Fix handling of stereo data

Stereo data was being incorrectly copied to the frame buffer. Fix
that and at the same time move the whole copy loop to C which should
hopefully improve performance.

cfuncs.go

@@ -58,10 +58,14 @@ get_decoder_rate(FLAC__StreamMetadata *metadata)
      return metadata->data.stream_info.sample_rate;
 }
 
-extern int
-get_audio_sample(const FLAC__int32 **buffer, int off, int ch)
+extern void
+get_audio_samples(int32_t *output, const FLAC__int32 **input,
+                  unsigned int blocksize, unsigned int channels)
 {
-     return buffer[ch][off];
+    unsigned int i, j, samples = blocksize * channels;
+    for (i = 0; i < blocksize; i++)
+        for (j = 0; j < channels; j++)
+            output[i * channels + j] = input[j][i];
 }
 
 */

libflac.go

@@ -60,8 +60,9 @@ get_decoder_depth(FLAC__StreamMetadata *metadata);
 extern int
 get_decoder_rate(FLAC__StreamMetadata *metadata);
 
-extern int
-get_audio_sample(const FLAC__int32 **buffer, int off, int ch);
+extern void
+get_audio_samples(int32_t *output, const FLAC__int32 **input,
+                  unsigned int blocksize, unsigned int channels);
 
 */
 import "C"
@@ -119,12 +120,8 @@ func decoderWriteCallback(d *C.FLAC__StreamDecoder, frame *C.FLAC__Frame, buffer
 	f.Channels = decoder.Channels
 	f.Depth = decoder.Depth
 	f.Rate = decoder.Rate
-	f.Buffer = make([]int32, blocksize)
-	for i := 0; i < blocksize; i++ {
-		for ch := 0; ch < decoder.Channels; ch++ {
-			f.Buffer[i + ch] = int32(C.get_audio_sample(buffer, C.int(i), C.int(ch)))
-		}
-	}
+	f.Buffer = make([]int32, blocksize * decoder.Channels)
+	C.get_audio_samples((*C.int32_t)(&f.Buffer[0]), buffer, C.uint(blocksize), C.uint(decoder.Channels))
 	return C.FLAC__STREAM_DECODER_WRITE_STATUS_CONTINUE
 }
 

libflac_test.go

@@ -134,3 +134,51 @@ func TestRoundTrip(t *testing.T) {
 
 	os.Remove(outputFile)
 }
+
+func TestRoundTripStereo(t *testing.T) {
+	a := assert.New(t)
+
+	inputFile := "data/sine16-12.flac"
+	outputFile := "data/test.flac"
+
+	d, err := NewDecoder(inputFile)
+
+	a.Equal(err, nil, "err is nil")
+
+	e, err := NewEncoder(outputFile, d.Channels, d.Depth, d.Rate)
+
+	samples := 0
+
+	for {
+		f, err := d.ReadFrame()
+		if (err == nil || err == io.EOF) {
+			if (f != nil) {
+				_ = e.WriteFrame(*f)
+				samples = samples + len(f.Buffer)
+			}
+		} else {
+			a.Equal(err, nil, "error reported")
+			break
+		}
+
+		if (err == io.EOF) {
+			break
+		}
+	}
+
+	a.Equal(samples, 400000, "all samples read")
+	d.Close()
+	e.Close()
+
+	inFile, err := os.Open(inputFile)
+	a.Equal(err, nil, "err is nil")
+	outFile, err := os.Open(outputFile)
+	a.Equal(err, nil, "err is nil")
+
+	inData, _ := ioutil.ReadAll(inFile)
+	outData, _ := ioutil.ReadAll(outFile)
+
+	a.Equal(md5.Sum(inData), md5.Sum(outData), "files md5sum the same")
+
+	os.Remove(outputFile)
+}