# frozen_string_literal: true

module WahWah
  module Mp3
    # mpeg frame header structure:
    #
    # Position   Length    Meaning
    # 0          11        Frame sync to find the header (all bits are always set)
    #
    # 11         2         Audio version ID
    #                      00 - MPEG Version 2.5 (unofficial extension of MPEG 2)
    #                      01 - reserved
    #                      10 - MPEG Version 2 (ISO/IEC 13818-3)
    #                      11 - MPEG Version 1 (ISO/IEC 11172-3)
    #
    # 13         2         Layer index
    #                      00 - reserved
    #                      01 - Layer III
    #                      10 - Layer II
    #                      11 - Layer I
    #
    # 15         1         Protection bit
    #
    # 16         4         Bitrate index, see FRAME_BITRATE_INDEX constant
    #
    # 20         2         Sampling rate index, see SAMPLE_RATE_INDEX constant
    #
    # 22         1         Padding bit
    #
    # 23         1         Private bit
    #
    # 24         2         Channel mode
    #                      00 - Stereo
    #                      01 - Joint Stereo (Stereo)
    #                      10 - Dual channel (Two mono channels)
    #                      11 - Single channel (Mono)
    #
    # 26         2         Mode extension (Only used in Joint Stereo)
    #
    # 28         1         Copyright bit (only informative)
    #
    # 29         1         Original bit (only informative)
    #
    # 30         2         Emphasis
    class MpegFrameHeader
      HEADER_SIZE = 4

      FRAME_BITRATE_INDEX = {
        'MPEG1 layer1' => [0, 32, 64, 96, 128, 160, 192, 224, 256, 288, 320, 352, 384, 416, 448, 0],
        'MPEG1 layer2' => [0, 32, 48, 56, 64, 80, 96, 112, 128, 160, 192, 224, 256, 320, 384, 0],
        'MPEG1 layer3' => [0, 32, 40, 48, 56, 64, 80, 96, 112, 128, 160, 192, 224, 256, 320, 0],

        'MPEG2 layer1' => [0, 32, 48, 56, 64, 80, 96, 112, 128, 144, 160, 176, 192, 224, 256, 0],
        'MPEG2 layer2' => [0, 8, 16, 24, 32, 40, 48, 56, 64, 80, 96, 112, 128, 144, 160, 0],
        'MPEG2 layer3' => [0, 8, 16, 24, 32, 40, 48, 56, 64, 80, 96, 112, 128, 144, 160, 0],

        'MPEG2.5 layer1' => [0, 32, 48, 56, 64, 80, 96, 112, 128, 144, 160, 176, 192, 224, 256, 0],
        'MPEG2.5 layer2' => [0, 8, 16, 24, 32, 40, 48, 56, 64, 80, 96, 112, 128, 144, 160, 0],
        'MPEG2.5 layer3' => [0, 8, 16, 24, 32, 40, 48, 56, 64, 80, 96, 112, 128, 144, 160, 0]
      }

      VERSIONS_INDEX = ['MPEG2.5', nil, 'MPEG2', 'MPEG1']
      LAYER_INDEX = [nil, 'layer3', 'layer2', 'layer1']
      CHANNEL_MODE_INDEX = ['Stereo', 'Joint Stereo', 'Dual Channel', 'Single Channel']

      SAMPLE_RATE_INDEX = {
        'MPEG1' => [44100, 48000, 32000],
        'MPEG2' => [22050, 24000, 16000],
        'MPEG2.5' => [11025, 12000, 8000]
      }

      SAMPLES_PER_FRAME_INDEX = {
        'MPEG1 layer1' => 384,
        'MPEG1 layer2' => 1152,
        'MPEG1 layer3' => 1152,

        'MPEG2 layer1' => 384,
        'MPEG2 layer2' => 1152,
        'MPEG2 layer3' => 576,

        'MPEG2.5 layer1' => 384,
        'MPEG2.5 layer2' => 1152,
        'MPEG2.5 layer3' => 576
      }

      attr_reader :version, :layer, :frame_bitrate, :channel_mode, :sample_rate

      def initialize(file_io, offset = 0)
        # mpeg frame header start with '11111111111' sync bits,
        # So look through file until find it.
        loop do
          file_io.rewind
          file_io.seek(offset)

          break if file_io.eof?

          header = file_io.read(HEADER_SIZE)
          sync_bits = header.unpack('B11').first

          if sync_bits == "#{'1' * 11}".b
            @header = header.unpack('B*').first
            @position = offset

            parse; break
          end

          offset += 1
        end
      end

      def valid?
        !@header.nil?
      end

      def position
        return 0 unless valid?
        @position
      end

      def kind
        return if @version.nil? && @layer.nil?
        "#{@version} #{@layer}"
      end

      def samples_per_frame
        SAMPLES_PER_FRAME_INDEX[kind]
      end

      private
        def parse
          return unless valid?

          @version = VERSIONS_INDEX[@header[11..12].to_i(2)]
          @layer = LAYER_INDEX[@header[13..14].to_i(2)]
          @frame_bitrate = FRAME_BITRATE_INDEX[kind]&.fetch(@header[16..19].to_i(2))
          @channel_mode = CHANNEL_MODE_INDEX[@header[24..25].to_i(2)]
          @sample_rate = SAMPLE_RATE_INDEX[@version]&.fetch(@header[20..21].to_i(2))
        end
    end
  end
end