effects: add Pan

effects/pan.go

@@ -0,0 +1,34 @@
+package effects
+
+import "github.com/faiface/beep"
+
+// Pan balances the wrapped Streamer between the left and the right channel. The Pan field value of
+// -1 means that both original channels go through the left channel. The value of +1 means the same
+// for the right channel. The value of 0 changes nothing.
+type Pan struct {
+	Streamer beep.Streamer
+	Pan      float64
+}
+
+// Stream streams the wrapped Streamer balanced by Pan.
+func (p *Pan) Stream(samples [][2]float64) (n int, ok bool) {
+	n, ok = p.Streamer.Stream(samples)
+	for i := range samples[:n] {
+		l := samples[i][0]
+		r := samples[i][1]
+		switch {
+		case p.Pan < 0:
+			samples[i][0] += -p.Pan * r
+			samples[i][1] -= -p.Pan * r
+		case p.Pan > 0:
+			samples[i][0] -= p.Pan * l
+			samples[i][1] += p.Pan * l
+		}
+	}
+	return n, ok
+}
+
+// Err propagates the wrapped Streamer's errors.
+func (p *Pan) Err() error {
+	return p.Streamer.Err()
+}