diff --git a/sub_crates/rmath/src/utils.rs b/sub_crates/rmath/src/utils.rs
index 051e956..7af13a3 100644
--- a/sub_crates/rmath/src/utils.rs
+++ b/sub_crates/rmath/src/utils.rs
@@ -1,6 +1,13 @@
 const TOP_BIT: u32 = 1 << 31;
 
 /// Compute how different two floats are in ulps.
+///
+/// Notes:
+/// - Treats 0.0 and -0.0 as zero ulps apart, and extends the
+///   implications of that to the rest of the numbers.  E.g. the numbers
+///   just above/below 0.0/-0.0 are only two ulps apart, not three.
+/// - Infinity is one ulp past float max, and converse for -infinity.
+/// - This doesn't handle NaNs in any really useful way.
 #[inline(always)]
 pub fn ulp_diff(a: f32, b: f32) -> u32 {
     let a = a.to_bits();
@@ -34,12 +41,20 @@ mod tests {
         assert_eq!(ulp_diff(0.0, 0.0), 0);
         assert_eq!(ulp_diff(0.0, -0.0), 0);
         assert_eq!(ulp_diff(1.0, 2.0), 1 << 23);
-        assert_eq!(ulp_diff(0.0, f32::from_bits(0.0f32.to_bits() + 1)), 1);
-        assert_eq!(ulp_diff(-0.0, f32::from_bits(0.0f32.to_bits() + 1)), 1);
+        assert_eq!(ulp_diff(2.0, 4.0), 1 << 23);
+        assert_eq!(ulp_diff(-1.0, -2.0), 1 << 23);
+        assert_eq!(ulp_diff(-2.0, -4.0), 1 << 23);
+        assert_eq!(ulp_diff(-1.0, 1.0), 0x7f000000);
+        assert_eq!(ulp_diff(0.0, 1.0), 0x3f800000);
+        assert_eq!(ulp_diff(-0.0, 1.0), 0x3f800000);
+        assert_eq!(ulp_diff(0.0, -1.0), 0x3f800000);
+        assert_eq!(ulp_diff(-0.0, -1.0), 0x3f800000);
         assert_eq!(
             ulp_diff(std::f32::INFINITY, -std::f32::INFINITY),
             0xff000000
         );
+        assert_eq!(ulp_diff(0.0, f32::from_bits(0.0f32.to_bits() + 1)), 1);
+        assert_eq!(ulp_diff(-0.0, f32::from_bits(0.0f32.to_bits() + 1)), 1);
     }
 
     #[test]