audio: Reduce magic numbers

exp/audio/audio.go

@@ -21,7 +21,6 @@ import (
 	"time"
 )
 
-// TODO: In JavaScript, mixing should be done by WebAudio for performance.
 type mixedPlayersStream struct {
 	context      *Context
 	writtenBytes int
@@ -34,12 +33,18 @@ func min(a, b int) int {
 	return b
 }
 
+const (
+	channelNum     = 2
+	bytesPerSample = 2
+	bitsPerSample  = bytesPerSample * 8
+)
+
 func (s *mixedPlayersStream) Read(b []byte) (int, error) {
 	s.context.Lock()
 	defer s.context.Unlock()
 
 	// TODO: 60 (FPS) is a magic number
-	bytesPerFrame := s.context.sampleRate * 4 / 60
+	bytesPerFrame := s.context.sampleRate * bytesPerSample * channelNum / 60
 	x := s.context.frames*bytesPerFrame + len(b)
 	if x <= s.writtenBytes {
 		return 0, nil

exp/audio/audio_js.go

@@ -75,9 +75,7 @@ func max64(a, b int64) int64 {
 }
 
 func (p *player) proceed() error {
-	const channelNum = 2
-	const bytesPerSample = channelNum * 16 / 8
-	bufferSize := p.sampleRate * bytesPerSample / 60
+	bufferSize := p.sampleRate * bytesPerSample * channelNum / 60
 	c := int64(p.context.Get("currentTime").Float() * float64(p.sampleRate))
 	if p.positionInSamples < c {
 		p.positionInSamples = c
@@ -85,7 +83,7 @@ func (p *player) proceed() error {
 	b := make([]byte, bufferSize)
 	n, err := p.src.Read(b)
 	if 0 < n {
-		buf := p.context.Call("createBuffer", channelNum, n/bytesPerSample, p.sampleRate)
+		buf := p.context.Call("createBuffer", channelNum, n/bytesPerSample/channelNum, p.sampleRate)
 		l := buf.Call("getChannelData", 0)
 		r := buf.Call("getChannelData", 1)
 		il, ir := toLR(b[:n])

exp/audio/audio_windows.go

@@ -78,12 +78,10 @@ type player struct {
 }
 
 func startPlaying(src io.Reader, sampleRate int) (*player, error) {
-	const numChannels = 2
-	const bitsPerSample = 16
-	const numBlockAlign = numChannels * bitsPerSample / 8
+	const numBlockAlign = channelNum * bitsPerSample / 8
 	f := C.WAVEFORMATEX{
 		wFormatTag:      C.WAVE_FORMAT_PCM,
-		nChannels:       numChannels,
+		nChannels:       channelNum,
 		nSamplesPerSec:  C.DWORD(sampleRate),
 		nAvgBytesPerSec: C.DWORD(sampleRate) * numBlockAlign,
 		wBitsPerSample:  bitsPerSample,