diff --git a/src/buffer.rs b/src/buffer.rs
index 09308e0..63eef0a 100644
--- a/src/buffer.rs
+++ b/src/buffer.rs
@@ -16,10 +16,7 @@ impl Buf {
 
     // Truncate x and y and return a corresponding index into the data slice.
     const fn index(&self, x: f32, y: f32) -> usize {
-        // x/y can come in negative, hence we shift them by width/height.
-        let i = (x + self.width as f32) as usize & (self.width - 1);
-        let j = (y + self.height as f32) as usize & (self.height - 1);
-        j * self.width + i
+        crate::util::index(self.width, self.height, x, y)
     }
 
     // Get the buffer value at a given position. The implementation effectively treats data as periodic, hence any finite position will produce a value.
diff --git a/src/grid.rs b/src/grid.rs
index 3ac5443..848e721 100644
--- a/src/grid.rs
+++ b/src/grid.rs
@@ -78,11 +78,8 @@ impl Grid {
     }
 
     // Truncate x and y and return a corresponding index into the data slice.
-    fn index(&self, x: f32, y: f32) -> usize {
-        // x/y can come in negative, hence we shift them by width/height.
-        let i = (x + self.width as f32) as usize & (self.width - 1);
-        let j = (y + self.height as f32) as usize & (self.height - 1);
-        j * self.width + i
+    const fn index(&self, x: f32, y: f32) -> usize {
+        crate::util::index(self.width, self.height, x, y)
     }
 
     // Add a value to the grid data at a given position.
diff --git a/src/util.rs b/src/util.rs
index cc43420..542bf9f 100644
--- a/src/util.rs
+++ b/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
+}