Fix BoundaryChainNoder to split chains at self-touch nodes

modules/core/src/main/java/org/locationtech/jts/noding/BasicSegmentString.java

@@ -29,6 +29,15 @@
 public class BasicSegmentString
 	implements SegmentString 
 {
+  public static BasicSegmentString substring(SegmentString segString, int start, int end) {
+    Coordinate[] pts = new Coordinate[end - start + 1];
+    int ipts = 0;
+    for (int i = start; i < end + 1; i++) {
+      pts[ipts++] = segString.getCoordinate(i).copy();
+    }
+    return new BasicSegmentString(pts, segString.getData());
+  }
+
   private Coordinate[] pts;
   private Object data;
 

modules/core/src/main/java/org/locationtech/jts/noding/BoundaryChainNoder.java

@@ -15,6 +15,7 @@
 import java.util.Collection;
 import java.util.HashSet;
 import java.util.List;
+import java.util.Set;
 
 import org.locationtech.jts.geom.Coordinate;
 import org.locationtech.jts.geom.LineSegment;
@@ -50,11 +51,17 @@ public BoundaryChainNoder() {
 
   @Override
   public void computeNodes(Collection segStrings) {
-    HashSet<Segment> segSet = new HashSet<Segment>();
+    HashSet<Segment> boundarySegSet = new HashSet<Segment>();
     BoundaryChainMap[] boundaryChains = new BoundaryChainMap[segStrings.size()];
-    addSegments(segStrings, segSet, boundaryChains);
-    markBoundarySegments(segSet);
+    addSegments(segStrings, boundarySegSet, boundaryChains);
+    markBoundarySegments(boundarySegSet);
     chainList = extractChains(boundaryChains);
+
+    //-- check for self-touching nodes and split chains at those nodes
+    Set<Coordinate> nodePts = findNodePts(chainList); 
+    if (nodePts.size() > 0) {
+      chainList = nodeChains(chainList, nodePts);
+    }
   }
 
   private static void addSegments(Collection<SegmentString> segStrings, HashSet<Segment> segSet, 
@@ -95,6 +102,56 @@ private static List<SegmentString> extractChains(BoundaryChainMap[] boundaryChai
     return chainList;
   }
 
+  private Set<Coordinate> findNodePts(List<SegmentString> segStrings) {
+    Set<Coordinate> interorVertices = new HashSet<Coordinate>();
+    Set<Coordinate> nodes = new HashSet<Coordinate>();
+    for (SegmentString ss : segStrings) {
+      //-- endpoints are nodes
+      nodes.add(ss.getCoordinate(0));
+      nodes.add(ss.getCoordinate(ss.size() - 1));
+
+      //-- check for duplicate interior points
+      for (int i = 1; i < ss.size() - 1; i++) {
+        Coordinate p = ss.getCoordinate(i);
+        if (interorVertices.contains(p)) {
+          nodes.add(p);
+        }
+        interorVertices.add(p);
+      }
+    }
+    return nodes;
+  }
+
+  private List<SegmentString> nodeChains(List<SegmentString> chains, Set<Coordinate> nodePts) {
+    List<SegmentString> nodedChains = new ArrayList<SegmentString>();
+    for (SegmentString chain : chains) {
+      nodeChain(chain, nodePts, nodedChains);
+    }
+    return nodedChains;
+  }
+
+  private void nodeChain(SegmentString chain, Set<Coordinate> nodePts, List<SegmentString> nodedChains) {
+    int start = 0;
+    while (start < chain.size() - 1) {
+      int end = findNodeIndex(chain, start, nodePts);
+      //-- if no interior nodes found, keep original chain
+      if (start == 0 && end == chain.size() - 1) {
+        nodedChains.add(chain);
+        return;
+      }
+      nodedChains.add(BasicSegmentString.substring(chain, start, end));
+      start = end;
+    }
+  }
+
+  private int findNodeIndex(SegmentString chain, int start, Set<Coordinate> nodePts) {
+    for (int i = start + 1; i < chain.size(); i++) {
+      if (nodePts.contains(chain.getCoordinate(i)))
+        return i;
+    }
+    return chain.size() - 1;
+  }
+
   @Override
   public Collection getNodedSubstrings() {
     return chainList;

modules/core/src/test/java/org/locationtech/jts/coverage/CoverageUnionTest.java

@@ -40,6 +40,27 @@ public void testEmpty() {
             );
   }
 
+  public void testHoleTouchingSide() {
+    checkUnion(
+        "GEOMETRYCOLLECTION (POLYGON ((1 9, 9 9, 9 6, 2 6, 1 9)), POLYGON ((1 1, 1 9, 2 6, 5 3, 9 6, 9 1, 1 1)))",
+        "POLYGON ((9 6, 9 1, 1 1, 1 9, 9 9, 9 6), (9 6, 2 6, 5 3, 9 6))"
+            );
+  }
+
+  public void testHolesTouchingSide() {
+    checkUnion(
+        "GEOMETRYCOLLECTION (POLYGON ((1 9, 9 9, 9 6, 5 7, 2 6, 1 9)), POLYGON ((1 1, 1 9, 2 6, 4 3, 5 7, 7 3, 9 6, 9 1, 1 1)))",
+        "POLYGON ((9 9, 9 6, 9 1, 1 1, 1 9, 9 9), (5 7, 7 3, 9 6, 5 7), (2 6, 4 3, 5 7, 2 6))"
+            );
+  }
+
+  public void testHolesTouching() {
+    checkUnion(
+        "GEOMETRYCOLLECTION (POLYGON ((1 9, 9 9, 9 6, 7 7, 5 7, 2 6, 1 9)), POLYGON ((1 1, 1 9, 2 6, 4 3, 5 7, 7 3, 7 7, 9 6, 9 1, 1 1)))",
+        "POLYGON ((9 9, 9 6, 9 1, 1 1, 1 9, 9 9), (5 7, 7 3, 7 7, 5 7), (2 6, 4 3, 5 7, 2 6))"
+            );
+  }
+
   private void checkUnion(String wktCoverage, String wktExpected) {
     Geometry covGeom = read(wktCoverage);
     Geometry[] coverage = toArray(covGeom);

modules/core/src/test/java/org/locationtech/jts/operation/overlayng/CoverageUnionTest.java

@@ -30,9 +30,25 @@ public void testPolygonsConcentricHalfDonuts( ) {
         "MULTIPOLYGON (((1 9, 6 9, 9 9, 9 1, 6 1, 1 1, 1 9), (2 8, 2 2, 6 2, 8 2, 8 8, 6 8, 2 8)), ((5 3, 3 3, 3 7, 5 7, 7 7, 7 3, 5 3), (5 4, 6 4, 6 6, 5 6, 4 6, 4 4, 5 4)))");
   }
 
-  public void testPolygonsNested( ) {
-    checkUnion("GEOMETRYCOLLECTION (POLYGON ((1 9, 9 9, 9 1, 1 1, 1 9), (3 7, 3 3, 7 3, 7 7, 3 7)), POLYGON ((3 7, 7 7, 7 3, 3 3, 3 7)))",
-        "POLYGON ((1 1, 1 9, 9 9, 9 1, 1 1))");
+  public void testHoleTouchingSide() {
+    checkUnion(
+        "GEOMETRYCOLLECTION (POLYGON ((1 9, 9 9, 9 6, 2 6, 1 9)), POLYGON ((1 1, 1 9, 2 6, 5 3, 9 6, 9 1, 1 1)))",
+        "POLYGON ((9 6, 9 1, 1 1, 1 9, 9 9, 9 6), (9 6, 2 6, 5 3, 9 6))"
+            );
+  }
+
+  public void testHolesTouchingSide() {
+    checkUnion(
+        "GEOMETRYCOLLECTION (POLYGON ((1 9, 9 9, 9 6, 5 7, 2 6, 1 9)), POLYGON ((1 1, 1 9, 2 6, 4 3, 5 7, 7 3, 9 6, 9 1, 1 1)))",
+        "POLYGON ((9 9, 9 6, 9 1, 1 1, 1 9, 9 9), (5 7, 7 3, 9 6, 5 7), (2 6, 4 3, 5 7, 2 6))"
+            );
+  }
+
+  public void testHolesTouching() {
+    checkUnion(
+        "GEOMETRYCOLLECTION (POLYGON ((1 9, 9 9, 9 6, 7 7, 5 7, 2 6, 1 9)), POLYGON ((1 1, 1 9, 2 6, 4 3, 5 7, 7 3, 7 7, 9 6, 9 1, 1 1)))",
+        "POLYGON ((9 9, 9 6, 9 1, 1 1, 1 9, 9 9), (5 7, 7 3, 7 7, 5 7), (2 6, 4 3, 5 7, 2 6))"
+            );
   }
 
   public void testPolygonsFormingHole( ) {
@@ -45,6 +61,13 @@ public void testPolygonsSquareGrid( ) {
         "POLYGON ((0 25, 0 50, 0 75, 0 100, 25 100, 50 100, 75 100, 100 100, 100 75, 100 50, 100 25, 100 0, 75 0, 50 0, 25 0, 0 0, 0 25))");
   }
 
+  public void testPolygonsNested( ) {
+    checkUnion("GEOMETRYCOLLECTION (POLYGON ((1 9, 9 9, 9 1, 1 1, 1 9), (3 7, 3 3, 7 3, 7 7, 3 7)), POLYGON ((3 7, 7 7, 7 3, 3 3, 3 7)))",
+        "POLYGON ((1 1, 1 9, 9 9, 9 1, 1 1))");
+  }
+
+  //------------------------------------------------------------
+
   /**
    * Sequential lines are still noded
    */
@@ -69,6 +92,8 @@ public void testLinesNetwork( ) {
         "MULTILINESTRING ((1 9, 3.1 8), (2 3, 4 3), (3.1 8, 5 7), (4 3, 5 3), (5 3, 5 7), (5 3, 7 4), (5 3, 8 1), (5 7, 7 8), (7 4, 9 5), (7 8, 9 9))");
   }
 
+  //=======================================================
+
   private void checkUnion(String wkt, String wktExpected) {
     Geometry coverage = read(wkt);
     Geometry expected = read(wktExpected);