If your walls and balls are both moving, you'll need to do collision detection by making the ball into a capsule (integrate its bounding sphere's position over current timestep) and make the wall into a quadrilateral (integrate wall's position over current timestep). If the ball's path intersects the quadrilateral, its velocity needs to be reflected at the intersection point. When your collision is dynamic like that, your performance isn't going to be O(1), which is what the poster was trying to achieve. You can make it sublinear, but when there are fewer than ~100 objects the speed gain isn't really worth the complexity. 100x100 is still only 10,000 ops, which can probably be done in javascript without performance problems. It's definitely a fun learning exercise though, and it will give you some valuable insight into how physics libraries work.