I wrote a multiplayer shooter in C++ without a game engine (~170k LoC). The entire simulation is fully deterministic across Windows, Linux, MacOS and the Web built via Emscripten, recently even aarch64. Compared to e.g. state snapshots, this is arguably the hardest networking strategy that's mainly used by RTS games to synchronize hundreds of units without bloating traffic.

The goal is to be the one free and open-source Esports game. Hypersomnia was recently played on a massive LAN tournament:

https://store.steampowered.com/news/app/2660970/view/4186736...

- The atlas packer I wrote for the game was reused in Assassin's Creed, Skydio and received 2 scientific references: https://github.com/TeamHypersomnia/rectpack2D

- There were significant challenges in making Box2D suitable for this, e.g. it comes without operator= so I had to implement complete cloning of the physical world for when I have to roll back the predicted world to reconcile it with the server state. Note it's not enough to just recreate the b2World from velocities and positions - you have to clone all contacts, generated trees, and all cached state so that the order of subsequent operations stays identical.

Read more about the interesting aspects of the game in the tech-highlights section: https://github.com/TeamHypersomnia/Hypersomnia#tech-highligh...