Trying to make more efficient

src/Body.java

@@ -37,12 +37,14 @@ public class Body {
      * Hint: see simulation loop in Simulation.java to find out how this is done.
      */
     public Vector gravitationalForce(Body b) {
-        Vector direction = b.massCenter.minus(massCenter);
+        Vector direction = b.massCenter.minus(this.massCenter);
         double distance = direction.length();
-        if (distance == 0) return new Vector();
+        if (distance == 0) return direction;
+
         direction.normalize();
-        double force = Simulation.G * mass * b.mass / (distance * distance);
-        return direction.times(force);
+        double force = Simulation.G * this.mass * b.mass / (distance * distance);
+        direction.mul(force);
+        return direction;
     }
 
     /**

src/Octree.java

@@ -36,11 +36,10 @@ public class Octree {
         }
     }
 
-    public void applyForces(Body[] bodies, double t) {
+    public void applyForces(Body[] bodies, Vector[] forces, double t) {
         if (root == null) return;
-        Vector[] forces = new Vector[bodies.length];
         for (int i = 0; i < bodies.length; i++) {
-            forces[i] = root.getForcesOnBody(bodies[i], t);
+            root.addForcesOnBody(bodies[i], forces[i], t);
         }
         for (int i = 0; i < bodies.length; i++) {
             bodies[i].move(forces[i]);
@@ -51,12 +50,6 @@ public class Octree {
         if (root == null) return;
         root.draw(cd);
     }
-
-    public int getNumberOfBalancedRegions() {
-        if (root == null || !(root instanceof OctreeNode r)) return 0;
-
-        return r.getNumberOfBalancedRegions();
-    }
 }
 
 abstract class OctreeItem {
@@ -74,6 +67,8 @@ abstract class OctreeItem {
 
     abstract protected Vector getForcesOnBody(Body body, double t);
 
+    abstract protected void addForcesOnBody(Body body, Vector force, double t);
+
     abstract protected void draw(CodeDraw cd);
 }
 
@@ -109,18 +104,24 @@ class OctreeNode extends OctreeItem {
 
     @Override
     protected Vector getForcesOnBody(Body body, double t) {
-        double r = pseudoBody.massCenter().distanceTo(body.massCenter());
+        Vector v = new Vector();
+        addForcesOnBody(body, v, t);
+        return v;
+    }
+
+    @Override
+    protected void addForcesOnBody(Body body, Vector force, double t) {
+        double r = center.distanceTo(body.massCenter());
         if (r == 0) {
-            return new Vector();
+            return;
         } else if (size / r < t) {
-            return body.gravitationalForce(pseudoBody);
+            force.add(body.gravitationalForce(pseudoBody));
+            return;
         }
-        Vector force = new Vector();
         for (OctreeItem child : children) {
             if (child == null) continue;
-            force.add(child.getForcesOnBody(body, t));
+            child.addForcesOnBody(body, force, t);
         }
-        return force;
     }
 
     @Override
@@ -155,26 +156,6 @@ class OctreeNode extends OctreeItem {
                 ((octNum & 4) != 0) ? -1 : 1)
                 .times(this.size / 4));
     }
-
-    private boolean isBalanced() {
-        for (OctreeItem i : children) {
-            if (!(i instanceof OctreeLeaf))
-                return false;
-        }
-        return true;
-    }
-
-    protected int getNumberOfBalancedRegions() {
-        if (isBalanced()) return 1;
-
-        int c = 0;
-        for (OctreeItem i : children) {
-            if (i instanceof OctreeNode n) {
-                c += n.getNumberOfBalancedRegions();
-            }
-        }
-        return c;
-    }
 }
 
 class OctreeLeaf extends OctreeItem {
@@ -183,7 +164,7 @@ class OctreeLeaf extends OctreeItem {
     public OctreeLeaf(Vector center, double size, Body body) {
         super(center, size);
         this.body = body;
-        this.pseudoBody = new Body(this.body);
+        //this.pseudoBody = new Body(this.body);
     }
 
     @Override
@@ -193,14 +174,21 @@ class OctreeLeaf extends OctreeItem {
 
     @Override
     protected Vector getForcesOnBody(Body body, double t) {
-        return body.gravitationalForce(pseudoBody);
+        return body.gravitationalForce(body);
+    }
+
+    @Override
+    protected void addForcesOnBody(Body body, Vector force, double t) {
+        force.add(getForcesOnBody(body, t));
     }
 
     @Override
     protected void draw(CodeDraw cd) {
+        /*
         Vector p1 = center.minus(new Vector(size / 2));
         Vector p2 = new Vector(size).minus(new Vector(Simulation.SECTION_SIZE / 2));
         cd.setColor(Color.GREEN);
         cd.drawRectangle(p1.getScreenX(cd), p1.getScreenY(cd), p2.getScreenX(cd), p2.getScreenY(cd));
+         */
     }
 }

src/Simulation.java

@@ -23,7 +23,7 @@ public class Simulation {
     // set some system parameters
     public static final double SECTION_SIZE = 2 * AU; // the size of the square region in space
     public static final int NUMBER_OF_BODIES = 10000;
-    public static final double OVERALL_SYSTEM_MASS = 2000 * SUN_MASS; // kilograms
+    public static final double OVERALL_SYSTEM_MASS = 20 * SUN_MASS; // kilograms
 
     public static void main(String[] args) {
         CodeDraw cd = new CodeDraw();
@@ -48,15 +48,34 @@ public class Simulation {
         }
 
         long seconds = 0;
+        Vector[] forces = new Vector[bodies.length];
+        for (int i = 0; i < forces.length; i++) forces[i] = new Vector();
+
         while (true) {
             seconds++;
 
+            /*
+            BodyLinkedList mergedBodies = new BodyLinkedList();
+            for (Body b1 : bodies) {
+                BodyLinkedList colliding = bodies.removeCollidingWith(b1);
+                for (Body b2 : colliding) {
+                    b1 = b1.merge(b2);
+                }
+                mergedBodies.addLast(b1);
+            }
+            bodies = mergedBodies;
+            */
+            //long n1 = System.nanoTime();
             Octree octree = new Octree(new Vector(0, 0, 0), SECTION_SIZE);
             for (Body body : bodies) {
                 octree.add(body);
             }
-            System.out.println(octree.getNumberOfBalancedRegions());
-            octree.applyForces(bodies, 100);
+            //long n2 = System.nanoTime();
+            for (Vector f : forces) f.reset();
+            octree.applyForces(bodies, forces, 1);
+            //long n3 = System.nanoTime();
+
+            //System.out.format("%8.3f µs, %8.3f µs\n", (n2 - n1) / 1000.0, (n3 - n2) / 1000.0);
 
             if ((seconds % 60) == 0) {
                 cd.clear(Color.BLACK);

src/Vector.java

@@ -58,6 +58,18 @@ public class Vector {
         return new Vector(x * d, y * d, z * d);
     }
 
+    public void mul(double d) {
+        this.x *= d;
+        this.y *= d;
+        this.z *= d;
+    }
+
+    public void div(double d) {
+        this.x /= d;
+        this.y /= d;
+        this.z /= d;
+    }
+
     /**
      * Returns the sum of this vector and -1*v.
      */
@@ -80,7 +92,7 @@ public class Vector {
      * Returns the length (norm) of this vector.
      */
     public double length() {
-        return distanceTo(new Vector());
+        return Math.sqrt(x * x + y * y + z * z);
     }
 
     /**
@@ -88,10 +100,13 @@ public class Vector {
      * The direction and orientation of the vector is not affected.
      */
     public void normalize() {
-        double length = length();
-        x /= length;
-        y /= length;
-        z /= length;
+        div(length());
+    }
+
+    public void reset() {
+        x = 0;
+        y = 0;
+        z = 0;
     }
 
     public double getScreenX(CodeDraw cd) {