add Envelope::is_empty (#190)

- [X] I agree to follow the project's [code of
conduct](https://github.com/georust/geo/blob/master/CODE_OF_CONDUCT.md).
- [x] I added an entry to `rstar/CHANGELOG.md` if knowledge of this
change could be valuable to users.
---

This just adds a little convenience method. With the default
implementation, this should be a non-breaking change.

---------

Co-authored-by: Adam Reichold <adamreichold@users.noreply.github.com>

Author: michaelkirk

rstar/CHANGELOG.md

@@ -5,6 +5,7 @@
 - Added nearest neighbor search with distance squared: `nearest_neighbor_with_distance_2` and `nearest_neighbors_with_distance_2` methods ([PR](https://github.com/georust/rstar/pull/191)).
 - Added `root` doc examples and traversal docs
 - Implemented `RTreeObject` for `Arc<T>` and `Rc<T>`
+- Added `Envelope::is_empty`. ([PR](https://github.com/georust/rstar/pull/190))
 - New `AABB::from_center` utility constructor
 
 ## Fixed

rstar/src/aabb.rs

@@ -126,6 +126,10 @@ where
         }
     }
 
+    fn is_empty(&self) -> bool {
+        self.lower.nth(0) > self.upper.nth(0)
+    }
+
     fn contains_point(&self, point: &P) -> bool {
         self.lower.all_component_wise(point, |x, y| x <= y)
             && self.upper.all_component_wise(point, |x, y| x >= y)
@@ -281,4 +285,13 @@ mod test {
         let aabb = AABB::from_points(&[(3., 3., 3.), (4., 4., 4.)]);
         assert_eq!(aabb, AABB::from_corners((3., 3., 3.), (4., 4., 4.)));
     }
+
+    #[test]
+    fn test_is_empty() {
+        let empty = AABB::<[f32; 2]>::new_empty();
+        assert!(empty.is_empty());
+
+        let not_empty = AABB::from_corners([1.0, 1.0], [1.0, 1.0]);
+        assert!(!not_empty.is_empty());
+    }
 }

rstar/src/envelope.rs

@@ -13,6 +13,11 @@ pub trait Envelope: Clone + PartialEq + ::core::fmt::Debug {
     /// Creates a new, empty envelope that does not encompass any child.
     fn new_empty() -> Self;
 
+    /// Returns true if there are no points in the Envelope
+    fn is_empty(&self) -> bool {
+        self == &Self::new_empty()
+    }
+
     /// Returns true if a point is contained within this envelope.
     fn contains_point(&self, point: &Self::Point) -> bool;