audio/wav: add (*Stream).SampleRate

Closes #2996

audio/wav/decode.go

@@ -28,6 +28,7 @@ import (
 type Stream struct {
 	inner      io.ReadSeeker
 	size       int64
+	sampleRate int
 }
 
 // Read is implementation of io.Reader's Read.
@@ -49,6 +50,11 @@ func (s *Stream) Length() int64 {
 	return s.size
 }
 
+// SampleRate returns the sample rate of the decoded stream.
+func (s *Stream) SampleRate() int {
+	return s.sampleRate
+}
+
 type stream struct {
 	src        io.Reader
 	headerSize int64
@@ -114,7 +120,7 @@ func (s *stream) Seek(offset int64, whence int) (int64, error) {
 // A Stream doesn't close src even if src implements io.Closer.
 // Closing the source is src owner's responsibility.
 func DecodeWithoutResampling(src io.Reader) (*Stream, error) {
-	s, _, err := decode(src)
+	s, err := decode(src)
 	if err != nil {
 		return nil, err
 	}
@@ -138,36 +144,37 @@ func DecodeWithoutResampling(src io.Reader) (*Stream, error) {
 // Resampling can be a very heavy task. Stream has a cache for resampling, but the size is limited.
 // Do not expect that Stream has a resampling cache even after whole data is played.
 func DecodeWithSampleRate(sampleRate int, src io.Reader) (*Stream, error) {
-	s, origSampleRate, err := decode(src)
+	s, err := decode(src)
 	if err != nil {
 		return nil, err
 	}
 
-	if sampleRate == origSampleRate {
+	if sampleRate == s.sampleRate {
 		return s, nil
 	}
 
-	r := convert.NewResampling(s.inner, s.size, origSampleRate, sampleRate)
+	r := convert.NewResampling(s.inner, s.size, s.sampleRate, sampleRate)
 	return &Stream{
 		inner:      r,
 		size:       r.Length(),
+		sampleRate: sampleRate,
 	}, nil
 }
 
-func decode(src io.Reader) (*Stream, int, error) {
+func decode(src io.Reader) (*Stream, error) {
 	buf := make([]byte, 12)
 	n, err := io.ReadFull(src, buf)
 	if n != len(buf) {
-		return nil, 0, fmt.Errorf("wav: invalid header")
+		return nil, fmt.Errorf("wav: invalid header")
 	}
 	if err != nil {
-		return nil, 0, err
+		return nil, err
 	}
 	if !bytes.Equal(buf[0:4], []byte("RIFF")) {
-		return nil, 0, fmt.Errorf("wav: invalid header: 'RIFF' not found")
+		return nil, fmt.Errorf("wav: invalid header: 'RIFF' not found")
 	}
 	if !bytes.Equal(buf[8:12], []byte("WAVE")) {
-		return nil, 0, fmt.Errorf("wav: invalid header: 'WAVE' not found")
+		return nil, fmt.Errorf("wav: invalid header: 'WAVE' not found")
 	}
 
 	// Read chunks
@@ -181,10 +188,10 @@ chunks:
 		buf := make([]byte, 8)
 		n, err := io.ReadFull(src, buf)
 		if n != len(buf) {
-			return nil, 0, fmt.Errorf("wav: invalid header")
+			return nil, fmt.Errorf("wav: invalid header")
 		}
 		if err != nil {
-			return nil, 0, err
+			return nil, err
 		}
 		headerSize += 8
 		size := int64(buf[4]) | int64(buf[5])<<8 | int64(buf[6])<<16 | int64(buf[7])<<24
@@ -192,19 +199,19 @@ chunks:
 		case bytes.Equal(buf[0:4], []byte("fmt ")):
 			// Size of 'fmt' header is usually 16, but can be more than 16.
 			if size < 16 {
-				return nil, 0, fmt.Errorf("wav: invalid header: maybe non-PCM file?")
+				return nil, fmt.Errorf("wav: invalid header: maybe non-PCM file?")
 			}
 			buf := make([]byte, size)
 			n, err := io.ReadFull(src, buf)
 			if n != len(buf) {
-				return nil, 0, fmt.Errorf("wav: invalid header")
+				return nil, fmt.Errorf("wav: invalid header")
 			}
 			if err != nil {
-				return nil, 0, err
+				return nil, err
 			}
 			format := int(buf[0]) | int(buf[1])<<8
 			if format != 1 {
-				return nil, 0, fmt.Errorf("wav: format must be linear PCM")
+				return nil, fmt.Errorf("wav: format must be linear PCM")
 			}
 			channelCount := int(buf[2]) | int(buf[3])<<8
 			switch channelCount {
@@ -213,11 +220,11 @@ chunks:
 			case 2:
 				mono = false
 			default:
-				return nil, 0, fmt.Errorf("wav: number of channels must be 1 or 2 but was %d", channelCount)
+				return nil, fmt.Errorf("wav: number of channels must be 1 or 2 but was %d", channelCount)
 			}
 			bitsPerSample = int(buf[14]) | int(buf[15])<<8
 			if bitsPerSample != 8 && bitsPerSample != 16 {
-				return nil, 0, fmt.Errorf("wav: bits per sample must be 8 or 16 but was %d", bitsPerSample)
+				return nil, fmt.Errorf("wav: bits per sample must be 8 or 16 but was %d", bitsPerSample)
 			}
 			sampleRate = int(buf[4]) | int(buf[5])<<8 | int(buf[6])<<16 | int(buf[7])<<24
 			headerSize += size
@@ -228,10 +235,10 @@ chunks:
 			buf := make([]byte, size)
 			n, err := io.ReadFull(src, buf)
 			if n != len(buf) {
-				return nil, 0, fmt.Errorf("wav: invalid header")
+				return nil, fmt.Errorf("wav: invalid header")
 			}
 			if err != nil {
-				return nil, 0, err
+				return nil, err
 			}
 			headerSize += size
 		}
@@ -252,7 +259,11 @@ chunks:
 			dataSize *= 2
 		}
 	}
-	return &Stream{inner: s, size: dataSize}, sampleRate, nil
+	return &Stream{
+		inner:      s,
+		size:       dataSize,
+		sampleRate: sampleRate,
+	}, nil
 }
 
 // Decode decodes WAV (RIFF) data to playable stream.