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Sounds like it's only a client. Still very impressive and might be helpful for some people, but not really where the performance bottleneck is most of the time.

The same thing done for a survival server implementation would be a lot more interesting and useful.

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Independent client (ie, not a patch on top of the Mojang client) with working physics is actually a sort of holy grail in the third party minecraft development community.

Very few implementations ever reach that level of maturity. Everyone and their mom gets the networking/keepalive/inventory stuff working. Mature clients maybe get gravity and basic movement working. A single digit number of implementations have gone farther than that.

Physics is the least documented corner of Minecraft. Effectively everything about lighting, entities, the map format, and the network protocol is well documented without having to dig through the decompiled source.

Not so with physics. No two physics implementations look alike because:

A) The Java Minecraft physics engine is a pretty tangled mess to begin with

B) Each implementation represents a unique attempt to research and re-implement that mess

What's interesting is that the Minecraft netcode is so bad that your client physics doesn't even need to be accurate or even correct for it to work. There is no verification that the client's inputs actually match the player's resulting position (not that they're even sent to the server). The client sends their position to the server, the server does some simple checks for flying, speed hacks, phasing through walls, and that's about it. Which is why there's all sorts of interesting movement cheats, and also why most servers run some kind of third-party anticheat plugin.

Client is more impressive than server in my opinion, so many people have already done tons of implementations for servers

The wiki.vg website has a list of quite a few server implementations, even some written in Rust! That site also has extensive documentation of the Minecraft protocol if you want to try writing a server or client yourself.

https://wiki.vg/Server_List

https://wiki.vg/Protocol

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