diff --git a/src/space_fill.rs b/src/space_fill.rs
index 67a7de9..39f417c 100644
--- a/src/space_fill.rs
+++ b/src/space_fill.rs
@@ -9,6 +9,7 @@ pub mod hilbert {
     ///
     /// x: The x coordinate.  Must be no greater than 2^16-1.
     /// y: The y coordinate.  Must be no greater than 2^16-1.
+    /// n: Basically the "resolution" of the curve, on one side.
     ///
     /// Returns the hilbert curve index corresponding to the (x,y) coordinates given.
     pub fn encode(x: u32, y: u32, n: u32) -> u32 {
@@ -33,6 +34,7 @@ pub mod hilbert {
     /// Convert hilbert curve index to (x,y).
     ///
     /// d: The hilbert curve index.
+    /// n: Basically the "resolution" of the curve, on one side.
     ///
     /// Returns the (x, y) coords at the given index.
     pub fn decode(d: u32, n: u32) -> (u32, u32) {
@@ -197,8 +199,8 @@ mod tests {
     #[test]
     fn hilbert_reversible() {
         let i = 0x4c8587a2;
-        let (x, y) = hilbert::decode(i);
-        let i2 = hilbert::encode(x, y);
+        let (x, y) = hilbert::decode(i, 1 << 16);
+        let i2 = hilbert::encode(x, y, 1 << 16);
 
         assert_eq!(i, i2);
     }