads
    |> Map.values()
    |> Enum.map(& &1.cut_candidate)
    |> Enum.max()
  end

  defp h264_pad?(pad_state), do: pad_state.cut_strategy == :h264_keyframe

  defp build_alignment_actions(state, reference) do
    with {:ok, selection} <- select_buffers(state.pads, reference),
         :ok <- validate_trim_limits(state.pads, selection, state.max_leading_trim) do
      actions =
        selection
        |> Enum.reduce([], fn {pad, %{forward_buffers: forward_buffers}}, acc ->
          if forward_buffers == [] do
            acc
          else
            [{:buffer, {output_pad(pad), forward_buffers}} | acc]
          end
        end)
        |> Enum.reverse()

      pads =
        Enum.reduce(state.pads, %{}, fn {pad, pad_state}, acc ->
          Map.put(acc, pad, %{pad_state | queue: :queue.new(), started?: true})
        end)

      {:ok, actions, pads, reference}
    else
      :not_ready -> :not_ready
      {:error, _reason} = error -> error
    end
  end

  defp select_buffers(pads, reference) do
    Enum.reduce_while(pads, {:ok, %{}}, fn {pad, pad_state}, {:ok, acc} ->
      buffers = :queue.to_list(pad_state.queue)

      case take_from_reference(buffers, pad_state.cut_strategy, reference) do
        :not_ready -> {:halt, :not_ready}
        selection -> {:cont, {:ok, Map.put(acc, pad, selection)}}
      end
    end)
  end

  defp validate_trim_limits(pads, selection, max_leading_trim) do
    Enum.reduce_while(selection, :ok, fn {pad, %{first_forward_ts: first_forward_ts}}, :ok ->
      pad_state = Map.fetch!(pads, pad)
      trimmed_duration = first_forward_ts - pad_state.first_ts

      if trimmed_duration > max_leading_trim do
        {:halt,
         {:error,
          """
          Alignment on pad #{inspect(pad)} requires trimming #{format_time(trimmed_duration)},
          exceeding max_leading_trim=#{format_time(max_leading_trim)}.
          """}}
      else
        {:cont, :ok}
      end
    end)
  end

  defp take_from_reference(buffers, cut_strategy, reference) do
    index =
      Enum.find_index(buffers, fn buffer ->
        ts = get_buffer_timestamp!(buffer)
        ts >= reference and cuttable?(cut_strategy, buffer)
      end)

    case index do
      nil ->
        :not_ready

      index ->
        {trimmed, forward} = Enum.split(buffers, index)
        [first_forward | _rest] = forward

        %{
          trimmed_count: length(trimmed),
          forward_buffers: forward,
          first_forward_ts: get_buffer_timestamp!(first_forward)
        }
    end
  end

  defp maybe_set_first_timestamp(%{first_ts: nil} = pad_state, ts),
    do: %{pad_state | first_ts: ts}

  defp maybe_set_first_timestamp(pad_state, _ts), do: pad_state

  defp maybe_set_cut_candidate(%{cut_candidate: nil} = pad_state, buffer, ts) do
    if cuttable?(pad_state.cut_strategy, buffer) do
      %{pad_state | cut_candidate: ts}
    else
      pad_state
    end
  end

  defp maybe_set_cut_candidate(pad_state, _buffer, _ts), do: pad_state

  defp cuttable?(:any, _buffer), do: true

  defp cuttable?(:h264_keyframe, %Buffer{metadata: %{h264: %{key_frame?: key_frame?}}})
       when is_boolean(key_frame?),
       do: key_frame?

  defp cuttable?(:h264_keyframe, buffer) do
    raise RuntimeError,
          """
          H264 buffer is missing keyframe metadata required for alignment: #{inspect(buffer.metadata)}.
          Ensure input comes from Membrane.H264.Parser with AU alignment.
          """
  end

  defp validate_stream_format!(:any, _format, _pad), do: :ok

  defp validate_stream_format!(:h264_keyframe, %Membrane.H264{} = format, pad) do
    if format.alignment == :au and format.nalu_in_metadata? do
      :ok
    else
      raise RuntimeError,
            """
            Pad #{inspect(pad)} requires parsed H264 input for alignment.
            Expected %Membrane.H264{alignment: :au, nalu_in_metadata?: true}, got: #{inspect(format)}
            """
    end
  end

  defp validate_stream_format!(:h264_keyframe, format, pad) do
    raise RuntimeError,
          """
          Pad #{inspect(pad)} is configured with :h264_keyframe strategy but got non-H264 format: #{inspect(format)}
          """
  end

  defp get_buffer_timestamp!(buffer) do
    case Buffer.get_dts_or_pts(buffer) do
      ts when is_integer(ts) -> ts
      _ -> raise RuntimeError, "Alignment requires buffers with DTS or PTS"
    end
  end

  defp output_pad({Membrane.Pad, :input, id}), do: {Membrane.Pad, :output, id}

  defp format_time(time_ns) when is_integer(time_ns) do
    "#{Float.round(time_ns / Membrane.Time.second(), 3)}s"
  end
end