From 159f8341776c76828ce82f8890748d3b6b95dded Mon Sep 17 00:00:00 2001
From: Jochen Topf <jochen@topf.org>
Date: Sun, 25 Sep 2022 12:16:15 +0200
Subject: [PATCH 1/2] Add support for area() on geometry collections

Simply add up the area of all member geometries.
---
 src/geom-functions.cpp          |  4 ++++
 tests/test-geom-collections.cpp | 16 ++++++++++++++++
 2 files changed, 20 insertions(+)

diff --git a/src/geom-functions.cpp b/src/geom-functions.cpp
index 7bf9917d4..1854d0489 100644
--- a/src/geom-functions.cpp
+++ b/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);
diff --git a/tests/test-geom-collections.cpp b/tests/test-geom-collections.cpp
index a7a91a981..ae4ce33b9 100644
--- a/tests/test-geom-collections.cpp
+++ b/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;

From 6327a635bd38e24b790fa75b53ecde631cd6a2c1 Mon Sep 17 00:00:00 2001
From: Jochen Topf <jochen@topf.org>
Date: Sun, 25 Sep 2022 12:43:44 +0200
Subject: [PATCH 2/2] Use area() implementation from boost::geometry

---
 src/geom-functions.cpp | 56 +++++++++---------------------------------
 1 file changed, 12 insertions(+), 44 deletions(-)

diff --git a/src/geom-functions.cpp b/src/geom-functions.cpp
index 1854d0489..250798834 100644
--- a/src/geom-functions.cpp
+++ b/src/geom-functions.cpp
@@ -13,6 +13,7 @@
 #include <algorithm>
 #include <cmath>
 #include <iterator>
+#include <numeric>
 #include <tuple>
 #include <utility>
 
@@ -334,52 +335,19 @@ geometry_t segmentize(geometry_t const &input, double max_segment_length)
     return output;
 }
 
-static double get_ring_area(ring_t const &ring) noexcept
-{
-    assert(ring.size() > 3);
-
-    double total = 0.0;
-    auto it = ring.begin();
-    auto prev = *it++;
-
-    while (it != ring.end()) {
-        auto const cur = *it;
-        total += prev.x() * cur.y() - cur.x() * prev.y();
-        prev = cur;
-        ++it;
-    }
-
-    return total;
-}
-
-static double get_polygon_area(polygon_t const &polygon)
-{
-    double total = get_ring_area(polygon.outer());
-
-    for (auto const &ring : polygon.inners()) {
-        total += get_ring_area(ring);
-    }
-
-    return total * 0.5;
-}
-
 double area(geometry_t const &geom)
 {
-    double total = 0.0;
-
-    if (geom.is_polygon()) {
-        total = get_polygon_area(geom.get<polygon_t>());
-    } else if (geom.is_multipolygon()) {
-        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);
+    return std::abs(geom.visit(
+        overloaded{[&](geom::nullgeom_t const & /*input*/) { return 0.0; },
+                   [&](geom::collection_t const &input) {
+                       return std::accumulate(input.cbegin(), input.cend(), 0.0,
+                                              [](double sum, auto const &geom) {
+                                                  return sum + area(geom);
+                                              });
+                   },
+                   [&](auto const &input) {
+                       return static_cast<double>(boost::geometry::area(input));
+                   }}));
 }
 
 double length(geometry_t const &geom)