dwelling-radio/pkg/oggtag/oggtag.go

package oggtag

/* oggtag is a naive implementation of OGG tag's reader that is just looking
for certain tag names ending with an = character, e.g. artist= and title=.
*/

import (
	"bytes"
	"io"
	"os"
	"strings"
	"time"
)

const (
	bufferLength = 6144
	OggS         = "OggS"
	OggSLen      = len(OggS)
	Vorbis       = "vorbis"
	VorbisLen    = len(Vorbis)
)

// OggFile holds a head of a file and a tail part conatining last granule.
type OggFile struct {
	bufHead, bufLast []byte
}

// NewOggFile reads a file and returns a new OggFile.
func NewOggFile(path string) (*OggFile, error) {
	f, err := os.Open(path)
	if err != nil {
		return nil, err
	}
	defer f.Close()

	of := &OggFile{}

	of.bufHead = make([]byte, bufferLength)

	if _, err := f.Read(of.bufHead); err != nil {
		return nil, err
	}

	of.bufLast = make([]byte, bufferLength)

	if _, err := f.Seek(-bufferLength, io.SeekEnd); err != nil {
		return nil, err
	}

	fst, err := f.Stat()
	if err != nil {
		return nil, err
	}

	for offset := int64(bufferLength); offset <= fst.Size(); offset += bufferLength {
		if _, err := f.Seek(-offset, io.SeekEnd); err != nil {
			return nil, err
		}

		if _, err := f.Read(of.bufLast); err != nil {
			return nil, err
		}

		if bytes.Contains(of.bufLast, []byte(OggS)) {
			break
		}
	}

	return of, nil
}

// GetTag is searching for a certain tag and returns its value or an empty string.
func (of *OggFile) GetTag(tag string) string {
	tagIdx := bytes.Index(of.bufHead, append([]byte{0, 0, 0}, (tag+"=")...))
	if tagIdx == -1 {
		if tagIdx = bytes.Index(of.bufHead, append([]byte{0, 0, 0}, (strings.ToUpper(tag)+"=")...)); tagIdx == -1 {
			if tagIdx = bytes.Index(of.bufHead, append([]byte{0, 0, 0}, (strings.Title(tag)+"=")...)); tagIdx == -1 {
				return ""
			}
		}
	}
	tagIdx += 3
	tagNameLen := len(tag) + 1
	valStart := tagIdx + tagNameLen
	valLen := int(of.bufHead[tagIdx-4]) - tagNameLen
	return string(of.bufHead[valStart : valStart+valLen])
}

// GetDuration returns song's duration in milliseconds.
func (of *OggFile) GetDuration() time.Duration {
	rateIdx := bytes.Index(of.bufHead, []byte(Vorbis)) +
		VorbisLen + 5
	rateBytes := of.bufHead[rateIdx : rateIdx+4]
	rate := int32(rateBytes[0]) + int32(rateBytes[1])<<8 +
		int32(rateBytes[2])<<16 + int32(rateBytes[3])<<24

	granuleIdx := bytes.LastIndex(of.bufLast, []byte(OggS)) +
		OggSLen + 2
	granuleBytes := of.bufLast[granuleIdx : granuleIdx+8]
	granule := int64(granuleBytes[0]) + int64(granuleBytes[1])<<8 +
		int64(granuleBytes[2])<<16 + int64(granuleBytes[3])<<24 +
		int64(granuleBytes[4])<<32 + int64(granuleBytes[5])<<40 +
		int64(granuleBytes[6])<<48 + int64(granuleBytes[7])<<56

	return time.Duration(granule*1000/int64(rate)) * time.Millisecond
}