audio: add Gain effect

audio/effects.go

@@ -0,0 +1,26 @@
+package audio
+
+// Gain amplifies the wrapped Streamer. The output of the wrapped Streamer gets multiplied by
+// 1+Gain.
+//
+// Note that gain is not equivalent to the human perception of volume. Human perception of volume is
+// roughly exponential, while gain only amplifies linearly.
+type Gain struct {
+	Streamer Streamer
+	Gain     float64
+}
+
+// Stream streams the wrapped Streamer amplified by Gain.
+func (g *Gain) Stream(samples [][2]float64) (n int, ok bool) {
+	n, ok = g.Streamer.Stream(samples)
+	for i := range samples[:n] {
+		samples[i][0] *= 1 + g.Gain
+		samples[i][1] *= 1 + g.Gain
+	}
+	return n, ok
+}
+
+// Err propagates the wrapped Streamer's errors.
+func (g *Gain) Err() error {
+	return g.Streamer.Err()
+}