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)

capi/geos_ts_c.cpp

@@ -4019,4 +4019,3 @@ extern "C" {
     }
 
 } /* extern "C" */
-

include/geos/triangulate/VoronoiDiagramBuilder.h

@@ -107,8 +107,8 @@ class GEOS_DLL VoronoiDiagramBuilder {
     std::unique_ptr<geom::GeometryCollection> getDiagram(const geom::GeometryFactory& geomFact);
 
     /** \brief
-     * Gets the faces of the computed diagram as a geom::GeometryCollection
-     * of {@link geom::LineString}s, clipped as specified.
+     * Gets the faces of the computed diagram as a {@link geom::MultiLineString},
+     * clipped as specified.
      *
      * @param geomFact the geometry factory to use to create the output
      * @return the faces of the diagram
@@ -132,4 +132,3 @@ class GEOS_DLL VoronoiDiagramBuilder {
 
 } //namespace geos.triangulate
 } //namespace geos
-

src/triangulate/VoronoiDiagramBuilder.cpp

@@ -138,7 +138,15 @@ VoronoiDiagramBuilder::getDiagramEdges(const geom::GeometryFactory& geomFact)
     }
     std::unique_ptr<geom::Geometry> clipPoly(geomFact.toGeometry(&diagramEnv));
     std::unique_ptr<Geometry> clipped(clipPoly->intersection(edges.get()));
-    return clipped;
+
+    if (dynamic_cast<LineString*>(clipped.get())) {
+        std::unique_ptr<std::vector<Geometry*>> lines(new std::vector<Geometry*>());
+        lines->push_back(clipped.release());
+        std::unique_ptr<MultiLineString> multi(geomFact.createMultiLineString(lines.release()));
+        return multi;
+    } else {
+        return clipped;
+    }
 }
 
 std::unique_ptr<geom::GeometryCollection>

tests/unit/triangulate/VoronoiTest.cpp

@@ -58,19 +58,23 @@ readTextOrHex(const char* wkt)
 
 //helper function for running triangulation
 void
-runVoronoi(const char* sitesWkt, const char* expectedWkt, const double tolerance)
+runVoronoi(const char* sitesWkt, const char* expectedWkt, const double tolerance, const bool onlyEdges = false)
 {
     WKTWriter writer;
     geos::triangulate::VoronoiDiagramBuilder builder;
     std::unique_ptr<Geometry> sites(readTextOrHex(sitesWkt));
     std::unique_ptr<Geometry> expected(readTextOrHex(expectedWkt));
-    std::unique_ptr<GeometryCollection> results;
+    std::unique_ptr<Geometry> results;
     const GeometryFactory& geomFact(*GeometryFactory::getDefaultInstance());
     builder.setSites(*sites);
 
     //set Tolerance:
     builder.setTolerance(tolerance);
-    results = builder.getDiagram(geomFact);
+    if (onlyEdges) {
+        results = builder.getDiagramEdges(geomFact);
+    } else {
+        results = builder.getDiagram(geomFact);
+    }
 
     results->normalize();
     expected->normalize();
@@ -232,4 +236,19 @@ void object::test<10>
     runVoronoi(wkt, expected, 0);
 }
 
+// Consistency in the return value for edges
+template<>
+template<>
+void object::test<11>
+()
+{
+    const char* wkt1 = "LINESTRING (10 10, 20 20)";
+    const char* expected1 = "MULTILINESTRING ((0 30, 30 0))";
+    const char* wkt2 = "LINESTRING (10 10, 20 20, 30 30)";
+    const char* expected2 = "MULTILINESTRING ((0 50, 50 0), (-10 40, 40 -10))";
+
+    runVoronoi(wkt1, expected1, 0, true);
+    runVoronoi(wkt2, expected2, 0, true);
+}
+
 } // namespace tut