fix test and comment

Signed-off-by: Matthew Katz <katz@spiraldb.com>

Author: Matthew Katz

vortex-array/src/arrays/listview/tests/density.rs

@@ -91,15 +91,15 @@ fn overlap_differential_exact_lower_than_estimate() -> VortexResult<()> {
 }
 
 #[test]
-fn empty_elements_returns_zero() -> VortexResult<()> {
+fn empty_elements_returns_one() -> VortexResult<()> {
     let mut ctx = test_execution_ctx();
     let lv = create_empty_elements_listview();
 
     let exact = lv.compute_density(&mut ctx)?;
     let est = lv.estimate_density(&mut ctx)?;
 
-    assert!(exact.abs() < EPS);
-    assert!(est.abs() < EPS);
+    assert!((exact - 1.0).abs() < EPS);
+    assert!((est - 1.0).abs() < EPS);
     Ok(())
 }
 

vortex-array/src/stats/array.rs

@@ -154,7 +154,7 @@ impl StatsSetRef<'_> {
     }
 
     /// Returns the value of `stat` by either fetching it from cache if it exists and is [`Precision::Exact`], or falling back to
-    /// computation.
+    /// computation. The underlying compute kernels will cache the computed stat in the latter case.
     pub fn compute_stat(&self, stat: Stat, ctx: &mut ExecutionCtx) -> VortexResult<Option<Scalar>> {
         // If it's already computed and exact, we can return it.
         if let Some(Precision::Exact(s)) = self.get(stat) {