#ID3v1 and genres

ID3v1 is the last 128 bytes of the file (ID3V1_SIZE): TAG and six fixed-width ISO-8859-1 fields. The byte layout is in the ID3 formats on disk.

def v1 as mp3.Id3v1 init mp3.readId3v1($data);
if ($v1.present) {
    io.printf("%s / %s / %s\n", $v1.artist, $v1.title, mp3.genreName($v1.genre));
}

Without a tag, readId3v1 returns present false, empty strings, track 0 and genre GENRE_NONE (255). Fields are read up to the first NUL and trimmed. genre is the raw byte, so it can be 192-254, which genreName maps to "".

#ID3v1.1

ID3v1.1 stores a track number in the last two bytes of the 30-byte comment: a NUL at comment byte 28 and a non-zero byte at 29. A tag with that marker reads with version11 true, track set and a 28-byte comment.

renderId3v1 always returns 128 bytes. It writes ID3v1.1 with a 28-byte comment when track is 1-255, and the plain 30-byte comment otherwise, so a track number outside 1-255 is dropped. present is ignored.

def v1 as mp3.Id3v1 init mp3.id3v1FromTag($tag);
def block as bytes init mp3.renderId3v1($v1);

#What is lost

id3v1FromTag keeps title, artist, album, year, commentText($tag), the track number and the genre. Everything else in a Tag is dropped. On render, text longer than its field is truncated, characters above U+00FF become ?, NULs are removed, and a genre outside 0-191 becomes GENRE_NONE. A genre name the table does not know also becomes GENRE_NONE.

$v1.title = "日本語";
mp3.readId3v1($written).title;   # "???"

$v1.title = "Ich war müde leer nicht mehr da";   # one character over
mp3.readId3v1($written).title;   # "Ich war müde leer nicht mehr d"

defaultOptions() does not write ID3v1. Turn it on for players that read nothing else:

def opts as mp3.WriteOptions init mp3.defaultOptions();
$opts.id3v1 = true;

optionsFor($track) writes ID3v1 when the file already had one.

#The genre table

GENRES holds 192 names (GENRE_COUNT): 0-79 are the original ID3v1 list, 80-125 the Winamp 1.x extensions, 126-147 Winamp 5.0 and 148-191 Winamp 5.6. 192 and up are unassigned; 255 is the conventional "no genre".

mp3.genreName(17);        # "Rock"
mp3.genreName(255);       # ""
mp3.genreNumber("rock");  # 17
mp3.genreNumber("Chiptune");   # -1

genreNumber ignores case and surrounding whitespace.

Index 133 is Afro-Punk, the name Winamp 5.6 gave the entry that replaced an earlier slur. Readers that emit the old spelling still agree on the number.

#TCON spellings

resolveGenre reads every form ID3v2 TCON has used:

Input is trimmed first. A name the table does not know, or a number outside it, is returned unchanged; a refinement after (n) wins over the number.

#Out of scope

The 227-byte TAG+ extended tag, Lyrics3 and APE tags are not implemented. They sit between the audio and the ID3v1 tag, and audioRange counts them as audio.