Support for numeric genres in id3v2

TCON

    The 'Content type', which previously was stored as a one byte numeric value
only, is now a numeric string. You may use one or several of the types as
ID3v1.1 did or, since the category list would be impossible to maintain with
accurate and up to date categories, define your own.

    References to the ID3v1 genres can be made by, as first byte, enter "("
followed by a number from the genres list (appendix A) and ended with a ")"
character. This is optionally followed by a refinement, e.g. "(21)" or
"(4)Eurodisco". Several references can be made in the same frame, e.g.
"(51)(39)". If the refinement should begin with a "(" character it should be
replaced with "((", e.g. "((I can figure out any genre)" or "(55)((I
think...)". The following new content types is defined in ID3v2 and is
implemented in the same way as the numerig content types, e.g. "(RX)".

To test it, use the id3v2 tool

% id3v2 -g 79 test.mp3
% id3v2 -l test.mp3| grep TCON
TCON (Content type): Hard Rock (79)
% ./tag test.mp3| grep Genre
 Genre: (79)

With the patch :
% go build && ./tag test.mp3| grep Genre
 Genre: Hard Rock

id3v2.go

@@ -7,9 +7,41 @@ package tag
 import (
 	"fmt"
 	"io"
+	"regexp"
 	"strconv"
+	"strings"
 )
 
+var id3v2Genres = [...]string{
+	"Blues", "Classic Rock", "Country", "Dance", "Disco", "Funk", "Grunge",
+	"Hip-Hop", "Jazz", "Metal", "New Age", "Oldies", "Other", "Pop", "R&B",
+	"Rap", "Reggae", "Rock", "Techno", "Industrial", "Alternative", "Ska",
+	"Death Metal", "Pranks", "Soundtrack", "Euro-Techno", "Ambient",
+	"Trip-Hop", "Vocal", "Jazz+Funk", "Fusion", "Trance", "Classical",
+	"Instrumental", "Acid", "House", "Game", "Sound Clip", "Gospel",
+	"Noise", "AlternRock", "Bass", "Soul", "Punk", "Space", "Meditative",
+	"Instrumental Pop", "Instrumental Rock", "Ethnic", "Gothic",
+	"Darkwave", "Techno-Industrial", "Electronic", "Pop-Folk",
+	"Eurodance", "Dream", "Southern Rock", "Comedy", "Cult", "Gangsta",
+	"Top 40", "Christian Rap", "Pop/Funk", "Jungle", "Native American",
+	"Cabaret", "New Wave", "Psychedelic", "Rave", "Showtunes", "Trailer",
+	"Lo-Fi", "Tribal", "Acid Punk", "Acid Jazz", "Polka", "Retro",
+	"Musical", "Rock & Roll", "Hard Rock", "Folk", "Folk-Rock",
+	"National Folk", "Swing", "Fast Fusion", "Bebob", "Latin", "Revival",
+	"Celtic", "Bluegrass", "Avantgarde", "Gothic Rock", "Progressive Rock",
+	"Psychedelic Rock", "Symphonic Rock", "Slow Rock", "Big Band",
+	"Chorus", "Easy Listening", "Acoustic", "Humour", "Speech", "Chanson",
+	"Opera", "Chamber Music", "Sonata", "Symphony", "Booty Bass", "Primus",
+	"Porn Groove", "Satire", "Slow Jam", "Club", "Tango", "Samba",
+	"Folklore", "Ballad", "Power Ballad", "Rhythmic Soul", "Freestyle",
+	"Duet", "Punk Rock", "Drum Solo", "A capella", "Euro-House", "Dance Hall",
+	"Goa", "Drum & Bass", "Club-House", "Hardcore", "Terror", "Indie",
+	"Britpop", "Negerpunk", "Polsk Punk", "Beat", "Christian Gangsta Rap",
+	"Heavy Metal", "Black Metal", "Crossover", "Contemporary Christian",
+	"Christian Rock ", "Merengue", "Salsa", "Thrash Metal", "Anime", "JPop",
+	"Synthpop",
+}
+
 // id3v2Header is a type which represents an ID3v2 tag header.
 type id3v2Header struct {
 	Version           Format
@@ -342,3 +374,27 @@ func ReadID3v2Tags(r io.ReadSeeker) (Metadata, error) {
 	}
 	return metadataID3v2{header: h, frames: f}, nil
 }
+
+//  id3v2genre parse a id3v2 genre tag and expand the numeric genres
+func id3v2genre(genre string) string {
+	c := true
+	for c {
+		orig := genre
+		re := regexp.MustCompile("(.*[^(]|.* |^)\\(([0-9]+)\\) *(.*)$")
+		if match := re.FindStringSubmatch(genre); len(match) > 0 {
+			if genreId, err := strconv.Atoi(match[2]); err == nil {
+				if genreId < len(id3v2Genres) {
+					genre = id3v2Genres[genreId]
+					if match[1] != "" {
+						genre = strings.TrimSpace(match[1]) + " " + genre
+					}
+					if match[3] != "" {
+						genre = genre + " " + match[3]
+					}
+				}
+			}
+		}
+		c = (orig != genre)
+	}
+	return strings.Replace(genre, "((", "(", -1)
+}