⬥ ID3 encoding/decoding library in Go ⬥
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package v1
import (
"bytes"
"encoding/binary"
"fmt"
"io"
"os"
"strconv"
"github.com/Unbewohnte/id3ed/util"
)
// https://id3.org/ID3v1 - documentation
type ID3v1Tags struct {
SongName string
Artist string
Album string
Year int
Comment string
Genre string
}
// Retrieves ID3v1 field values of provided io.ReadSeeker (usually a file)
func GetID3v1Tags(rs io.ReadSeeker) (*ID3v1Tags, 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 := util.Read(rs, 3)
if err != nil {
return nil, err
}
if !bytes.Equal(tag, []byte(ID3v1IDENTIFIER)) {
// no TAG, given file does not use ID3v1
return nil, fmt.Errorf("does not use ID3v1: expected %s; got %s", ID3v1IDENTIFIER, tag)
}
songname, err := util.ReadToString(rs, 30)
if err != nil {
return nil, err
}
artist, err := util.ReadToString(rs, 30)
if err != nil {
return nil, err
}
album, err := util.ReadToString(rs, 30)
if err != nil {
return nil, err
}
yearStr, err := util.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 := util.ReadToString(rs, 30)
if err != nil {
return nil, err
}
genreByte, err := util.Read(rs, 1)
if err != nil {
return nil, err
}
genreInt, err := util.BytesToInt(genreByte)
if err != nil {
return nil, err
}
genre, exists := id3v1genres[int(genreInt)]
if !exists {
genre = ""
}
return &ID3v1Tags{
SongName: songname,
Artist: artist,
Album: album,
Year: year,
Comment: comment,
Genre: genre,
}, nil
}
// Writes given ID3v1.0 tags to given io.WriteSeeker.
// NOTE: will not remove already existing ID3v1 tag if it`s present,
// use ⁕WriteToFile⁕ method if you`re working with REAL mp3 files !!!
func (tags *ID3v1Tags) Write(dst io.WriteSeeker) error {
dst.Seek(0, io.SeekEnd)
// TAG
_, err := dst.Write([]byte(ID3v1IDENTIFIER))
if err != nil {
return err
}
// Song name
err = util.WriteToExtent(dst, []byte(tags.SongName), 30)
if err != nil {
return err
}
// Artist
err = util.WriteToExtent(dst, []byte(tags.Artist), 30)
if err != nil {
return err
}
// Album
err = util.WriteToExtent(dst, []byte(tags.Album), 30)
if err != nil {
return err
}
// Year
err = util.WriteToExtent(dst, []byte(fmt.Sprint(tags.Year)), 4)
if err != nil {
return err
}
// Comment
err = util.WriteToExtent(dst, []byte(tags.Comment), 30)
if err != nil {
return err
}
// Genre
genreCode := util.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 = dst.Write(genrebyte)
if err != nil {
return err
}
return nil
}
// Checks for existing ID3v1 tag in file, if present - removes it and replaces with provided tags
func (tags *ID3v1Tags) WriteToFile(f *os.File) error {
defer f.Close()
// check for existing ID3v1 tag
f.Seek(-int64(ID3v1SIZE), io.SeekEnd)
tag, err := util.Read(f, 3)
if err != nil {
return err
}
if !bytes.Equal(tag, []byte(ID3v1IDENTIFIER)) {
// no existing tag, just write given tags
err = tags.Write(f)
if err != nil {
return err
}
return nil
}
// does contain ID3v1 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
}