Implemented OggReadFile() func to get raw data from an OGG file. Also a func OggGetDuration() was implemented that returns duration of a song.

pkg/oggtag/oggtag.go

@@ -6,9 +6,57 @@ for certain tag names ending with an = character, e.g. artist= and title=.
 
 import (
 	"bytes"
+	"io"
+	"os"
 	"strings"
+	"time"
 )
 
+const bufferLength = 4096
+const OggMagicSequence = "OggS"
+const OggVorbisSequence = "vorbis"
+
+// OggReadFile returns a head of an OGG file, and a buffer contains last frame.
+func OggReadFile(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(OggMagicSequence)) {
+			break
+		}
+	}
+
+	return
+}
+
 // OggGetTag is searching for a certain tag in a given buffer buf and returns
 // its value.
 //
@@ -29,3 +77,22 @@ func OggGetTag(buf []byte, tag string) string {
 	valLen := int(buf[tagIdx-4]) - tagNameLen
 	return string(buf[valStart : valStart+valLen])
 }
+
+// OggGetDuration returns song's duration in milliseconds.
+func OggGetDuration(buf_start, buf_end []byte) time.Duration {
+	rateIdx := bytes.Index(buf_start, []byte(OggVorbisSequence)) +
+		len(OggVorbisSequence) + 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(OggMagicSequence)) +
+		len(OggMagicSequence) + 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
+}

pkg/oggtag/oggtag_test.go

@@ -1,6 +1,7 @@
 package oggtag
 
 import (
+	"bytes"
 	"os"
 	"testing"
 )
@@ -33,3 +34,31 @@ func BenchmarkOggGetTag(b *testing.B) {
 		OggGetTag(buf, "artist")
 	}
 }
+
+func TestOggReadFile(t *testing.T) {
+	bs, bf, err := OggReadFile(sampleSong)
+	if err != nil {
+		t.Error(err)
+	}
+
+	t.Log("bs = ", len(bs), "; bf =", len(bf), "bf(OggS) =", bytes.LastIndex(bf, []byte("OggS")))
+}
+
+func BenchmarkOggGetDuration(b *testing.B) {
+	bs, bf, err := OggReadFile(sampleSong)
+	if err != nil {
+		b.Error(err)
+	}
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		OggGetDuration(bs, bf)
+	}
+}
+
+func TestOggGetDuration(t *testing.T) {
+	bs, bf, err := OggReadFile(sampleSong)
+	if err != nil {
+		t.Error(err)
+	}
+	t.Log(OggGetDuration(bs, bf))
+}