[utils] add various crossfading things

crates/utils/src/crossfade.rs

@@ -0,0 +1,30 @@
+/// Linear crossfade
+/// x is the crossfading param, [0, 1]
+/// Returns (fade_in, fade_out)
+pub fn lin_crossfade(x: f32) -> (f32, f32) {
+    (x, 1.0 - x)
+}
+
+// next two are from https://signalsmith-audio.co.uk/writing/2021/cheap-energy-crossfade/
+
+/// Amplitude preserving crossfade
+/// x is the crossfading param, [0, 1]
+/// Returns (fade_in, fade_out)
+pub fn ap_crossfade(x: f32) -> (f32, f32) {
+    let fin = x * x * (3.0 - 2.0 * x);
+
+    (fin, 1.0 - fin)
+}
+
+/// Energy preserving crossfade
+/// x is the crossfading param, [0, 1]
+/// Returns (fade_in, fade_out)
+pub fn ep_crossfade(x: f32) -> (f32, f32) {
+    let x2 = 1.0 - x;
+    let a = x * x2;
+    let b = a * (1.0 + 1.4186 * a);
+    let c = b + x;
+    let d = b + x2;
+
+    (c * c, d * d)
+}

crates/utils/src/lib.rs

@@ -1,4 +1,5 @@
 pub mod buffers;
+pub mod crossfade;
 pub mod delay;
 pub mod envelope;
 pub mod logs;