mpeg_ts

defmodule MPEG.TS.PSI do
  @behaviour MPEG.TS.Unmarshaler

  @moduledoc """
  Program Specific Information payload. Supported tables are PMT and PAT.
  """

  @type header_t :: %{
          table_id: 0..3 | 16..31,
          section_syntax_indicator: boolean,
          section_length: 0..1021,
          transport_stream_id: 0..65_535,
          version_number: 0..31,
          current_next_indicator: boolean,
          section_number: 0..255,
          last_section_number: 0..255
        }
  @type t :: %__MODULE__{header: header_t(), table: bitstring()}
  defstruct [:header, :table]

  @crc_length 4
  @remaining_header_length 5

  @impl true
  def is_unmarshable?(data, is_start_unit) do
    case unmarshal_header(data, is_start_unit) do
      {:ok, _} -> true
      _ -> false
    end
  end

  @impl true
  def unmarshal(data, is_start_unit) do
    with {:ok, {header, data}} <- unmarshal_header(data, is_start_unit) do
      content_length = header.section_length - @crc_length - @remaining_header_length

      case data do
        <<raw_data::binary-size(content_length), _crc::@crc_length-binary, _::binary>> ->
          {:ok, %__MODULE__{header: header, table: raw_data}}

        _ ->
          {:error, :invalid_data}
      end
    end
  end

  def unmarshal_header(<<0::8, data::bitstring>>, true), do: unmarshal_header(data)
  def unmarshal_header(_, true), do: {:error, :invalid_header}

  def unmarshal_header(data, false), do: unmarshal_header(data)

  def unmarshal_header(<<
        table_id::8,
        section_syntax_indicator::1,
        0::1,
        _r1::2,
        # section length starts with 00
        0::2,
        section_length::10,
        transport_stream_id::16,
        _r2::2,
        version_number::5,
        current_next_indicator::1,
        section_number::8,
        last_section_number::8,
        rest::binary
      >>) do
    header = %{
      table_id: table_id,
      section_syntax_indicator: section_syntax_indicator == 1,
      section_length: section_length,
      transport_stream_id: transport_stream_id,
      version_number: version_number,
      current_next_indicator: current_next_indicator == 1,
      section_number: section_number,
      last_section_number: last_section_number
    }

    {:ok, {header, rest}}
  end

  def unmarshal_header(_), do: {:error, :invalid_header}

  defimpl MPEG.TS.Marshaler do
    import Bitwise

    @crc_length 4
    @remaining_header_length 5

    def marshal(%{header: header, table: table}) do
      section_length =
        if header.section_syntax_indicator,
          do: byte_size(table) + @remaining_header_length + @crc_length,
          else: byte_size(table) + @crc_length

      psi_header =
        <<header.table_id::8, bool_to_int(header.section_syntax_indicator)::1,
          _private_bit = 0::1, _reserved = 0b11::2, 0::2, section_length::10>>

      long_header =
        if header.section_syntax_indicator do
          <<header.transport_stream_id::16, 0b11::2, header.version_number::5,
            bool_to_int(header.current_next_indicator)::1, header.section_number::8,
            header.last_section_number::8>>
        else
          <<>>
        end

      payload = psi_header <> long_header <> table
      <<0, payload::binary, crc32(payload)::32>>
    end

    defp bool_to_int(true), do: 1
    defp bool_to_int(_), do: 0

    defp crc32(data) do
      data
      |> :binary.bin_to_list()
      |> Enum.reduce(0xFFFFFFFF, fn byte, crc ->
        crc = bxor(crc, byte <<< 24)

        Enum.reduce(0..7, crc, fn _i, crc ->
          if (crc &&& 0x80000000) == 0, do: crc <<< 1, else: bxor(crc <<< 1, 0x104C11DB7)
        end)
      end)
    end
  end
end