class FormatParser::WAVParser
  include FormatParser::IOUtils
  include FormatParser::DSL

  natures :audio
  formats :wav

  def call(io)
    # Read the RIFF header. Chunk descriptor should be RIFF, the size should 
    # contain the size of the entire file in bytes minus 8 bytes for the 
    # two fields not included in this count: chunk_id and size.
    chunk_id, size, riff_type = safe_read(io, 12).unpack('a4la4')

    # The chunk_id and riff_type should be RIFF and WAVE respectively
    return unless chunk_id == 'RIFF' && riff_type == 'WAVE'

    # There are no restrictions upon the order of the chunks within a WAVE file,
    # with the exception that the Format chunk must precede the Data chunk.
    # The specification does not require the Format chunk to be the first chunk
    # after the RIFF header.
    # http://soundfile.sapp.org/doc/WaveFormat/
    # For WAVE files containing PCM audio format we parse the 'fmt ' and
    # 'data' chunks while for non PCM audio formats the 'fmt ' and 'fact'
    # chunks. In the latter case the order fo appearence of the chunks is
    # arbitrary.
    fmt_processed = false
    fact_processed = false
    fmt_data = {}
    total_sample_frames = 0
    loop do
      chunk_type, chunk_size = safe_read(io, 8).unpack('a4l')
      case chunk_type
      when 'fmt ' # watch out: the chunk ID of the format chunk ends with a space
        fmt_data = unpack_fmt_chunk(io, chunk_size)
        if fmt_data[:audio_format] != 1 and fact_processed
          return process_non_pcm(fmt_data, total_sample_frames)
        end
        fmt_processed = true
      when 'data'
        return unless fmt_processed # the 'data' chunk cannot preceed the 'fmt ' chunk
        return process_pcm(fmt_data, chunk_size) if fmt_data[:audio_format] == 1
        safe_skip(io, chunk_size)
      when 'fact'
        total_sample_frames = safe_read(io, 4).unpack('l').first
        safe_skip(io, chunk_size - 4)
        if fmt_processed and fmt_data[:audio_format] != 1
          return process_non_pcm(fmt_data, total_sample_frames)
        end
        fact_processed = true
      else # Skip this chunk until a known chunk is encountered
        safe_skip(io, chunk_size)
      end
    end
  end

  def unpack_fmt_chunk(io, chunk_size)
    # The size of the fmt chunk is at least 16 bytes. If the format tag's value is not
    # 1 compression might be in use for storing the data
    # and the fmt chunk might contain extra fields appended to it.
    # The last 4 fields of the fmt tag are always:
    # * unsigned short     channels
    # * unsigned long      samples per sec
    # * unsigned long      average bytes per sec
    # * unsigned short     block align
    # * unsigned short     bits per sample

    fmt_info = safe_read(io, 16).unpack('S_2I2S_2')
    safe_skip(io, chunk_size - 16) # skip the extra fields

    {
      audio_format:    fmt_info[0],
      channels:        fmt_info[1],
      sample_rate:     fmt_info[2],
      byte_rate:       fmt_info[3],
      block_align:     fmt_info[4],
      bits_per_sample: fmt_info[5],
    }
  end

  def process_pcm(fmt_data, data_size)
    return unless fmt_data[:channels] > 0 and fmt_data[:bits_per_sample] > 0
    sample_frames = data_size / (fmt_data[:channels] * fmt_data[:bits_per_sample] / 8)
    file_info(fmt_data, sample_frames)
  end

  def process_non_pcm(fmt_data, total_sample_frames)
    file_info(fmt_data, total_sample_frames)
  end

  def file_info(fmt_data, sample_frames)
    return unless fmt_data[:sample_rate] > 0
    duration_in_seconds = sample_frames / fmt_data[:sample_rate].to_f
    FormatParser::Audio.new(
      format: :wav,
      num_audio_channels: fmt_data[:channels],
      audio_sample_rate_hz: fmt_data[:sample_rate],
      media_duration_frames: sample_frames,
      media_duration_seconds: duration_in_seconds,
    )
  end

  FormatParser.register_parser_constructor self
end