Add buffer artifact removal heuristic for single-element inputs (#1161)

Author: dr-jts

modules/core/src/main/java/org/locationtech/jts/operation/buffer/BufferBuilder.java

@@ -27,6 +27,7 @@
 import org.locationtech.jts.geom.GeometryFactory;
 import org.locationtech.jts.geom.LineString;
 import org.locationtech.jts.geom.Location;
+import org.locationtech.jts.geom.Polygon;
 import org.locationtech.jts.geom.Position;
 import org.locationtech.jts.geom.PrecisionModel;
 import org.locationtech.jts.geom.TopologyException;
@@ -176,10 +177,42 @@ public Geometry buffer(Geometry g, double distance)
       return createEmptyResultGeometry();
     }
 
+    /**
+     *  Heuristic to remove artifacts caused by topology robustness problems
+     *  or buffer curve generation anomalies.
+     *  Uses fact that for distance > 0 single-element inputs must create single element buffers.
+     *  This does not hold if distance <= 0;
+     *  distance = 0 can create multipolygon results due to topology collapse,
+     *  and distance < 0 may erode polygons so they are disconnected.
+     */
+    if (distance > 0 && g.getNumGeometries() == 1 && resultPolyList.size() > 1) {
+      resultPolyList = keepLargestArea(resultPolyList);
+    }
+    
     Geometry resultGeom = geomFact.buildGeometry(resultPolyList);
     return resultGeom;
   }
 
+  private static List<Polygon> keepLargestArea(List<Polygon> polyList) {
+    Polygon largest = findLargestArea(polyList);
+    List<Polygon> largestPolyList = new ArrayList<Polygon>();
+    largestPolyList.add(largest);
+    return largestPolyList;
+  }
+
+  private static Polygon findLargestArea(List<Polygon> polyList) {
+    double largestArea = 0;
+    Polygon largest = null;
+    for (Polygon poly : polyList) {
+      double polyArea = poly.getArea();
+      if (largest == null || polyArea > largestArea) {
+        largest = poly;
+        largestArea = polyArea;
+      }
+    }
+    return largest;
+  }
+
   private Noder getNoder(PrecisionModel precisionModel)
   {
     if (workingNoder != null) return workingNoder;

modules/core/src/test/java/org/locationtech/jts/operation/buffer/BufferTest.java

@@ -668,6 +668,20 @@ public void testBufferByZeroKeepsAllElements() {
 "MULTIPOLYGON (((24 95.239, 24 96, 24 99, 24.816 99, 24 95.239)), ((3 90, 3 93, 3 96, 3 99, 21 99, 21 96, 21 93, 21 90, 3 90)))");    
   }
 
+  //-- see https://github.com/libgeos/geos/issues/1321
+  public void testArtifactsRemovedFromLineBuffer() {
+    String wkt = "LINESTRING (640734.77510795 216861.236982439, 640733.832969266 216862.938074642, 640732.814325629 216865.039674844, 640731.225095251 216869.012141189, 640729.979984761 216871.879095724, 640729.445974092 216873.02148841, 640729.002794006 216873.679857725, 640728.952197105 216873.745389857, 640728.676962154 216874.089814544)";
+    checkBufferNumGeometries(wkt, 100, 1);
+  }
+  
+  //-- see https://github.com/r-spatial/sf/issues/2552
+  public void testArtifactsRemovedFromLineBufferFlatEnd() {
+    String wkt = "LINESTRING (245184.6 6045650, 245193.3 6045649, 245201.7 6045651, 245204.3 6045653)";
+    Geometry geom = read(wkt);
+    Geometry buf = BufferOp.bufferOp(geom, 50, bufParamEndCapFlat());
+    assertEquals(1, buf.getNumGeometries());
+  }
+  
   //===================================================
 
   private static BufferParameters bufParamRoundMitre(double mitreLimit) {
@@ -677,6 +691,12 @@ private static BufferParameters bufParamRoundMitre(double mitreLimit) {
     return param;
   }
 
+  private static BufferParameters bufParamEndCapFlat() {
+    BufferParameters param = new BufferParameters();
+    param.setEndCapStyle(BufferParameters.CAP_FLAT);
+    return param;
+  }
+  
   private void checkBuffer(String wkt, double dist, BufferParameters param, String wktExpected) {
     Geometry geom = read(wkt);
     Geometry result = BufferOp.bufferOp(geom, dist, param);
@@ -706,7 +726,7 @@ private void checkBufferHasHole(String wkt, double dist, boolean isHoleExpected)
   private void checkBufferNumGeometries(String wkt, double dist, int numExpected) {
     Geometry a = read(wkt);
     Geometry result = a.buffer(dist);
-    assertTrue(numExpected == result.getNumGeometries());
+    assertEquals(numExpected, result.getNumGeometries());
   }
 
   private boolean hasHole(Geometry geom) {