audio/mp3: Add Decoder.SampleRate

audio/mp3/decode_notjs.go

@@ -99,23 +99,32 @@ func (s *source) getFilepos() int {
 
 type Decoder struct {
 	source     *source
+	sampleRate int
 	length     int64
 	buf        []uint8
 	frame      *frame
 	eof        bool
 }
 
-func (d *Decoder) Read(buf []uint8) (int, error) {
-	for len(d.buf) == 0 && !d.eof {
+func (d *Decoder) read() error {
 	var err error
 	d.frame, err = d.source.readNextFrame(d.frame)
 	if err != nil {
 		if err == io.EOF {
 			d.eof = true
 		}
-			return 0, err
+		return err
 	}
 	d.buf = append(d.buf, d.frame.decodeL3()...)
+	return nil
+}
+
+// Read is io.Reader's Read.
+func (d *Decoder) Read(buf []uint8) (int, error) {
+	for len(d.buf) == 0 && !d.eof {
+		if err := d.read(); err != nil {
+			return 0, err
+		}
 	}
 	if d.eof {
 		return 0, io.EOF
@@ -125,6 +134,13 @@ func (d *Decoder) Read(buf []uint8) (int, error) {
 	return n, nil
 }
 
+// SampleRate returns the sample rate like 44100.
+//
+// Note that the sample rate is retrieved from the first frame.
+func (d *Decoder) SampleRate() int {
+	return d.sampleRate
+}
+
 // Length returns the total size in bytes.
 //
 // Length returns -1 when the total size is not available
@@ -160,5 +176,9 @@ func decode(r io.Reader) (*Decoder, error) {
 		}
 		d.length = l
 	}
+	if err := d.read(); err != nil {
+		return nil, err
+	}
+	d.sampleRate = samplingFrequency[d.frame.header.sampling_frequency]
 	return d, nil
 }

audio/mp3/types.go

@@ -110,7 +110,7 @@ var mpeg1Bitrates = map[mpeg1Layer][15]int{
 	},
 }
 
-var samplingFrequency = [3]int{44100, 48000, 32000}
+var samplingFrequency = []int{44100, 48000, 32000}
 
 func (h *mpeg1FrameHeader) frameSize() int {
 	return (144*mpeg1Bitrates[h.layer][h.bitrate_index])/