101 lines
2.7 KiB
Go
101 lines
2.7 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 bufferLength = 4096
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const MagicSequence = "OggS"
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const MagicSequenceLen = len(MagicSequence)
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const VorbisSequence = "vorbis"
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const VorbisSequenceLen = len(VorbisSequence)
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// OggReadFile returns a head of an OGG file, and a buffer contains last frame.
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func ReadFile(path string) (buf_start, buf_end []byte, _ error) {
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f, err := os.Open(path)
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if err != nil {
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return nil, nil, err
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}
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buf_start = make([]byte, bufferLength)
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if _, err := f.Read(buf_start); err != nil {
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return nil, nil, err
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}
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buf_end = make([]byte, bufferLength)
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if _, err := f.Seek(-bufferLength, io.SeekEnd); err != nil {
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return nil, 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, 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, nil, err
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}
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if _, err := f.Read(buf_end); err != nil {
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return nil, nil, err
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}
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if bytes.Contains(buf_end, []byte(MagicSequence)) {
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break
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}
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}
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return
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}
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// OggGetTag is searching for a certain tag in a given buffer buf and returns
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// its value.
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//
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// It is a simple implementation that doesn't parse a header and instead just
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// looking for a certain tag preceded with three zero bytes and ends with
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// an = character.
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func GetTag(buf []byte, tag string) string {
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tagIdx := bytes.Index(buf, append([]byte{0, 0, 0}, (tag+"=")...))
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if tagIdx == -1 {
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tagIdx = bytes.Index(buf, append([]byte{0, 0, 0}, (strings.ToUpper(tag)+"=")...))
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if 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(buf[tagIdx-4]) - tagNameLen
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return string(buf[valStart : valStart+valLen])
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}
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// OggGetDuration returns song's duration in milliseconds.
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func GetDuration(buf_start, buf_end []byte) time.Duration {
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rateIdx := bytes.Index(buf_start, []byte(VorbisSequence)) +
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VorbisSequenceLen + 5
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rateBytes := buf_start[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(buf_end, []byte(MagicSequence)) +
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MagicSequenceLen + 2
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granuleBytes := buf_end[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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