⬥ ID3 encoding/decoding library in Go ⬥
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package id3ed
import (
"bytes"
"encoding/binary"
"fmt"
"io"
"os"
"strconv"
)
type ID3v11Tags struct {
SongName string
Artist string
Album string
Year int
Comment string
Track int
Genre string
}
// Retrieves ID3v1.1 field values of provided io.ReadSeeker
func GetID3v11Tags(rs io.ReadSeeker) (*ID3v11Tags, error) {
// set reader to the last 128 bytes
_, err := rs.Seek(-int64(ID3v1SIZE), io.SeekEnd)
if err != nil {
return nil, fmt.Errorf("could not seek: %s", err)
}
tag, err := read(rs, 3)
if err != nil {
return nil, err
}
if !bytes.Equal(tag, []byte("TAG")) {
// no TAG, given file does not use ID3v1
return nil, fmt.Errorf("does not use ID3v1: expected %s; got %s", "TAG", tag)
}
songname, err := readToString(rs, 30)
if err != nil {
return nil, err
}
artist, err := readToString(rs, 30)
if err != nil {
return nil, err
}
album, err := readToString(rs, 30)
if err != nil {
return nil, err
}
yearStr, err := readToString(rs, 4)
if err != nil {
return nil, err
}
year, err := strconv.Atoi(yearStr)
if err != nil {
return nil, fmt.Errorf("could not convert yearbytes into int: %s", err)
}
comment, err := readToString(rs, 28)
if err != nil {
return nil, err
}
// skip 1 null byte
_, err = read(rs, 1)
if err != nil {
return nil, err
}
trackByte, err := read(rs, 1)
if err != nil {
return nil, err
}
track, err := binary.ReadVarint(bytes.NewBuffer(trackByte))
if err != nil {
return nil, err
}
genreByte, err := read(rs, 1)
if err != nil {
return nil, err
}
genreInt, err := binary.ReadVarint(bytes.NewBuffer(genreByte))
if err != nil {
return nil, err
}
genre, exists := id3v1genres[int(genreInt)]
if !exists {
genre = ""
}
return &ID3v11Tags{
SongName: songname,
Artist: artist,
Album: album,
Year: year,
Comment: comment,
Track: int(track),
Genre: genre,
}, nil
}
// Writes given ID3v1.1 tags to dst
func (tags *ID3v11Tags) Write(dst io.WriteSeeker) error {
dst.Seek(0, io.SeekEnd)
// TAG
_, err := dst.Write([]byte("TAG"))
if err != nil {
return fmt.Errorf("could not write to dst: %s", err)
}
// Song name
err = writeToExtent(dst, []byte(tags.SongName), 30)
if err != nil {
return fmt.Errorf("could not write to dst: %s", err)
}
// Artist
err = writeToExtent(dst, []byte(tags.Artist), 30)
if err != nil {
return fmt.Errorf("could not write to dst: %s", err)
}
// Album
err = writeToExtent(dst, []byte(tags.Album), 30)
if err != nil {
return fmt.Errorf("could not write to dst: %s", err)
}
// Year
err = writeToExtent(dst, []byte(fmt.Sprint(tags.Year)), 4)
if err != nil {
return fmt.Errorf("could not write to dst: %s", err)
}
// Comment
err = writeToExtent(dst, []byte(tags.Comment), 28)
if err != nil {
return fmt.Errorf("could not write to dst: %s", err)
}
_, err = dst.Write([]byte{0})
if err != nil {
return fmt.Errorf("could not write to dst: %s", err)
}
// Track
trackBytes := make([]byte, 1)
binary.PutVarint(trackBytes, int64(tags.Track))
// binary.BigEndian.PutUint16(trackBytes, uint16(tags.Track))
_, err = dst.Write(trackBytes)
if err != nil {
return fmt.Errorf("could not write to dst: %s", err)
}
//Genre
genreCode := getKey(id3v1genres, tags.Genre)
if genreCode == -1 {
// if no genre found - encode genre code as 255
genreCode = ID3v1INVALIDGENRE
}
genrebyte := make([]byte, 1)
binary.PutVarint(genrebyte, int64(genreCode))
err = writeToExtent(dst, genrebyte, 1)
if err != nil {
return fmt.Errorf("could not write to dst: %s", err)
}
return nil
}
// Checks for existing ID3v1.1 tag in file, if present - removes it and replaces with provided tags
func (tags *ID3v11Tags) WriteToFile(f *os.File) error {
defer f.Close()
// check for existing ID3v1.1 tag
f.Seek(-int64(ID3v1SIZE), io.SeekEnd)
tag, err := read(f, 3)
if err != nil {
return err
}
if !bytes.Equal(tag, []byte("TAG")) {
// no existing tag, just write given tags
err = tags.Write(f)
if err != nil {
return err
}
return nil
}
// does contain ID3v1.1 tag. Removing it
fStats, err := f.Stat()
if err != nil {
return err
}
err = f.Truncate(fStats.Size() - int64(ID3v1SIZE))
if err != nil {
return nil
}
// writing new tags
err = tags.Write(f)
if err != nil {
return err
}
return nil
}