Merge pull request #1735 from joto/geom-geometry-n2

Add geometry_n() function to get the nth geometry of a multi geometry

Author: lonvia

src/geom-functions.cpp

@@ -59,6 +59,61 @@ std::size_t num_geometries(geometry_t const &geom)
 
 namespace {
 
+class geometry_n_visitor
+{
+public:
+    geometry_n_visitor(geometry_t *output, std::size_t n)
+    : m_output(output), m_n(n)
+    {}
+
+    void operator()(nullgeom_t const & /*input*/) const { m_output->reset(); }
+
+    void operator()(geom::collection_t const &input) const
+    {
+        *m_output = input[m_n - 1];
+    }
+
+    template <typename T>
+    void operator()(geom::multigeometry_t<T> const &input) const
+    {
+        m_output->set<T>() = input[m_n - 1];
+    }
+
+    template <typename T>
+    void operator()(T const &input) const
+    {
+        m_output->set<T>() = input;
+    }
+
+private:
+    geometry_t *m_output;
+    std::size_t m_n;
+
+}; // class geometry_n_visitor
+
+} // anonymous namespace
+
+void geometry_n(geometry_t *output, geometry_t const &input, std::size_t n)
+{
+    auto const num = num_geometries(input);
+    if (n < 1 || n > num) {
+        output->reset();
+        return;
+    }
+
+    input.visit(geometry_n_visitor{output, n});
+    output->set_srid(input.srid());
+}
+
+geometry_t geometry_n(geometry_t const &input, std::size_t n)
+{
+    geom::geometry_t output{};
+    geometry_n(&output, input, n);
+    return output;
+}
+
+namespace {
+
 void set_to_same_type(geometry_t *output, geometry_t const &input)
 {
     input.visit(overloaded{[&](auto in) { output->set<decltype(in)>(); }});

src/geom-functions.hpp

@@ -64,6 +64,30 @@ std::string_view geometry_type(geometry_t const &geom);
  */
 std::size_t num_geometries(geometry_t const &geom);
 
+/**
+ * Get a copy of the nth geometry out of a (multi)geometry.
+ *
+ * For non-multi geometries and n==1 a copy of the single geometry is returned.
+ * If n is out or range a null geometry is returned.
+ *
+ * \param output Pointer to output geometry.
+ * \param input Input geometry.
+ * \param n Number of geometry to get (1-based).
+ */
+void geometry_n(geometry_t *output, geometry_t const &input, std::size_t n);
+
+/**
+ * Get a copy of the nth geometry out of a (multi)geometry.
+ *
+ * For non-multi geometries and n==1 a copy of the single geometry is returned.
+ * If n is out or range a null geometry is returned.
+ *
+ * \param input Input geometry.
+ * \param n Number of geometry to get (1-based).
+ * \returns The geometry.
+ */
+geometry_t geometry_n(geometry_t const &input, std::size_t n);
+
 /**
  * Transform a geometry in 4326 into some other projection.
  *

tests/test-geom-collections.cpp

@@ -26,6 +26,7 @@ TEST_CASE("geometry collection with point", "[NoDB]")
     REQUIRE(num_geometries(geom) == 1);
     REQUIRE(area(geom) == Approx(0.0));
     REQUIRE_THROWS(centroid(geom));
+    REQUIRE(geometry_n(geom, 1) == geom::geometry_t{geom::point_t{1, 1}});
 }
 
 TEST_CASE("geometry collection with several geometries", "[NoDB]")
@@ -41,6 +42,10 @@ TEST_CASE("geometry collection with several geometries", "[NoDB]")
     REQUIRE(num_geometries(geom) == 3);
     REQUIRE(area(geom) == Approx(0.0));
     REQUIRE_THROWS(centroid(geom));
+    REQUIRE(geometry_n(geom, 1) == geom::geometry_t{geom::point_t{1, 1}});
+    REQUIRE(geometry_n(geom, 2) ==
+            geom::geometry_t{geom::linestring_t{{1, 1}, {2, 2}}});
+    REQUIRE(geometry_n(geom, 3) == geom::geometry_t{geom::point_t{2, 2}});
 }
 
 TEST_CASE("create_collection from OSM data", "[NoDB]")

tests/test-geom-linestrings.cpp

@@ -46,6 +46,7 @@ TEST_CASE("line geometry", "[NoDB]")
     REQUIRE(area(geom) == Approx(0.0));
     REQUIRE(geometry_type(geom) == "LINESTRING");
     REQUIRE(centroid(geom) == geom::geometry_t{geom::point_t{1.5, 1.5}});
+    REQUIRE(geometry_n(geom, 1) == geom);
 }
 
 TEST_CASE("create_linestring from OSM data", "[NoDB]")

tests/test-geom-multilinestrings.cpp

@@ -200,6 +200,8 @@ TEST_CASE("create_multilinestring from four lines forming two rings", "[NoDB]")
     REQUIRE(ml.num_geometries() == 2);
     REQUIRE(ml[0] == expected[0]);
     REQUIRE(ml[1] == expected[1]);
+    REQUIRE(geometry_n(geom, 1).get<geom::linestring_t>() == expected[0]);
+    REQUIRE(geometry_n(geom, 2).get<geom::linestring_t>() == expected[1]);
 }
 
 TEST_CASE("create_multilinestring from Y shape", "[NoDB]")

tests/test-geom-multipoints.cpp

@@ -37,9 +37,9 @@ TEST_CASE("multipoint_t with a single point", "[NoDB]")
 
 TEST_CASE("multipoint_t with several points", "[NoDB]")
 {
-    geom::point_t const p0{1, 1};
-    geom::point_t const p1{2, 1};
-    geom::point_t const p2{3, 1};
+    geom::point_t p0{1, 1};
+    geom::point_t p1{2, 1};
+    geom::point_t p2{3, 1};
 
     geom::geometry_t geom{geom::multipoint_t{}};
     auto &mp = geom.get<geom::multipoint_t>();
@@ -56,4 +56,8 @@ TEST_CASE("multipoint_t with several points", "[NoDB]")
     REQUIRE(mp[0] == p0);
     REQUIRE(mp[1] == p1);
     REQUIRE(mp[2] == p2);
+
+    REQUIRE(geometry_n(geom, 1) == geom::geometry_t{std::move(p0)});
+    REQUIRE(geometry_n(geom, 2) == geom::geometry_t{std::move(p1)});
+    REQUIRE(geometry_n(geom, 3) == geom::geometry_t{std::move(p2)});
 }

tests/test-geom-multipolygons.cpp

@@ -27,6 +27,9 @@ TEST_CASE("multipolygon geometry with single outer, no inner", "[NoDB]")
     REQUIRE(num_geometries(geom) == 1);
     REQUIRE(area(geom) == Approx(1.0));
     REQUIRE(centroid(geom) == geom::geometry_t{geom::point_t{0.5, 0.5}});
+    REQUIRE(geometry_n(geom, 1) ==
+            geom::geometry_t{geom::polygon_t{
+                geom::ring_t{{0, 0}, {0, 1}, {1, 1}, {1, 0}, {0, 0}}}});
 }
 
 TEST_CASE("multipolygon geometry with two polygons", "[NoDB]")

tests/test-geom-null.cpp

@@ -21,4 +21,5 @@ TEST_CASE("null geometry", "[NoDB]")
     REQUIRE(area(geom) == 0.0);
     REQUIRE(geometry_type(geom) == "NULL");
     REQUIRE(centroid(geom).is_null());
+    REQUIRE(geometry_n(geom, 1).is_null());
 }

tests/test-geom-points.cpp

@@ -53,6 +53,7 @@ TEST_CASE("create_point from OSM data", "[NoDB]")
     REQUIRE(num_geometries(geom) == 1);
     REQUIRE(area(geom) == Approx(0.0));
     REQUIRE(centroid(geom) == geom::geometry_t{geom::point_t{1.1, 2.2}});
+    REQUIRE(geometry_n(geom, 1) == geom);
     REQUIRE(geom.get<geom::point_t>() == geom::point_t{1.1, 2.2});
 }
 

tests/test-geom-polygons.cpp

@@ -24,6 +24,7 @@ TEST_CASE("polygon geometry without inner", "[NoDB]")
     REQUIRE(area(geom) == Approx(1.0));
     REQUIRE(geometry_type(geom) == "POLYGON");
     REQUIRE(centroid(geom) == geom::geometry_t{geom::point_t{0.5, 0.5}});
+    REQUIRE(geometry_n(geom, 1) == geom);
 }
 
 TEST_CASE("polygon geometry without inner (reverse)", "[NoDB]")