Added mbz subpackage for extracting MusicBrainz-specific tags.

This is a rewrite of pull request #11 which was a first attempt at this (thanks @xhenner for starting this).

mbz/mbz.go

@@ -0,0 +1,125 @@
+// Package mbz extracts MusicBrainz Picard-specific tags from general tag metadata.
+// See https://picard.musicbrainz.org/docs/mappings/ for more information.
+package mbz
+
+import (
+	"strings"
+
+	"github.com/dhowden/tag"
+)
+
+// Info is a structure which contains MusicBrainz identifier information.
+type Info struct {
+	AcoustID     string
+	Album        string
+	AlbumArtist  string
+	Artist       string
+	ReleaseGroup string
+	Track        string
+}
+
+// Supported MusicBrainz tag names
+const (
+	TagAcoustID     = "acoustid_id"
+	TagAlbum        = "musicbrainz_albumid"
+	TagAlbumArtist  = "musicbrainz_albumartistid"
+	TagArtist       = "musicbrainz_artistid"
+	TagReleaseGroup = "musicbrainz_releasegroupid"
+	TagTrack        = "musicbrainz_recordingid"
+)
+
+// UFIDProviderURL is the URL that we match inside a UFID tag.
+const UFIDProviderURL = "http://musicbrainz.org"
+
+// Mapping between the internal picard tag names and aliases.
+var tags = map[string]string{
+	TagAcoustID:     "Acoustid Id",
+	TagAlbum:        "MusicBrainz Album Id",
+	TagAlbumArtist:  "MusicBrainz Album Artist Id",
+	TagArtist:       "MusicBrainz Artist Id",
+	TagReleaseGroup: "MusicBrainz Release Group Id",
+	TagTrack:        "MusicBrainz Track Id",
+}
+
+func (i *Info) set(t, v string) {
+	switch t {
+	case TagAcoustID:
+		i.AcoustID = v
+	case TagAlbum:
+		i.Album = v
+	case TagAlbumArtist:
+		i.AlbumArtist = v
+	case TagArtist:
+		i.Artist = v
+	case TagReleaseGroup:
+		i.ReleaseGroup = v
+	case TagTrack:
+		i.Track = v
+	}
+}
+
+// Set the MusicBrainz tag to the given value.
+func (i *Info) Set(t, v string) {
+	if _, ok := tags[t]; ok {
+		i.set(t, v)
+		return
+	}
+
+	for k, tt := range tags {
+		if tt == t {
+			i.set(k, v)
+			return
+		}
+	}
+}
+
+// extractID3 attempts to extract MusicBrainz Picard tags from m.Raw(), where m.Format
+// is assumed to be a supported version of ID3.
+func extractID3(m tag.Metadata) *Info {
+	var txxx, ufid string
+	switch m.Format() {
+	case tag.ID3v2_2:
+		txxx, ufid = "TXX", "UFI"
+	case tag.ID3v2_3, tag.ID3v2_4:
+		txxx, ufid = "TXXX", "UFID"
+	}
+
+	i := &Info{}
+	for k, v := range m.Raw() {
+		switch {
+		case strings.HasPrefix(k, txxx):
+			if str, ok := v.(*tag.Comm); ok {
+				i.Set(str.Description, str.Text)
+			}
+		case strings.HasPrefix(k, ufid):
+			if id, ok := v.(*tag.UFID); ok {
+				if id.Provider == UFIDProviderURL {
+					i.Set(TagTrack, string(id.Identifier))
+				}
+			}
+		}
+	}
+	return i
+}
+
+// extractMP4Vorbis attempts to extract MusicBrainz Picard tags from m.Raw(), where m.Format
+// is assumed to be MP4 or VORBIS.
+func extractMP4Vorbis(m tag.Metadata) *Info {
+	i := &Info{}
+	for t, v := range m.Raw() {
+		if s, ok := v.(string); ok {
+			i.Set(t, s)
+		}
+	}
+	return i
+}
+
+// Extract tags created by MusicBrainz Picard which can be used with with the MusicBrainz and LastFM APIs.
+// See https://picard.musicbrainz.org/docs/mappings/ for more information.
+func Extract(m tag.Metadata) *Info {
+	switch m.Format() {
+	case tag.ID3v2_2, tag.ID3v2_3, tag.ID3v2_4:
+		return extractID3(m)
+	}
+	return extractMP4Vorbis(m)
+}