105 lines
2.5 KiB
Go
105 lines
2.5 KiB
Go
package oggtag
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/* oggtag is a naive implementation of OGG tag's reader that is just looking
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for certain tag names ending with an = character, e.g. artist= and title=.
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*/
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import (
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"bytes"
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"io"
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"os"
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"strings"
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"time"
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)
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const (
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bufferLength = 6144
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OggS = "OggS"
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OggSLen = len(OggS)
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Vorbis = "vorbis"
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VorbisLen = len(Vorbis)
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)
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// OggFile holds a head of a file and a tail part conatining last granule.
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type OggFile struct {
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bufHead, bufTail []byte
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}
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// NewOggFile reads a file and returns a new OggFile.
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func NewOggFile(path string) (*OggFile, error) {
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f, err := os.Open(path)
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if err != nil {
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return nil, err
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}
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defer f.Close()
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of := &OggFile{}
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of.bufHead = make([]byte, bufferLength)
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if _, err := f.Read(of.bufHead); err != nil {
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return nil, err
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}
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of.bufTail = make([]byte, bufferLength)
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if _, err := f.Seek(-bufferLength, io.SeekEnd); err != nil {
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return nil, err
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}
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fst, err := f.Stat()
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if err != nil {
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return nil, err
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}
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for offset := int64(bufferLength); offset <= fst.Size(); offset += bufferLength {
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if _, err := f.Seek(-offset, io.SeekEnd); err != nil {
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return nil, err
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}
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if _, err := f.Read(of.bufTail); err != nil {
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return nil, err
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}
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if bytes.Contains(of.bufTail, []byte(OggS)) {
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break
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}
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}
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return of, nil
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}
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// GetTag is searching for a certain tag and returns its value or an empty string.
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func (of *OggFile) GetTag(tag string) string {
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tagIdx := bytes.Index(of.bufHead, append([]byte{0, 0, 0}, (tag+"=")...))
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if tagIdx == -1 {
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if tagIdx = bytes.Index(of.bufHead, append([]byte{0, 0, 0}, (strings.ToUpper(tag)+"=")...)); tagIdx == -1 {
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return ""
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}
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}
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tagIdx += 3
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tagNameLen := len(tag) + 1
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valStart := tagIdx + tagNameLen
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valLen := int(of.bufHead[tagIdx-4]) - tagNameLen
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return string(of.bufHead[valStart : valStart+valLen])
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}
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// GetDuration returns song's duration in milliseconds.
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func (of *OggFile) GetDuration() time.Duration {
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rateIdx := bytes.Index(of.bufHead, []byte(Vorbis)) +
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VorbisLen + 5
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rateBytes := of.bufHead[rateIdx : rateIdx+4]
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rate := int32(rateBytes[0]) + int32(rateBytes[1])<<8 +
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int32(rateBytes[2])<<16 + int32(rateBytes[3])<<24
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granuleIdx := bytes.LastIndex(of.bufTail, []byte(OggS)) +
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OggSLen + 2
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granuleBytes := of.bufTail[granuleIdx : granuleIdx+8]
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granule := int64(granuleBytes[0]) + int64(granuleBytes[1])<<8 +
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int64(granuleBytes[2])<<16 + int64(granuleBytes[3])<<24 +
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int64(granuleBytes[4])<<32 + int64(granuleBytes[5])<<40 +
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int64(granuleBytes[6])<<48 + int64(granuleBytes[7])<<56
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return time.Duration(granule*1000/int64(rate)) * time.Millisecond
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}
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