Merge #379

379: add union operation for geometry r=metasim a=zredb

- [X] I agree to follow the project's [code of conduct](https://github.com/georust/gdal/blob/master/CODE_OF_CONDUCT.md).
- [X] I added an entry to `CHANGES.md` if knowledge of this change could be valuable to users.
---
This is my first time to commit code to this repository. I see that vecotr only has an intersection operation at present, so I try to add a union operation to it. If it can be accepted, I will add other operations in the same way later.


Co-authored-by: zhb <[email protected]>

CHANGES.md

@@ -1,6 +1,9 @@
 # Changes
 
 ## Unreleased
+- Added `Geometry::union`
+
+  - <https://github.com/georust/gdal/pull/379>
 
 - Added `Geometry::from_gml`
 

src/vector/ops/set.rs

@@ -14,8 +14,8 @@ impl Geometry {
     /// otherwise the result might be wrong.
     ///
     /// # Returns
-    /// Some(Geometry) if both Geometries contain pointers
-    /// None if either geometry is missing the gdal pointer, or there is an error.
+    /// `Some(geometry)` if both Geometries contain pointers
+    /// `None` if either geometry is missing the GDAL pointer, or there is an error.
     ///
     /// See: [`OGR_G_Intersection`](https://gdal.org/api/vector_c_api.html#_CPPv418OGR_G_Intersection12OGRGeometryH12OGRGeometryH)
     pub fn intersection(&self, other: &Self) -> Option<Self> {
@@ -32,6 +32,35 @@ impl Geometry {
         }
         Some(unsafe { Geometry::with_c_geometry(ogr_geom, true) })
     }
+
+    /// Compute union.
+    ///
+    /// Generates a new geometry which is the region of union of
+    /// the two geometries operated on.
+    /// Geometry validity is not checked. In case you are unsure of the
+    /// validity of the input geometries, call IsValid() before,
+    /// otherwise the result might be wrong.
+    ///
+    /// # Returns
+    /// `Some(geometry)` if both Geometries contain pointers.
+    /// `None` if either geometry is missing the GDAL pointer, or there is an error.
+    ///
+    /// See: [`OGR_G_Union`](https://gdal.org/api/vector_c_api.html#_CPPv419OGR_G_UnionCascaded12OGRGeometryH)
+    pub fn union(&self, other: &Self) -> Option<Self> {
+        if !self.has_gdal_ptr() {
+            return None;
+        }
+        if !other.has_gdal_ptr() {
+            return None;
+        }
+        unsafe {
+            let ogr_geom = gdal_sys::OGR_G_Union(self.c_geometry(), other.c_geometry());
+            if ogr_geom.is_null() {
+                return None;
+            }
+            Some(Geometry::with_c_geometry(ogr_geom, true))
+        }
+    }
 }
 
 #[cfg(test)]
@@ -84,4 +113,50 @@ mod tests {
 
         assert_eq!(inter.unwrap().area(), 0.0);
     }
+
+    #[test]
+    #[allow(clippy::float_cmp)]
+    fn test_union_success() {
+        let geom =
+            Geometry::from_wkt("POLYGON ((0.0 10.0, 0.0 0.0, 10.0 0.0, 10.0 10.0, 0.0 10.0))")
+                .unwrap();
+        let other = Geometry::from_wkt("POLYGON ((1 -5, 1 1, -5 1, -5 -5, 1 -5))").unwrap();
+
+        let res = geom.union(&other);
+
+        assert!(res.is_some());
+
+        let res = res.unwrap();
+
+        assert_eq!(res.area(), 135.0);
+    }
+
+    #[test]
+    fn test_union_no_gdal_ptr() {
+        let geom =
+            Geometry::from_wkt("POLYGON ((0.0 10.0, 0.0 0.0, 10.0 0.0, 10.0 10.0, 0.0 10.0))")
+                .unwrap();
+        let other = unsafe { Geometry::lazy_feature_geometry() };
+
+        let res = geom.union(&other);
+
+        assert!(res.is_none());
+    }
+
+    #[test]
+    #[allow(clippy::float_cmp)]
+    fn test_union_no_intersects() {
+        let geom =
+            Geometry::from_wkt("POLYGON ((0.0 5.0, 0.0 0.0, 5.0 0.0, 5.0 5.0, 0.0 5.0))").unwrap();
+
+        let other =
+            Geometry::from_wkt("POLYGON ((15.0 15.0, 15.0 20.0, 20.0 20.0, 20.0 15.0, 15.0 15.0))")
+                .unwrap();
+
+        let res = geom.union(&other);
+
+        assert!(res.is_some());
+
+        assert_eq!(res.unwrap().area(), 50.0);
+    }
 }