Add support for area() on geometry collections

Simply add up the area of all member geometries.

Author: joto

src/geom-functions.cpp

@@ -373,6 +373,10 @@ double area(geometry_t const &geom)
         for (auto const &polygon : geom.get<multipolygon_t>()) {
             total += get_polygon_area(polygon);
         }
+    } else if (geom.is_collection()) {
+        for (auto const &sgeom : geom.get<collection_t>()) {
+            total += area(sgeom);
+        }
     }
 
     return std::abs(total);

tests/test-geom-collections.cpp

@@ -72,6 +72,22 @@ TEST_CASE("geometry collection with several geometries", "[NoDB]")
     REQUIRE(geometry_n(geom, 3) == geom::geometry_t{geom::point_t{2, 2}});
 }
 
+TEST_CASE("geometry collection with polygon", "[NoDB]")
+{
+    geom::geometry_t geom{geom::collection_t{}};
+    auto &c = geom.get<geom::collection_t>();
+
+    c.add_geometry(geom::geometry_t{geom::point_t{1, 1}});
+    c.add_geometry(geom::geometry_t{
+        geom::polygon_t{geom::ring_t{{1, 1}, {1, 2}, {2, 2}, {2, 1}, {1, 1}}}});
+
+    REQUIRE(geometry_type(geom) == "GEOMETRYCOLLECTION");
+    REQUIRE(num_geometries(geom) == 2);
+    REQUIRE(area(geom) == Approx(1.0));
+    REQUIRE(length(geom) == Approx(0.0));
+    REQUIRE(centroid(geom) == geom::geometry_t{geom::point_t{1.5, 1.5}});
+}
+
 TEST_CASE("create_collection from OSM data", "[NoDB]")
 {
     test_buffer_t buffer;