class Entity {
	static double r;
	double x, y;
	int cellX() {
		return (int)(x/r);
	}
	int cellY() {
		return (int)(y/r);
	}
	List<Entity> neighbors() {
		List<Entity> ans = new List<Entity>();
		int cx = cellX(), cy = cellY();
		for(int xi=cx-1; xi<=cx+1; xi+=1) {
			for(int yi=cy-1; yi<=cy+1; yi+=1) {
				for(Entity e : grid[xi][yi]) {
					if (e != this && this.distanceFrom(e) < r)
						ans.add(e);
				}
			}
		}
		return ans;
	}
	void move(int newX, int newY) {
		grid[cellX()][cellY()].remove(this);
		this.x = newX;
		this.y = newY;
		grid[cellX()][cellY()].add(this);
	}
}

List<Entity>[][] grid;
grid = new List<Entity>[width][height];