add Dup compositor

compositors.go

@@ -137,3 +137,37 @@ func Mix(s ...Streamer) Streamer {
 		return n, ok
 	})
 }
+
+// Dup returns two Streamers which both stream the same data as the original s. The two Streamers
+// can't be used concurrently without synchronization.
+func Dup(s Streamer) (t, u Streamer) {
+	var tBuf, uBuf [][2]float64
+	return &dup{&tBuf, &uBuf, s}, &dup{&uBuf, &tBuf, s}
+}
+
+type dup struct {
+	myBuf, itsBuf *[][2]float64
+	s             Streamer
+}
+
+func (d *dup) Stream(samples [][2]float64) (n int, ok bool) {
+	buf := *d.myBuf
+	n = copy(samples, buf)
+	ok = len(buf) > 0
+	buf = buf[n:]
+	samples = samples[n:]
+	*d.myBuf = buf
+
+	if len(samples) > 0 {
+		sn, sok := d.s.Stream(samples)
+		n += sn
+		ok = ok || sok
+		*d.itsBuf = append(*d.itsBuf, samples[:sn]...)
+	}
+
+	return n, ok
+}
+
+func (d *dup) Err() error {
+	return d.s.Err()
+}

compositors_test.go

@@ -109,3 +109,29 @@ func TestMix(t *testing.T) {
 		t.Error("Mix not working correctly")
 	}
 }
+
+func TestDup(t *testing.T) {
+	for i := 0; i < 7; i++ {
+		s, data := randomDataStreamer(rand.Intn(1e5) + 1e4)
+		st, su := beep.Dup(s)
+
+		var tData, uData [][2]float64
+		for {
+			buf := make([][2]float64, rand.Intn(1e4))
+			tn, tok := st.Stream(buf)
+			tData = append(tData, buf[:tn]...)
+
+			buf = make([][2]float64, rand.Intn(1e4))
+			un, uok := su.Stream(buf)
+			uData = append(uData, buf[:un]...)
+
+			if !tok && !uok {
+				break
+			}
+		}
+
+		if !reflect.DeepEqual(data, tData) || !reflect.DeepEqual(data, uData) {
+			t.Error("Dup not working correctly")
+		}
+	}
+}