Return linestring or multilinestring from create_multilinestring()

create_multilinestring() has a new optional parameter `force_multi`
defaulted to true. If this is set to false, the function will return a
linestring geometry if that's possible.

Author: joto

src/geom-from-osm.cpp

@@ -96,27 +96,37 @@ geometry_t create_polygon(osmium::Way const &way)
 }
 
 void create_multilinestring(geometry_t *geom,
-                            osmium::memory::Buffer const &ways)
+                            osmium::memory::Buffer const &ways_buffer,
+                            bool force_multi)
 {
-    auto &mls = geom->set<multilinestring_t>();
-
-    for (auto const &way : ways.select<osmium::Way>()) {
-        linestring_t line;
+    auto ways = ways_buffer.select<osmium::Way>();
+    if (ways.size() == 1 && !force_multi) {
+        auto &line = geom->set<linestring_t>();
+        auto &way = *ways.begin();
         fill_point_list(&line, way.nodes());
-        if (line.size() >= 2U) {
-            mls.add_geometry(std::move(line));
+        if (line.size() < 2U) {
+            geom->reset();
+        }
+    } else {
+        auto &multiline = geom->set<multilinestring_t>();
+        for (auto const &way : ways) {
+            linestring_t line;
+            fill_point_list(&line, way.nodes());
+            if (line.size() >= 2U) {
+                multiline.add_geometry(std::move(line));
+            }
+        }
+        if (multiline.num_geometries() == 0) {
+            geom->reset();
         }
-    }
-
-    if (mls.num_geometries() == 0) {
-        geom->reset();
     }
 }
 
-geometry_t create_multilinestring(osmium::memory::Buffer const &ways)
+geometry_t create_multilinestring(osmium::memory::Buffer const &ways,
+                                  bool force_multi)
 {
     geometry_t geom{};
-    create_multilinestring(&geom, ways);
+    create_multilinestring(&geom, ways, force_multi);
     return geom;
 }
 

src/geom-from-osm.hpp

@@ -101,7 +101,8 @@ void create_polygon(geometry_t *geom, osmium::Way const &way);
  * \param ways Buffer containing all the input ways.
  */
 void create_multilinestring(geometry_t *geom,
-                            osmium::memory::Buffer const &ways);
+                            osmium::memory::Buffer const &ways,
+                            bool force_multi = true);
 
 /**
  * Create a multilinestring geometry from a bunch of ways (usually this
@@ -115,7 +116,8 @@ void create_multilinestring(geometry_t *geom,
  * \returns The created geometry.
  */
 [[nodiscard]] geometry_t
-create_multilinestring(osmium::memory::Buffer const &ways);
+create_multilinestring(osmium::memory::Buffer const &ways,
+                       bool force_multi = true);
 
 /**
  * Create a (multi)polygon geometry from a relation and member ways.

src/geom-functions.cpp

@@ -448,6 +448,11 @@ static void add_nodes_to_linestring(linestring_t *linestring, ITERATOR it,
 
 void line_merge(geometry_t *output, geometry_t const &input)
 {
+    if (input.is_linestring()) {
+        *output = input;
+        return;
+    }
+
     if (!input.is_multilinestring()) {
         output->reset();
         return;

src/geom-functions.hpp

@@ -159,19 +159,23 @@ double area(geometry_t const &geom);
 std::vector<geometry_t> split_multi(geometry_t &&geom, bool split_multi = true);
 
 /**
- * Merge lines in a multilinestring end-to-end as far as possible. Always
- * returns a multilinestring unless there is an error or the input geometry
- * is a nullgeom_t, in which case nullgeom_t is returned.
+ * Merge lines in a multilinestring end-to-end as far as possible.
+ *
+ * * Returns a multilinestring if the input is a multilinestring.
+ * * Returns a copy of the input if it is a linestring.
+ * * Returns nullgeom_t otherwise.
  *
  * \param output Pointer to output geometry.
  * \param input Input geometry.
  */
 void line_merge(geometry_t *output, geometry_t const &input);
 
 /**
- * Merge lines in a multilinestring end-to-end as far as possible. Always
- * returns a multilinestring unless there is an error or the input geometry
- * is a nullgeom_t, in which case nullgeom_t is returned.
+ * Merge lines in a multilinestring end-to-end as far as possible.
+ *
+ * * Returns a multilinestring if the input is a multilinestring.
+ * * Returns a copy of the input if it is a linestring.
+ * * Returns nullgeom_t otherwise.
  *
  * \param input Input geometry.
  * \returns Result multilinestring.

src/output-flex.cpp

@@ -892,7 +892,8 @@ int output_flex_t::app_as_multilinestring()
     m_relation_cache.add_members(middle());
 
     auto *geom = create_lua_geometry_object(lua_state());
-    geom::create_multilinestring(geom, m_relation_cache.members_buffer());
+    geom::create_multilinestring(geom, m_relation_cache.members_buffer(),
+                                 false);
 
     return 1;
 }

tests/test-geom-multilinestrings.cpp

@@ -36,6 +36,26 @@ TEST_CASE("create_multilinestring with single line", "[NoDB]")
     REQUIRE(ml[0] == expected);
 }
 
+TEST_CASE("create_multilinestring with single line and no force_multi",
+          "[NoDB]")
+{
+    geom::linestring_t const expected{{1, 1}, {2, 1}};
+
+    test_buffer_t buffer;
+    buffer.add_way("w20 Nn10x1y1,n11x2y1");
+
+    auto const geom =
+        geom::line_merge(geom::create_multilinestring(buffer.buffer(), false));
+
+    REQUIRE(geom.is_linestring());
+    REQUIRE(geometry_type(geom) == "LINESTRING");
+    REQUIRE(num_geometries(geom) == 1);
+    REQUIRE(area(geom) == Approx(0.0));
+    auto const &l = geom.get<geom::linestring_t>();
+    REQUIRE(l.num_geometries() == 1);
+    REQUIRE(l == expected);
+}
+
 TEST_CASE("create_multilinestring with single line forming a ring", "[NoDB]")
 {
     geom::linestring_t const expected{{1, 1}, {2, 1}, {2, 2}, {1, 1}};