OverlayEdge: Don't skip first point when adding coordinates

Signed-off-by: Dan Baston <dbaston@gmail.com>

Author: dbaston

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

@@ -147,7 +147,7 @@ public Coordinate[] getCoordinatesOriented() {
    */
   public void addCoordinates(CoordinateList coords)
   {
-    boolean isFirstEdge = coords.size() > 0;
+    boolean isFirstEdge = coords.isEmpty();
     if (direction) {
       int startIndex = 1;
       if (isFirstEdge) startIndex = 0;

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

@@ -489,6 +489,15 @@ public void testPolygonLineHorizontalIntersection() {
     Geometry actual = intersection(a, b);
     checkEqual(expected, actual);    
   }
- 
+
+  /**
+   * Tests that overlay produces stable (non-rotated) results.
+   * See https://github.com/locationtech/jts/issues/865
+   */
+  public void testRingsNonRotated() {
+    Geometry a = read("POLYGON ((1 9, 9 9, 9 1, 1 1, 1 9))");
+    Geometry actual = intersection(a, a);
+    checkEqualExact(a, actual);
+  }
 
 }