Add function for creating multipoint geometries from relations

This adds the C++ create_multipoint() function analogous to the
create_multilinestring() and create_multipolygon() functions.

Author: joto

src/geom-from-osm.cpp

@@ -95,6 +95,37 @@ geometry_t create_polygon(osmium::Way const &way)
     return geom;
 }
 
+void create_multipoint(geometry_t *geom, osmium::memory::Buffer const &buffer)
+{
+    auto nodes = buffer.select<osmium::Node>();
+    if (nodes.size() == 1) {
+        auto const location = nodes.cbegin()->location();
+        if (location.valid()) {
+            geom->set<point_t>() = point_t{location};
+        } else {
+            geom->reset();
+        }
+    } else {
+        auto &multiline = geom->set<multipoint_t>();
+        for (auto const &node : nodes) {
+            auto const location = node.location();
+            if (location.valid()) {
+                multiline.add_geometry(point_t{location});
+            }
+        }
+        if (multiline.num_geometries() == 0) {
+            geom->reset();
+        }
+    }
+}
+
+geometry_t create_multipoint(osmium::memory::Buffer const &buffer)
+{
+    geometry_t geom{};
+    create_multipoint(&geom, buffer);
+    return geom;
+}
+
 void create_multilinestring(geometry_t *geom,
                             osmium::memory::Buffer const &buffer,
                             bool force_multi)

src/geom-from-osm.hpp

@@ -89,6 +89,33 @@ void create_polygon(geometry_t *geom, osmium::Way const &way);
  */
 [[nodiscard]] geometry_t create_polygon(osmium::Way const &way);
 
+/**
+ * Create a multipoint geometry from a bunch of nodes (usually this would be
+ * used for member nodes of a relation). The result is multipoint geometry
+ * (or a point geometry if there is only one point).
+ *
+ * If there are no (valid) points, a null geometry is returned.
+ *
+ * \param geom Pointer to an existing geometry which will be used as output.
+ * \param ways Buffer containing all the input ways. Object types other than
+ *             ways in the buffer are ignored.
+ */
+void create_multipoint(geometry_t *geom, osmium::memory::Buffer const &buffer);
+
+/**
+ * Create a multipoint geometry from a bunch of nodes (usually this would be
+ * used for member nodes of a relation). The result is multipoint geometry
+ * (or a point geometry if there is only one point).
+ *
+ * If there are no (valid) points, a null geometry is returned.
+ *
+ * \param ways Buffer containing all the input nodes. Object types other than
+ *             nodes in the buffer are ignored.
+ * \returns The created geometry.
+ */
+[[nodiscard]] geometry_t
+create_multipoint(osmium::memory::Buffer const &buffer);
+
 /**
  * Create a multilinestring geometry from a bunch of ways (usually this
  * would be used for member ways of a relation). The result is always a

tests/test-geom-multipoints.cpp

@@ -67,3 +67,31 @@ TEST_CASE("multipoint_t with several points", "[NoDB]")
     REQUIRE(geometry_n(geom, 2) == geom::geometry_t{std::move(p1)});
     REQUIRE(geometry_n(geom, 3) == geom::geometry_t{std::move(p2)});
 }
+
+TEST_CASE("create_multipoint from OSM data", "[NoDB]")
+{
+    test_buffer_t buffer;
+    buffer.add_node("n10 x1 y0");
+    buffer.add_way("w20 Nn1x1y1,n2x2y1");
+    buffer.add_node("n11 x1 y1");
+    buffer.add_node("n12 x3 y2");
+    buffer.add_way("w21 Nn3x10y10,n4x10y11");
+    buffer.add_node("n13 x3 y1");
+    buffer.add_relation("r30 Mw20@");
+
+    auto const geom = geom::create_multipoint(buffer.buffer());
+
+    REQUIRE(geometry_type(geom) == "MULTIPOINT");
+    REQUIRE(dimension(geom) == 0);
+    REQUIRE(num_geometries(geom) == 4);
+
+    auto const &c = geom.get<geom::multipoint_t>();
+    REQUIRE(c[0] == geom::point_t{1, 0});
+    REQUIRE(c[1] == geom::point_t{1, 1});
+    REQUIRE(c[2] == geom::point_t{3, 2});
+    REQUIRE(c[3] == geom::point_t{3, 1});
+
+    REQUIRE(area(geom) == Approx(0.0));
+    REQUIRE(length(geom) == Approx(0.0));
+    REQUIRE(centroid(geom) == geom::geometry_t{geom::point_t{2, 1}});
+}