Class: FormatParser::MP3Parser

Inherits:
Object
  • Object
show all
Includes:
IOUtils
Defined in:
lib/parsers/mp3_parser.rb

Defined Under Namespace

Modules: ID3Extraction Classes: InvalidDeepFetch, MP3Info, MPEGFrame, TagWrapper, VBRHeader

Constant Summary collapse

MAX_FRAMES_TO_SCAN =

We limit the number of MPEG frames we scan to obtain our duration estimation

500
SAMPLES_PER_FRAME =

Default frame size for mp3

1152
ZIP_LOCAL_ENTRY_SIGNATURE =

For some edge cases

"PK\x03\x04\x14\x00".b
PNG_HEADER_BYTES = 
[137, 80, 78, 71, 13, 10, 26, 10].pack('C*')
MAGIC_LE = 
[0x49, 0x49, 0x2A, 0x0].pack('C4')
MAGIC_BE = 
[0x4D, 0x4D, 0x0, 0x2A].pack('C4')
TIFF_HEADER_BYTES = 
[MAGIC_LE, MAGIC_BE]
MP3_MIME_TYPE = 
'audio/mpeg'

Instance Method Summary collapse

Methods included from IOUtils

#safe_read, #safe_skip

Instance Method Details

#call(raw_io) ⇒ Object 



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# File 'lib/parsers/mp3_parser.rb', line 61

def call(raw_io)
  io = FormatParser::IOConstraint.new(raw_io)

  # Special case: some ZIPs (Office documents) did detect as MP3s.
  # To avoid having that happen, we check for the PKZIP signature -
  # local entry header signature - at the very start of the file.
  # If the file is too small safe_read will fail too and the parser
  # will terminate here. Same with PNGs. In the future
  # we should implement "confidence" for MP3 as of all our formats
  # it is by far the most lax.
  header = safe_read(io, 8)
  return if header.start_with?(ZIP_LOCAL_ENTRY_SIGNATURE)
  return if header.start_with?(PNG_HEADER_BYTES)

  io.seek(0)
  return if TIFF_HEADER_BYTES.include?(safe_read(io, 4))

  # Read all the ID3 tags (or at least attempt to)
  io.seek(0)
  id3v1 = ID3Extraction.attempt_id3_v1_extraction(io)
  tags = [id3v1, ID3Extraction.attempt_id3_v2_extraction(io)].compact

  io.seek(0) if tags.empty?

  # Compute how many bytes are occupied by the actual MPEG frames
  ignore_bytes_at_tail = id3v1 ? 128 : 0
  ignore_bytes_at_head = io.pos
  bytes_used_by_frames = io.size - ignore_bytes_at_head - ignore_bytes_at_tail

  io.seek(ignore_bytes_at_head)

  maybe_xing_header, initial_frames = parse_mpeg_frames(io)

  return if initial_frames.empty?

  first_frame = initial_frames.first

  id3tags_hash = with_id3tag_local_configs { blend_id3_tags_into_hash(*tags) }

  file_info = FormatParser::Audio.new(
    format: :mp3,
    # media_duration_frames is omitted because the frames
    # in MPEG are not the same thing as in a movie file - they
    # do not tell anything of substance
    num_audio_channels: first_frame.channels,
    audio_sample_rate_hz: first_frame.sample_rate,
    intrinsics: id3tags_hash.merge(id3tags: tags),
    content_type: MP3_MIME_TYPE,
  )

  extra_file_attirbutes = fetch_extra_attributes_from_id3_tags(id3tags_hash)

  extra_file_attirbutes.each do |name, value|
    file_info.send(:"#{name}=", value)
  end

  if maybe_xing_header
    duration = maybe_xing_header.frames * SAMPLES_PER_FRAME / first_frame.sample_rate.to_f
    _bit_rate = maybe_xing_header.byte_count * 8 / duration / 1000
    file_info.media_duration_seconds = duration
    return file_info
  end

  # Estimate duration using the frames we did parse - to have an exact one
  # we would need to have all the frames and thus read most of the file
  _avg_bitrate = float_average_over(initial_frames, :frame_bitrate)
  avg_frame_size = float_average_over(initial_frames, :frame_length)
  avg_sample_rate = float_average_over(initial_frames, :sample_rate)

  est_frame_count = bytes_used_by_frames / avg_frame_size
  est_samples = est_frame_count * SAMPLES_PER_FRAME
  est_duration_seconds = est_samples / avg_sample_rate

  # Safeguard for i.e. some JPEGs being recognized as MP3
  # to prevent ambiguous recognition
  return if est_duration_seconds == Float::INFINITY

  file_info.media_duration_seconds = est_duration_seconds
  file_info
end

#likely_match?(filename) ⇒ Boolean

Returns:

  • (Boolean) 


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# File 'lib/parsers/mp3_parser.rb', line 57

def likely_match?(filename)
  filename =~ /\.mp3$/i
end