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 = 4096
const MagicSequence = "OggS"
const MagicSequenceLen = len(MagicSequence)
const VorbisSequence = "vorbis"
const VorbisSequenceLen = len(VorbisSequence)

// OggReadFile returns a head of an OGG file, and a buffer contains last frame.
func ReadFile(path string) (buf_start, buf_end []byte, _ error) {
	f, err := os.Open(path)
	if err != nil {
		return nil, nil, err
	}

	buf_start = make([]byte, bufferLength)

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

	buf_end = make([]byte, bufferLength)

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

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

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

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

		if bytes.Contains(buf_end, []byte(MagicSequence)) {
			break
		}
	}

	return
}

// OggGetTag is searching for a certain tag in a given buffer buf and returns
// its value.
//
// It is a simple implementation that doesn't parse a header and instead just
// looking for a certain tag preceded with three zero bytes and ends with
// an = character.
func GetTag(buf []byte, tag string) string {
	tagIdx := bytes.Index(buf, append([]byte{0, 0, 0}, (tag+"=")...))
	if tagIdx == -1 {
		tagIdx = bytes.Index(buf, append([]byte{0, 0, 0}, (strings.ToUpper(tag)+"=")...))
		if tagIdx == -1 {
			return ""
		}
	}
	tagIdx += 3
	tagNameLen := len(tag) + 1
	valStart := tagIdx + tagNameLen
	valLen := int(buf[tagIdx-4]) - tagNameLen
	return string(buf[valStart : valStart+valLen])
}

// OggGetDuration returns song's duration in milliseconds.
func GetDuration(buf_start, buf_end []byte) time.Duration {
	rateIdx := bytes.Index(buf_start, []byte(VorbisSequence)) +
		VorbisSequenceLen + 5
	rateBytes := buf_start[rateIdx : rateIdx+4]
	rate := int32(rateBytes[0]) + int32(rateBytes[1])<<8 +
		int32(rateBytes[2])<<16 + int32(rateBytes[3])<<24

	granuleIdx := bytes.LastIndex(buf_end, []byte(MagicSequence)) +
		MagicSequenceLen + 2
	granuleBytes := buf_end[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
}