Add SegmentString default methods

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

@@ -36,8 +36,66 @@ public interface SegmentString
    */
   public void setData(Object data);
 
+  /**
+   * Gets the number of coordinates in this segment string.
+   * 
+   * @return the number of coordinates
+   */
   public int size();
+
+  /**
+   * Gets the segment string coordinate at a given index.
+   *  
+   * @param i the coordinate index
+   * @return the coordinate at the index
+   */
   public Coordinate getCoordinate(int i);
+
+  /**
+   * Gets the coordinates in this segment string.
+   * 
+   * @return the coordinates as an array
+   */
   public Coordinate[] getCoordinates();
+
+  /**
+   * Tests if a segment string is a closed ring.
+   * 
+   * @return true if the segment string is closed
+   */
   public boolean isClosed();
+
+  /**
+   * Gets the previous vertex in a ring from a vertex index.
+   * 
+   * @param ringSS a segment string forming a ring
+   * @param index the vertex index
+   * @return the previous vertex in the ring
+   * 
+   * @see #isClosed
+   */
+  public default Coordinate prevInRing(int index) {
+    int prevIndex = index - 1;
+    if (prevIndex < 0) {
+      prevIndex = size() - 2;
+    }
+    return getCoordinate( prevIndex );
+  }
+
+  /**
+   * Gets the next vertex in a ring from a vertex index.
+   * 
+   * @param ringSS a segment string forming a ring
+   * @param index the vertex index
+   * @return the next vertex in the ring
+   * 
+   * @see #isClosed
+   */
+  public default Coordinate nextInRing(int index) {
+    int nextIndex = index + 1;
+    if (nextIndex > size() - 1) {
+      nextIndex = 1;
+    }
+    return getCoordinate( nextIndex );
+  }
 }

modules/core/src/test/java/org/locationtech/jts/noding/SegmentStringTest.java

@@ -0,0 +1,52 @@
+/*
+ * Copyright (c) 2024 Martin Davis.
+ *
+ * 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.noding;
+
+import org.locationtech.jts.geom.Coordinate;
+import org.locationtech.jts.geom.Geometry;
+
+import test.jts.GeometryTestCase;
+
+public class SegmentStringTest  extends GeometryTestCase {
+
+  public static void main(String[] args) {
+    junit.textui.TestRunner.run(SegmentStringTest.class);
+  }  
+
+  public SegmentStringTest(String name) {
+    super(name);
+  }
+
+  public void testNextInRing() {
+    SegmentString ss = create("LINESTRING(0 0, 1 2, 3 1, 0 0)");
+    assertTrue(ss.isClosed());
+    checkEqualXY(ss.nextInRing(0), new Coordinate(1, 2));
+    checkEqualXY(ss.nextInRing(1), new Coordinate(3, 1));
+    checkEqualXY(ss.nextInRing(2), new Coordinate(0, 0));
+    checkEqualXY(ss.nextInRing(3), new Coordinate(1, 2));
+  }
+
+  public void testPrevInRing() {
+    SegmentString ss = create("LINESTRING(0 0, 1 2, 3 1, 0 0)");
+    assertTrue(ss.isClosed());
+    checkEqualXY(ss.prevInRing(0), new Coordinate(3, 1));
+    checkEqualXY(ss.prevInRing(1), new Coordinate(0, 0));
+    checkEqualXY(ss.prevInRing(2), new Coordinate(1, 2));
+    checkEqualXY(ss.prevInRing(3), new Coordinate(3, 1));
+  }
+
+  private SegmentString create(String wkt) {
+    Geometry geom = read(wkt);
+    return new BasicSegmentString(geom.getCoordinates(), null);
+  }
+
+}