> The system was deterministic enough ...

I wrote about it here many times over the years but in 1991 I wrote a little DOS game (and I had a publisher and a deal but it never came out and yet it's how my career started but that's another story) and at some point I had an "impossible to find" bug because it was so hard to reproduce.

So I modified my game engine to be entirely deterministic: I'd record "random seed + player input + frame at which user(s) [two players but non-networked] input was happening". With that I could make tiny save files and replay (and I did find my "impossible to find" bug thanks to that).

First time I remember someone talking about it was a Gamasutra article by an Age of Empire dev (article which another poster already mentioned here in this thread): they had a 100% deterministic engine. FWIW I wrote an email to the author of that article back then and we discussed deterministic game engines.

Warcraft 3 definitely had a deterministic game engine: save files, even for 8 players (networked) games were tiny. But then you had another issue: when units, over different patches, would be "nerfed" to balance the game (or any other engine change really), your replay files wouldn't play correctly anymore. The game wouldn't bother shipping with older engines: no backward compatibility for replay files.

I had a fully deterministic game engine in 1991 and, funnily enough, a few days ago with the help of Claude Code CLI / Sonnet 4.6 I compiled that old game of mine again (I may put it on a public repo one day): I still had the source files and assets after all those years, but not the tooling anymore (no more MASM / no more linker) so I had to "fight" a bit (for example I had not one but two macros who now clashed with macros/functions used by the assembler: "incbin" and another one I forgot) to be able to compile it again (now using UASM, to compile for DOS but from Linux).

Another fun sidenote... A very good friends of mine wrote "World Rally Fever" (published by Team 17) and I was a beta tester of the game. Endless discussion with my friend because I was pissed off for his engine was so "non-deterministic" than hitting the Turbo button on my 486 (I think it was a 486) while I was playing the game would change the behavior of the (computer) opponents.

https://youtu.be/NhRQWNqbvTk

To me a deterministic game engine, unless you're a massively networked multi-player game, just makes sense.

Blizzard could do it for Warcraft 3 in 2002 for up to 8 players and hundreds of units. Several games had it already in the nineties.

It simplifies everything and I'd guesstimate something like 99% of all the game out there that don't do it could actually do it.

But it touches to something much more profound: state and how programmers think about state and reproducibility. Hint: most don't think about that at all.

Some do though: I was watching a Clojure conf vid the other day and they often keep hammering that "view is a function of state". And it is. That's how things are. It was true in 1991 when I wrote my DOS game, it was true for Age of Empire, Warcraft 3 and many other games. And it is still true today.

But we're in 2026 and there are still many devs insisting that "functional programming sucks" and that we should bow to the mutability gods for that is the only way and they'll fight you to death if you dare to say that "view <- fn(state)".

This explains that.