Preserve line order in OverlayNG (#665)

* Fix OverlayNG edge ordering to preserve order of input lines

Signed-off-by: Martin Davis <mtnclimb@gmail.com>

modules/core/src/main/java/org/locationtech/jts/operation/overlayng/EdgeMerger.java

@@ -12,13 +12,11 @@
 package org.locationtech.jts.operation.overlayng;
 
 import java.util.ArrayList;
-import java.util.Collection;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 
 import org.locationtech.jts.util.Assert;
-import org.locationtech.jts.util.Debug;
 
 /**
  * Performs merging on the noded edges of the input geometries.
@@ -39,25 +37,22 @@
  * no other coincident edge, or if all coincident edges have the same direction).
  * This ensures that the overlay output line direction will be as consistent
  * as possible with input lines.
+ * <p>
+ * The merger also preserves the order of the edges in the input.
+ * This means that for polygon-line overlay
+ * the result lines will be in the same order as in the input
+ * (possibly with multiple result lines for a single input line).
  * 
  * @author mdavis
  *
  */
 class EdgeMerger {
-
+ 
   public static List<Edge> merge(List<Edge> edges) {
-    EdgeMerger merger = new EdgeMerger(edges);
-    return merger.merge();
-  }
+    // use a list to collect the final edges, to preserve order
+    List<Edge> mergedEdges = new ArrayList<Edge>();
+    Map<EdgeKey, Edge> edgeMap = new HashMap<EdgeKey, Edge>();
 
-  private Collection<Edge> edges;
-  private Map<EdgeKey, Edge> edgeMap = new HashMap<EdgeKey, Edge>();
-
-  public EdgeMerger(List<Edge> edges) {
-    this.edges = edges;
-  }
-
-  public ArrayList<Edge> merge() {
     for (Edge edge : edges) {
       EdgeKey edgeKey = EdgeKey.create(edge);
       Edge baseEdge = edgeMap.get(edgeKey);
@@ -66,6 +61,7 @@ public ArrayList<Edge> merge() {
         edgeMap.put(edgeKey, edge);
         //Debug.println("edge added: " + edge);
         //Debug.println(edge.toLineString());
+        mergedEdges.add(edge);
       }
       else {
         // found an existing edge
@@ -80,7 +76,7 @@ public ArrayList<Edge> merge() {
         //Debug.println(edge.toLineString());
       }
     }
-    return new ArrayList<Edge>(edgeMap.values());
+    return mergedEdges;
   }
 
 }

modules/core/src/main/java/org/locationtech/jts/operation/overlayng/LineBuilder.java

@@ -305,6 +305,10 @@ private void addResultLinesRings() {
    * by the start edge direction. This implies
    * that if all edges are reversed, the created line
    * will be reversed to match.
+   * This ensures the orientation of linework is faithful to the input
+   * in the case of polygon-line overlay.
+   * However, this does not provide a consistent orientation 
+   * in the case of line-line intersection(where A and B might have different orientations).
    * (Other more complex strategies would be possible.
    * E.g. using the direction of the majority of segments,
    * or preferring the direction of the A edges.)

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

@@ -569,6 +569,14 @@ public void testPolygonFlatCollapseIntersection() {
     checkEqual(expected, actual);
   }
 
+  public void testPolygonLineIntersectionOrder() {
+    Geometry a = read("POLYGON ((1 1, 1 9, 9 9, 9 7, 3 7, 3 3, 9 3, 9 1, 1 1))");
+    Geometry b = read("MULTILINESTRING ((2 10, 2 0), (4 10, 4 0))");
+    Geometry expected = read("MULTILINESTRING ((2 9, 2 1), (4 9, 4 7), (4 3, 4 1))");
+    Geometry actual = intersection(a, b, 1);
+    checkEqualExact(expected, actual);    
+  }
+
   //============================================================
 
 

modules/core/src/test/java/test/jts/GeometryTestCase.java

@@ -57,7 +57,7 @@ protected GeometryTestCase(String name, CoordinateSequenceFactory coordinateSequ
 
   /**
    * Checks that the normalized values of the expected and actual
-   * geometries are exactly equals.
+   * geometries are exactly equal.
    * 
    * @param expected the expected value
    * @param actual the actual value
@@ -72,6 +72,21 @@ protected void checkEqual(Geometry expected, Geometry actual) {
     assertTrue(equal);
   }
 
+  /**
+   * Checks that the values of the expected and actual
+   * geometries are exactly equal.
+   * 
+   * @param expected the expected value
+   * @param actual the actual value
+   */
+  protected void checkEqualExact(Geometry expected, Geometry actual) {
+    boolean equal = actual.equalsExact(expected);
+    if (! equal) {
+      System.out.format(CHECK_EQUAL_FAIL, expected, actual );
+    }
+    assertTrue(equal);
+  }
+
   protected void checkEqual(Geometry expected, Geometry actual, double tolerance) {
     Geometry actualNorm = actual.norm();
     Geometry expectedNorm = expected.norm();