// HasID3V1Tag returns true if an ID3v1 tag is present at the end of r.
func HasID3v1Tag(r readerutil.SizeReaderAt) (bool, error) {
if r.Size() < ID3v1TagLength {
return false, nil
}
buf := make([]byte, len(id3v1Magic), len(id3v1Magic))
if _, err := r.ReadAt(buf, r.Size()-ID3v1TagLength); err != nil {
return false, fmt.Errorf("Failed to read ID3v1 data: %v", err)
}
if bytes.Equal(buf, id3v1Magic) {
return true, nil
}
return false, nil
}
// GetMPEGAudioDuration reads the first frame in r and returns the audio length with millisecond precision.
// Format details are at http://www.codeproject.com/Articles/8295/MPEG-Audio-Frame-Header.
func GetMPEGAudioDuration(r readerutil.SizeReaderAt) (time.Duration, error) {
var header uint32
if err := binary.Read(io.NewSectionReader(r, 0, r.Size()), binary.BigEndian, &header); err != nil {
return 0, fmt.Errorf("Failed to read MPEG frame header: %v", err)
}
getBits := func(startBit, numBits uint) uint32 {
return (header << startBit) >> (32 - numBits)
}
if getBits(0, 11) != 0x7ff {
return 0, errors.New("Missing sync bits in MPEG frame header")
}
var version mpegVersion
var ok bool
if version, ok = mpegVersionsById[getBits(11, 2)]; !ok {
return 0, errors.New("Invalid MPEG version index")
}
var layer mpegLayer
if layer, ok = mpegLayersByIndex[getBits(13, 2)]; !ok {
return 0, errors.New("Invalid MPEG layer index")
}
bitrate := mpegBitrates[version][layer][getBits(16, 4)]
if bitrate == 0 {
return 0, errors.New("Invalid MPEG bitrate")
}
samplingRate := mpegSamplingRates[version][getBits(20, 2)]
if samplingRate == 0 {
return 0, errors.New("Invalid MPEG sample rate")
}
samplesPerFrame := mpegSamplesPerFrame[version][layer]
var xingHeaderStart int64 = 4
// Skip "side information".
if getBits(24, 2) == 0x3 { // Channel mode; 0x3 is mono.
xingHeaderStart += 17
} else {
xingHeaderStart += 32
}
// Skip 16-bit CRC if present.
if getBits(15, 1) == 0x0 { // 0x0 means "has protection".
xingHeaderStart += 2
}
b := make([]byte, 12, 12)
if _, err := r.ReadAt(b, xingHeaderStart); err != nil {
return 0, fmt.Errorf("Unable to read Xing header at %d: %v", xingHeaderStart, err)
}
var ms int64
if bytes.Equal(b[0:4], xingHeaderName) || bytes.Equal(b[0:4], infoHeaderName) {
r := bytes.NewReader(b[4:])
var xingFlags uint32
binary.Read(r, binary.BigEndian, &xingFlags)
if xingFlags&0x1 == 0x0 {
return 0, fmt.Errorf("Xing header at %d lacks number of frames", xingHeaderStart)
}
var numFrames uint32
binary.Read(r, binary.BigEndian, &numFrames)
ms = int64(samplesPerFrame) * int64(numFrames) * 1000 / int64(samplingRate)
} else {
// Okay, no Xing VBR header. Assume that the file has a constant bitrate.
// (The other alternative is to read the whole file and examine each frame.)
ms = r.Size() / int64(bitrate) * 8
}
return time.Duration(ms) * time.Millisecond, nil
}
