flac: add constant encoding of samples

cmd/wav2flac/main.go

@@ -100,23 +100,14 @@ func wav2flac(wavPath string, force bool) error {
 	}
 
 	subframes := make([]*frame.Subframe, nchannels)
-	subHdr := frame.SubHeader{
-		// Specifies the prediction method used to encode the audio sample of the
-		// subframe.
-		Pred: frame.PredVerbatim,
-		// Prediction order used by fixed and FIR linear prediction decoding.
-		Order: 0,
-		// Wasted bits-per-sample.
-		Wasted: 0,
-	}
 	for i := range subframes {
 		subframe := &frame.Subframe{
-			SubHeader: subHdr,
-			Samples:   make([]int32, nsamplesPerChannel),
+			Samples: make([]int32, nsamplesPerChannel),
 		}
 		subframes[i] = subframe
 	}
-	for j := 0; !dec.EOF(); j++ {
+	for frameNum := 0; !dec.EOF(); frameNum++ {
+		fmt.Println("frame number:", frameNum)
 		// Decode WAV samples.
 		n, err := dec.PCMBuffer(buf)
 		if err != nil {
@@ -126,6 +117,16 @@ func wav2flac(wavPath string, force bool) error {
 			break
 		}
 		for _, subframe := range subframes {
+			subHdr := frame.SubHeader{
+				// Specifies the prediction method used to encode the audio sample of the
+				// subframe.
+				Pred: frame.PredVerbatim,
+				// Prediction order used by fixed and FIR linear prediction decoding.
+				Order: 0,
+				// Wasted bits-per-sample.
+				Wasted: 0,
+			}
+			subframe.SubHeader = subHdr
 			subframe.NSamples = n / nchannels
 			subframe.Samples = subframe.Samples[:subframe.NSamples]
 		}
@@ -133,7 +134,21 @@ func wav2flac(wavPath string, force bool) error {
 			subframe := subframes[i%nchannels]
 			subframe.Samples[i/nchannels] = int32(sample)
 		}
-		fmt.Println("j:", j)
+		// Check if the subframe may be encoded as constant; when all samples are
+		// the same.
+		for _, subframe := range subframes {
+			sample := subframe.Samples[0]
+			constant := true
+			for _, s := range subframe.Samples[1:] {
+				if sample != s {
+					constant = false
+				}
+			}
+			if constant {
+				fmt.Println("constant method")
+				subframe.SubHeader.Pred = frame.PredConstant
+			}
+		}
 
 		// Encode FLAC frame.
 		channels, err := getChannels(nchannels)

encode_subframe.go

@@ -18,20 +18,20 @@ func encodeSubframe(bw bitio.Writer, hdr frame.Header, subframe *frame.Subframe)
 
 	// Encode audio samples.
 	switch subframe.Pred {
-	//case frame.PredConstant:
-	//	if err := encodeConstantSamples(bw, samples); err != nil {
-	//		return errutil.Err(err)
-	//	}
+	case frame.PredConstant:
+		if err := encodeConstantSamples(bw, hdr.BitsPerSample, subframe.Samples); err != nil {
+			return errutil.Err(err)
+		}
 	case frame.PredVerbatim:
 		if err := encodeVerbatimSamples(bw, hdr, subframe.Samples); err != nil {
 			return errutil.Err(err)
 		}
 	//case frame.PredFixed:
-	//	if err := encodeFixedSamples(bw, samples, subframe.Order); err != nil {
+	//	if err := encodeFixedSamples(bw, hdr, subframe.Samples, subframe.Order); err != nil {
 	//		return errutil.Err(err)
 	//	}
 	//case frame.PredFIR:
-	//	if err := encodeFIRSamples(bw, samples, subframe.Order); err != nil {
+	//	if err := encodeFIRSamples(bw, hdr, subframe.Samples, subframe.Order); err != nil {
 	//		return errutil.Err(err)
 	//	}
 	default:
@@ -92,6 +92,23 @@ func encodeSubframeHeader(bw bitio.Writer, hdr frame.SubHeader) error {
 	return nil
 }
 
+// --- [ Constant samples ] ----------------------------------------------------
+
+// encodeConstantSamples stores the given constant sample, writing to bw.
+func encodeConstantSamples(bw bitio.Writer, bps byte, samples []int32) error {
+	sample := samples[0]
+	for _, s := range samples[1:] {
+		if sample != s {
+			return errutil.Newf("constant sample mismatch; expected %v, got %v", sample, s)
+		}
+	}
+	// Unencoded constant value of the subblock, n = frame's bits-per-sample.
+	if err := bw.WriteBits(uint64(sample), bps); err != nil {
+		return errutil.Err(err)
+	}
+	return nil
+}
+
 // --- [ Verbatim samples ] ----------------------------------------------------
 
 // encodeVerbatimSamples stores the given samples verbatim (uncompressed),