// Insertion sort for Sweep and Prune
    public void insertionSort(Comparable[] a)
    {
         for( int p = 1; p < ballCount; p++ )
         {
             Comparable tmp = a[ p ];
             int j = p;

             for( ; j > 0 && tmp.compareTo( a[ j - 1 ] ) < 0; j-- )
                 a[ j ] = a[ j - 1 ];

             a[ j ] = tmp;
         }
    }


    public void checkCollisions()
    {
        insertionSort(balls);


        // Check for collision with walls
        for (int i = 0; i < ballCount; i++)
        {
        //  System.out.println("Ball #" + i + ": " + (balls[i].position.getX() - balls[i].getRadius()));

            if (balls[i].position.getX() - balls[i].getRadius() < 0)
            {
                balls[i].position.setX(balls[i].getRadius()); // Place ball against edge
                balls[i].velocity.setX(-(balls[i].velocity.getX() * Constants.restitution)); // Reverse direction and account for friction
                balls[i].velocity.setY(balls[i].velocity.getY() * Constants.restitution);
            }
            else if (balls[i].position.getX() + balls[i].getRadius() > getWidth()) // Right Wall
            {
                balls[i].position.setX(getWidth() - balls[i].getRadius());      // Place ball against edge
                balls[i].velocity.setX(-(balls[i].velocity.getX() * Constants.restitution)); // Reverse direction and account for friction
                balls[i].velocity.setY((balls[i].velocity.getY() * Constants.restitution));
            }

            if (balls[i].position.getY() - balls[i].getRadius() < 0)                // Top Wall
            {
                balls[i].position.setY(balls[i].getRadius());               // Place ball against edge
                balls[i].velocity.setY(-(balls[i].velocity.getY() * Constants.restitution)); // Reverse direction and account for friction
                balls[i].velocity.setX((balls[i].velocity.getX() * Constants.restitution));
            }
            else if (balls[i].position.getY() + balls[i].getRadius() > getHeight()) // Bottom Wall
            {
                balls[i].position.setY(getHeight() - balls[i].getRadius());     // Place ball against edge
                balls[i].velocity.setY(-(balls[i].velocity.getY() * Constants.restitution));    // Reverse direction and account for friction
                balls[i].velocity.setX((balls[i].velocity.getX() * Constants.restitution));
            }

            // Ball to Ball collision
            for(int j = i + 1; j < ballCount; j++)
            {
                if ((balls[i].position.getX() + balls[i].getRadius()) < (balls[j].position.getX() - balls[j].getRadius()))
                        break;

                if((balls[i].position.getY() + balls[i].getRadius()) < (balls[j].position.getY() - balls[j].getRadius()) ||
                   (balls[j].position.getY() + balls[j].getRadius()) < (balls[i].position.getY() - balls[i].getRadius()))
                        continue;

                balls[i].resolveCollision(balls[j]);

            }
        }

    }