For example, the advection of the density field it was quite eye opening to see how the reverse lookup worked, and then how the sampling is done by bi-linear filtering. On the flip side, the pressure projection I really have no idea what's going on. That, was utterly a black box for me. It has _something_ do do with the gradient of the velocity field, or so it seems, so maybe another look through the code would help.
It's been fun, perhaps I can find a simple sph simulation I could cobble together.