replace common index code

src/util.rs

@@ -2,3 +2,12 @@
 pub fn wrap(x: f32, max: f32) -> f32 {
     x - max * ((x > max) as i32 as f32 - (x < 0.0_f32) as i32 as f32)
 }
+
+// Truncate x and y and return a corresponding index into the data slice.
+#[inline]
+pub const fn index(width: usize, height: usize, x: f32, y: f32) -> usize {
+    // x/y can come in negative, hence we shift them by width/height.
+    let i = (x + width as f32) as usize & (width - 1);
+    let j = (y + height as f32) as usize & (height - 1);
+    j * width + i
+}