Fix buffer element erosion for negative distance and remove overlay deps

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

@@ -40,10 +40,6 @@
 import org.locationtech.jts.noding.MCIndexNoder;
 import org.locationtech.jts.noding.Noder;
 import org.locationtech.jts.noding.SegmentString;
-import org.locationtech.jts.operation.overlay.OverlayNodeFactory;
-import org.locationtech.jts.operation.overlay.PolygonBuilder;
-
-
 
 /**
  * Builds the buffer geometry for a given input geometry and precision model.
@@ -167,7 +163,7 @@ public Geometry buffer(Geometry g, double distance)
     boolean isNodingValidated = distance == 0.0;
     computeNodedEdges(bufferSegStrList, precisionModel, isNodingValidated);
 
-    graph = new PlanarGraph(new OverlayNodeFactory());
+    graph = new PlanarGraph(new BufferNodeFactory());
     graph.addEdges(edgeList.getEdges());
 
     List subgraphList = createSubgraphs(graph);

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

@@ -0,0 +1,36 @@
+/*
+ * Copyright (c) 2016 Vivid Solutions.
+ *
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License 2.0
+ * and Eclipse Distribution License v. 1.0 which accompanies this distribution.
+ * The Eclipse Public License is available at http://www.eclipse.org/legal/epl-v20.html
+ * and the Eclipse Distribution License is available at
+ *
+ * http://www.eclipse.org/org/documents/edl-v10.php.
+ */
+package org.locationtech.jts.operation.buffer;
+
+/**
+ * @version 1.7
+ */
+import org.locationtech.jts.geom.Coordinate;
+import org.locationtech.jts.geomgraph.DirectedEdgeStar;
+import org.locationtech.jts.geomgraph.Node;
+import org.locationtech.jts.geomgraph.NodeFactory;
+import org.locationtech.jts.geomgraph.PlanarGraph;
+
+/**
+ * Creates nodes for use in the {@link PlanarGraph}s constructed during
+ * buffer operations.
+ *
+ * @version 1.7
+ */
+class BufferNodeFactory
+  extends NodeFactory
+{
+  public Node createNode(Coordinate coord)
+  {
+    return new Node(coord, new DirectedEdgeStar());
+  }
+}

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

@@ -0,0 +1,86 @@
+/*
+ * Copyright (c) 2016 Vivid Solutions.
+ *
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License 2.0
+ * and Eclipse Distribution License v. 1.0 which accompanies this distribution.
+ * The Eclipse Public License is available at http://www.eclipse.org/legal/epl-v20.html
+ * and the Eclipse Distribution License is available at
+ *
+ * http://www.eclipse.org/org/documents/edl-v10.php.
+ */
+package org.locationtech.jts.operation.buffer;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import org.locationtech.jts.geom.GeometryFactory;
+import org.locationtech.jts.geomgraph.DirectedEdge;
+import org.locationtech.jts.geomgraph.DirectedEdgeStar;
+import org.locationtech.jts.geomgraph.EdgeRing;
+import org.locationtech.jts.geomgraph.Node;
+
+/**
+ * A ring of {@link DirectedEdge}s which may contain nodes of degree > 2.
+ * A <tt>MaximalEdgeRing</tt> may represent two different spatial entities:
+ * <ul>
+ * <li>a single polygon possibly containing inversions (if the ring is oriented CW)
+ * <li>a single hole possibly containing exversions (if the ring is oriented CCW)
+ * </ul>
+ * If the MaximalEdgeRing represents a polygon,
+ * the interior of the polygon is strongly connected.
+ * <p>
+ * These are the form of rings used to define polygons under some spatial data models.
+ * However, under the OGC SFS model, {@link MinimalEdgeRing}s are required.
+ * A MaximalEdgeRing can be converted to a list of MinimalEdgeRings using the
+ * {@link #buildMinimalRings() } method.
+ *
+ * @version 1.7
+ * @see org.locationtech.jts.operation.buffer.MinimalEdgeRing
+ */
+class MaximalEdgeRing
+  extends EdgeRing
+{
+
+  public MaximalEdgeRing(DirectedEdge start, GeometryFactory geometryFactory) {
+    super(start, geometryFactory);
+  }
+
+  public DirectedEdge getNext(DirectedEdge de)
+  {
+    return de.getNext();
+  }
+  public void setEdgeRing(DirectedEdge de, EdgeRing er)
+  {
+    de.setEdgeRing(er);
+  }
+
+  /**
+   * For all nodes in this EdgeRing,
+   * link the DirectedEdges at the node to form minimalEdgeRings
+   */
+  public void linkDirectedEdgesForMinimalEdgeRings()
+  {
+    DirectedEdge de = startDe;
+    do {
+      Node node = de.getNode();
+      ((DirectedEdgeStar) node.getEdges()).linkMinimalDirectedEdges(this);
+      de = de.getNext();
+    } while (de != startDe);
+  }
+
+  public List buildMinimalRings()
+  {
+    List minEdgeRings = new ArrayList();
+    DirectedEdge de = startDe;
+    do {
+      if (de.getMinEdgeRing() == null) {
+        EdgeRing minEr = new MinimalEdgeRing(de, geometryFactory);
+        minEdgeRings.add(minEr);
+      }
+      de = de.getNext();
+    } while (de != startDe);
+    return minEdgeRings;
+  }
+
+}

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

@@ -0,0 +1,44 @@
+/*
+ * Copyright (c) 2016 Vivid Solutions.
+ *
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License 2.0
+ * and Eclipse Distribution License v. 1.0 which accompanies this distribution.
+ * The Eclipse Public License is available at http://www.eclipse.org/legal/epl-v20.html
+ * and the Eclipse Distribution License is available at
+ *
+ * http://www.eclipse.org/org/documents/edl-v10.php.
+ */
+package org.locationtech.jts.operation.buffer;
+
+import org.locationtech.jts.geom.GeometryFactory;
+import org.locationtech.jts.geomgraph.DirectedEdge;
+import org.locationtech.jts.geomgraph.Edge;
+import org.locationtech.jts.geomgraph.EdgeRing;
+
+/**
+ * A ring of {@link Edge}s with the property that no node
+ * has degree greater than 2.  These are the form of rings required
+ * to represent polygons under the OGC SFS spatial data model.
+ *
+ * @version 1.7
+ * @see org.locationtech.jts.operation.buffer.MaximalEdgeRing
+ */
+class MinimalEdgeRing
+  extends EdgeRing
+{
+
+  public MinimalEdgeRing(DirectedEdge start, GeometryFactory geometryFactory) {
+    super(start, geometryFactory);
+  }
+
+  public DirectedEdge getNext(DirectedEdge de)
+  {
+    return de.getNextMin();
+  }
+  public void setEdgeRing(DirectedEdge de, EdgeRing er)
+  {
+    de.setMinEdgeRing(er);
+  }
+
+}