proposal 2: multilinesting

Improve consistency of GEOSVoronoiDiagram
**change**

the getDiagramEdges now only returns a MultiLineString, whether there is
one or many lines. Which reduces to two the number of possible kind of
geometries given by `GEOSVoronoiDiagram`

**reason**

I've been working on adding `voronoi_diagram` to RGeo. And looking at
the [Shapely PR for voronoi diagram][1] and [mine in RGeo][2], it seems
like the `GEOSVoronoiDiagram` CAPI method causes confusion by returning
three possible kind of geometries:

- `GeometryCollection` (when `onlyEdges` is false)
- `MultiLineString` (`onlyEdges` is true and there are at least two lines)
- `LineString` (`onlyEdges` is true and there is only one line)

The parameter `onlyEdges` changes the outcome, which is ok to reason
about. However, the fact that we could have either a LineString or a
MultiLineString is way harder to work around in my opinion.

[1]: shapely/shapely#881 (comment)
[2]: rgeo/rgeo#334 (comment)

include/geos/triangulate/VoronoiDiagramBuilder.h

@@ -107,13 +107,13 @@ class GEOS_DLL VoronoiDiagramBuilder {
     std::unique_ptr<geom::GeometryCollection> getDiagram(const geom::GeometryFactory& geomFact);
 
     /** \brief
-     * Gets the faces of the computed diagram as a {@link geom::GeometryCollection}
+     * Gets the faces of the computed diagram as a {@link geom::MultiLineString}
      * of {@link geom::LineString}s, clipped as specified.
      *
      * @param geomFact the geometry factory to use to create the output
      * @return the faces of the diagram
      */
-    std::unique_ptr<geom::GeometryCollection> getDiagramEdges(const geom::GeometryFactory& geomFact);
+    std::unique_ptr<geom::MultiLineString> getDiagramEdges(const geom::GeometryFactory& geomFact);
 
 private:
 

src/triangulate/VoronoiDiagramBuilder.cpp

@@ -123,33 +123,30 @@ VoronoiDiagramBuilder::getDiagram(const geom::GeometryFactory& geomFact)
     return ret;
 }
 
-std::unique_ptr<GeometryCollection>
+std::unique_ptr<MultiLineString>
 VoronoiDiagramBuilder::getDiagramEdges(const geom::GeometryFactory& geomFact)
 {
     create();
 
     if (!subdiv) {
-        return geomFact.createGeometryCollection();
+        return geomFact.createMultiLineString();
     }
 
     std::unique_ptr<geom::MultiLineString> edges = subdiv->getVoronoiDiagramEdges(geomFact);
     if(edges->isEmpty()) {
-        return geomFact.createGeometryCollection();
+        return edges.release();
     }
     std::unique_ptr<geom::Geometry> clipPoly(geomFact.toGeometry(&diagramEnv));
     std::unique_ptr<Geometry> clipped(clipPoly->intersection(edges.get()));
 
     switch (clipped->getGeometryTypeId()) {
         case GEOS_LINESTRING: {
-            std::vector<std::unique_ptr<Geometry>> vec;
-            vec.reserve(1);
-            vec.emplace_back(clipped.release());
-            return geomFact.createGeometryCollection(std::move(vec));
+            std::vector<std::unique_ptr<LineString>> lines;
+            lines.emplace_back(static_cast<LineString*>(clipped.release()));
+            return geomFact.createMultiLineString(std::move(lines));
         }
         case GEOS_MULTILINESTRING: {
-            return geomFact.createGeometryCollection(
-                static_cast<MultiLineString*>(clipped.get())->releaseGeometries()
-            );
+            return clipped
         }
         default: {
             throw util::GEOSException("Unknown state");

tests/unit/triangulate/VoronoiTest.cpp

@@ -237,25 +237,19 @@ void object::test<10>
     runVoronoi(wkt, expected, 0);
 }
 
-// Consistency in the return value.
+// Consistency in the return value for edges
 template<>
 template<>
 void object::test<11>
 ()
 {
-    const char* wkt0 = "MULTIPOINT ((150 200))";
-    const char* expected0 = "GEOMETRYCOLLECTION EMPTY";
     const char* wkt1 = "LINESTRING (10 10, 20 20)";
-    const char* expected1 = "GEOMETRYCOLLECTION (POLYGON ((0 30, 30 30, 30 0, 0 30)), POLYGON ((0 0, 0 30, 30 0, 0 0)))";
-    const char* expected2 = "GEOMETRYCOLLECTION (LINESTRING (0 30, 30 0))";
+    const char* expected1 = "MULTILINESTRING ((0 30, 30 0))";
     const char* wkt2 = "LINESTRING (10 10, 20 20, 30 30)";
-    const char* expected3 = "GEOMETRYCOLLECTION (LINESTRING (0 50, 50 0), LINESTRING (-10 40, 40 -10))";
+    const char* expected2 = "MULTILINESTRING ((0 50, 50 0), (-10 40, 40 -10))";
 
-    runVoronoi(wkt0, expected0, 0, false);
-    runVoronoi(wkt0, expected0, 0, true);
-    runVoronoi(wkt1, expected1, 0, false);
-    runVoronoi(wkt1, expected2, 0, true);
-    runVoronoi(wkt2, expected3, 0, true);
+    runVoronoi(wkt1, expected1, 0, true);
+    runVoronoi(wkt2, expected2, 0, true);
 }
 
 } // namespace tut