It looks like it's using a fluid solver. So, it's not that every particle is interacting with every other particle, but that there's a flow vector being establi

It looks like it's using a fluid solver. So, it's not that every particle is interacting with every other particle, but that there's a flow vector being established at each point on the screen and that's being added to each particle's position. Disclaimer: I haven't actually analyzed the source.