Optimize Node.collideWith with no additional CollisionResults allocation

jme3-core/src/main/java/com/jme3/collision/DummyCollisionResults.java

@@ -0,0 +1,77 @@
+/*
+ * Copyright (c) 2009-2014 jMonkeyEngine
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ * * Redistributions of source code must retain the above copyright
+ *   notice, this list of conditions and the following disclaimer.
+ *
+ * * Redistributions in binary form must reproduce the above copyright
+ *   notice, this list of conditions and the following disclaimer in the
+ *   documentation and/or other materials provided with the distribution.
+ *
+ * * Neither the name of 'jMonkeyEngine' nor the names of its contributors
+ *   may be used to endorse or promote products derived from this software
+ *   without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
+ * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+ * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+ * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+ * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
+ * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
+ * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+ * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+package com.jme3.collision;
+
+import java.util.Collections;
+import java.util.Iterator;
+import java.util.List;
+
+/**
+ * <code>DummyCollisionResults</code> is an empty "dummy" collection returned as a result of a 
+ * collision detection operation done by {@link Collidable}.
+ * Its purpose is to avoid allocation of CollisionResults and its content, 
+ * when collisionResults are out of our interest.
+ * Using the same API Collidable.collideWith() instead of possibly some new countCollideWith() implementation
+ * is chosen to keep the collision detection code "DRY", @see <a href="http://en.wikipedia.org/wiki/Don%27t_repeat_yourself">Don't repeat yourself</a>
+ * 
+ * @author Bebul
+ */
+public class DummyCollisionResults extends CollisionResults {
+  // the singleton for DummyCollisionResults is sufficient
+  private static DummyCollisionResults instance = null; 
+  
+  // exists only to defeat instantiation
+  protected DummyCollisionResults() {}
+  
+  // classic singleton instance
+  public static DummyCollisionResults getInstance() {
+    if ( instance == null ) {
+      instance = new DummyCollisionResults();
+    }
+    return instance;
+  }
+  
+  // Dummy implementation of CollisionResults
+  // All methods can be dummy except for iterator
+  @Override public void clear(){}
+  // even dummy iterator should return valid iterator
+  @Override public Iterator<CollisionResult> iterator() {
+    List<CollisionResult> dumbCompiler = Collections.emptyList();            
+    return dumbCompiler.iterator();
+  }
+  @Override public void addCollision(CollisionResult result){}
+  @Override public int size(){ return 0; }
+  @Override public CollisionResult getClosestCollision(){ return null; }
+  @Override public CollisionResult getFarthestCollision(){ return null; }
+  @Override public CollisionResult getCollision(int index){ return null; }
+  @Override public CollisionResult getCollisionDirect(int index){ return null; }
+}

jme3-core/src/main/java/com/jme3/scene/Node.java

@@ -34,6 +34,7 @@ package com.jme3.scene;
 import com.jme3.bounding.BoundingVolume;
 import com.jme3.collision.Collidable;
 import com.jme3.collision.CollisionResults;
+import com.jme3.collision.DummyCollisionResults;
 import com.jme3.export.JmeExporter;
 import com.jme3.export.JmeImporter;
 import com.jme3.export.Savable;
@@ -491,13 +492,17 @@ public class Node extends Spatial implements Savable {
         int total = 0;
         
         // optimization: try collideWith BoundingVolume to avoid possibly redundant tests on children
+        // number 4 in condition is somewhat arbitrary. When there is only one child, the boundingVolume test is redundant at all. 
+        // The idea is when there are few children, it can be too expensive to test boundingVolume first.
         if (children.size() > 4)
         {
           BoundingVolume bv = this.getWorldBound();
           if (bv==null) return 0;
 
-          CollisionResults bvColRes = new CollisionResults();
+          // use DummyCollisionResults to avoid allocation while making the call to any BoundingVolume.collideWith possible
-          if (bv.collideWith(other, bvColRes) == 0) return 0;
+          // moreover, the DummyCollisionResults operations are empty, so faster
+          DummyCollisionResults dummyColRes = DummyCollisionResults.getInstance();
+          if (bv.collideWith(other, dummyColRes) == 0) return 0;
         }
         for (Spatial child : children.getArray()){
             total += child.collideWith(other, results);