iir: copy_within is better than rotate_right

dsp/src/iir.rs

@@ -159,10 +159,12 @@ impl IIR {
     /// * `xy` - Current filter state.
     /// * `x0` - New input.
     pub fn update(&self, xy: &mut IIRState, x0: f32) -> f32 {
+        let n = self.ba.len();
+        debug_assert!(xy.len() == n);
         // `xy` contains       x0 x1 y0 y1 y2
         // Increment time      x1 x2 y1 y2 y3
         // Rotate              y3 x1 x2 y1 y2
-        xy.rotate_right(1);
+        xy.copy_within(0..n - 1, 1);  // unrolls better than xy.rotate_right(1)
         // Store x0            x0 x1 x2 y1 y2
         xy[0] = x0;
         // Compute y0 by multiply-accumulate
@@ -170,7 +172,7 @@ impl IIR {
         // Limit y0
         let y0 = max(self.y_min, min(self.y_max, y0));
         // Store y0            x0 x1 y0 y1 y2
-        xy[xy.len() / 2] = y0;
+        xy[n / 2] = y0;
         y0
     }
 }