Introduce Cycle::with_crossfade

src/cycle.rs

@@ -1,18 +1,36 @@
 use alloc::sync::Arc;
 use core::cell::Cell;
 
-use crate::{frame, math::Float, Frame, Frames, Seek, Signal};
+use crate::{frame, frame::lerp, math::Float, Frame, Frames, Seek, Signal};
 
 /// Loops [`Frames`] end-to-end to construct a repeating signal
+///
+/// To avoid glitching at the loop point, the underlying `Frames` *must* be specially prepared to
+/// provide a smooth transition.
+#[derive(Clone)]
 pub struct Cycle<T> {
     /// Current playback time, in samples
     cursor: Cell<f32>,
     frames: Arc<Frames<T>>,
 }
 
 impl<T> Cycle<T> {
+    /// Construct cycle from `frames`, smoothing the loop point over `crossfade_size` seconds
+    ///
+    /// Suitable for use with arbitrary audio,
+    pub fn with_crossfade(crossfade_size: f32, rate: u32, frames: &[T]) -> Self
+    where
+        T: Frame + Copy,
+    {
+        let mut frames = frames.iter().copied().collect::<alloc::vec::Vec<_>>();
+        let frames = apply_crossfade((crossfade_size * rate as f32) as usize, &mut frames);
+        Self::new(Frames::from_slice(rate, frames))
+    }
+
     /// Construct cycle from `frames`
-    // TODO: Crossfade
+    ///
+    /// `frames` *must* be specially constructed to loop seamlessly. For arbitrary audio, use
+    /// [`with_crossfade`](Self::with_crossfade) instead.
     pub fn new(frames: Arc<Frames<T>>) -> Self {
         Self {
             cursor: Cell::new(0.0),
@@ -52,6 +70,18 @@ impl<T: Frame + Copy> Seek for Cycle<T> {
     }
 }
 
+/// Prepare arbitrary frames for glitch-free use in `Cycle`
+fn apply_crossfade<T: Frame + Copy>(size: usize, frames: &mut [T]) -> &mut [T] {
+    let end = frames.len() - size;
+    for i in 0..size {
+        let a = frames[end + i];
+        let b = frames[i];
+        let t = (i + 1) as f32 / (size + 1) as f32;
+        frames[i] = lerp(&a, &b, t);
+    }
+    &mut frames[..end]
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -74,4 +104,14 @@ mod tests {
         s.sample(1.0, &mut buf[2..]);
         assert_eq!(buf, [1.0, 2.0, 3.0, 1.0, 2.0]);
     }
+
+    #[test]
+    fn crossfade() {
+        let mut frames = [0.0, 0.0, 0.0, 0.0, 0.0, 1.0, 1.0, 1.0, 1.0];
+        let faded = apply_crossfade(4, &mut frames);
+        assert_eq!(faded.len(), 5);
+        for i in 0..5 {
+            assert!((faded[i] - (5 - (i + 1)) as f32 / 5.0).abs() < 1e-3);
+        }
+    }
 }