tag

You can install command-line tools to demonstrate tag extraction and audio summing:

1. Install the tools: go install github.com/dhowden/tag/cmd/tag@latest
2. Use ./tag <filename> to extract metadata.
3. Use ./sum <filename> to calculate the metadata-invariant SHA1 checksum.

$ go install github.com/dhowden/tag/cmd/tag@latest
$ ./tag 11\ High\ Hopes.m4a
$ ./sum 11\ High\ Hopes.m4a

Use tag.ReadFrom(r io.ReadSeeker) to detect and parse metadata from an audio file (such as an *os.File). The function returns a Metadata interface which provides a consistent API regardless of whether the underlying format is MP3, MP4, OGG, or FLAC.

m, err := tag.ReadFrom(f)
if err != nil {
	log.Fatal(err)
}
log.Print(m.Format()) // The detected format.
log.Print(m.Title())  // The title of the track

The oggDemuxer is an internal mechanism used by ReadOGGTags to parse OGG pages and reconstruct packets.

It handles:

1. Page Validation: Verifies the OggS magic number and validates the CRC32 checksum of the page header, segment table, and segment data.
2. Packet Reconstruction: OGG packets can span multiple pages. The demuxer uses the SerialNumber and the Flags (specifically the continuation bit) to buffer and reassemble fragmented packets.
3. Segment Handling: It processes the segment table to correctly slice the segmentsData into individual packets.

Note: While oggDemuxer is available in the package, it is primarily intended to support the high-level ReadOGGTags API.

When calling ReadID3v2Tags, the returned Metadata is an interface that encapsulates the parsed ID3v2 header and frames. The frames are stored in a map where keys are the frame names (e.g., TXXX, APIC, COMM) and values are the parsed content of those frames.

Common frame types handled include:

• Text Frames (starting with T): Parsed via readTFrame.
• Encoded Text Frames (starting with W): Parsed via readWFrame.
• User Defined Text Frames (TXXX, TXX): Parsed with description support.
• Comments/Lyrics (COMM, COM, USLT, ULT): Parsed with language and encoding support.
• Images (APIC, PIC): Parsed as picture frames.
• UFID (UFID, UFI): Parsed as Universal Unique Identifier frames.

When tag.ReadFrom cannot identify a metadata format or find any tags, it returns the ErrNoTagsFound error. You should check for this specific error to distinguish between a file that simply lacks metadata and a file that failed to parse due to other I/O issues.

var ErrNoTagsFound = errors.New("no tags found")

// Usage pattern:
m, err := tag.ReadFrom(f)
if err == tag.ErrNoTagsFound {
	// Handle file with no metadata
}

The -mbz flag uses the mbz.Extract(m) function to retrieve MusicBrainz-specific metadata from the provided tag.Metadata. The output is formatted as an indented JSON string.

tag -mbz filename.mp3

When using the -raw flag, the tool iterates through the map returned by m.Raw() and prints each key and value. Note that *tag.Picture values are printed using %v instead of the more verbose %#v to maintain readability.

tag -raw filename.mp3

The metadataID3v2 type implements the Metadata interface for ID3v2 tags. It provides methods to retrieve common audio metadata such as title, artist, album, and more. The library automatically handles the mapping of these fields to the correct ID3v2 frame names based on the tag version (ID3v2.2, ID3v2.3, or ID3v2.4).

// Assuming 'm' is an instance of Metadata implementing metadataID3v2
fmt.Println(m.Title())
fmt.Println(m.Artist())
fmt.Println(m.Album())
fmt.Println(m.AlbumArtist())
fmt.Println(m.Composer())
fmt.Println(m.Genre())
fmt.Println(m.Year())
fmt.Println(m.Track())
fmt.Println(m.Disc())
fmt.Println(m.Lyrics())
fmt.Println(m.Comment())

pic := m.Picture()
if pic != nil {
    // use picture
}

The Metadata interface returned by ReadAtoms for MP4 files provides the following methods to retrieve common tags:

• Title() string: Returns the track title.
• Artist() string: Returns the performer/artist.
• Album() string: Returns the album name.
• AlbumArtist() string: Returns the album artist.
• Composer() string: Returns the composer.
• Genre() string: Returns the genre.
• Year() int: Returns the year (extracted from the first 4 characters of the year atom).
• Track() (int, int): Returns the current track number and the total number of tracks in the album.
• Disc() (int, int): Returns the current disc number and the total number of discs.
• Lyrics() string: Returns the lyrics if present.
• Comment() string: Returns the comment field.
• Picture() *Picture: Returns a *Picture object containing image data (e.g., from the covr atom) if available.

Use tag.ReadFrom(r) to automatically detect and parse metadata from an io.ReadSeeker (such as an *os.File). The function identifies the format (ID3v1, ID3v2, MP4, FLAC, etc.) and returns a Metadata interface for accessing track information. If the format cannot be identified, it returns ErrNoTagsFound.

m, err := tag.ReadFrom(f)
if err != nil {
	log.Fatal(err)
}
log.Print(m.Format()) // The detected format.
log.Print(m.Title()) // The title of the track