Those guys all interacted with each other and the environment (though the engine was designed to do the interactions in slices, where 1/N of the guys would be checked each frame, but N was not high, like 4 or 5 maybe?)
Just 2 days ago I ran across my all my written notes from GDC 2002 (the only GDC I've gone to; I went by myself for fun) with that specific session, and I had your name written down. (I just thought that was a funny coincidence.) It was very memorable, especially the one with the Doom guy doing the Robotron thing with the camera zooming out. Late last year, Casey Muratori also mentioned it on his handmade hero webcast.
In particle simulations interactions can happen at a distance, so at every time step all particles need to be checked with all other nearby particles to get their distances and compute the force of their interaction. It looks like your simulation is a hard-sphere type interaction where you're only checking to see if two particles bump into each other and is considerably faster to compute.
No, it's not really different from a hard-sphere type interaction. It's the same thing. But that doesn't matter in this context, because in the system we built, our problem was harder/more-general because we were simulating 'guys', meaning you can program an arbitrary interaction into your guys (is guy type X near guy type Y, if so what happens?), and those 'guys' also interacted with a 2D height field terrain.
Keep in mind this was all on computers from the year 2002, which were pretty damn slow compared to computers today. Today you could do a lot of guys.
> though the engine was designed to do the interactions in slices, where 1/N of the guys would be checked each frame, but N was not high, like 4 or 5 maybe?
I'm assuming they all still moved, and the checking was just for change-of-independent-behaviour-pattern?
Yes, everyone moved every frame. If I remember correctly, they would get linearly extrapolated until next time the update for that guy ran. I think there was also an option to do the terrain interaction every frame, to avoid any perception of guys sinking into the terrain slightly.
