private Set<Particle> getSingleChunck(Particle current) {
		Set<Particle> impacting = new HashSet<Particle>();
		impacting.add(current);
		while (true) {
			Set<Particle> tmp = new HashSet<Particle>();
			for (Particle pi : impacting) {
				tmp.addAll(pi.impacting);
			}
			boolean changed = impacting.addAll(tmp);
			if (!changed) {
				break;
			}
		}
		// now impacting have all the chunk of collapsing particles
		return impacting;
	}