A 1.8.9 anticheat I have worked on some time ago for quite a while, never finished it. Posting this to show how a simulation anticheat can work. I'm slowly working on a better multi version anticheat with things I learned while writing this anticheat.
This anticheat is not meant to be compiled or used its only here for educational purposes about simulation engines.
Theres some nice concepts in the anticheat like SplitState/ConfirmableState which allows a specific state to be in 2 states at once to account for transaction splitting. (nearly every block is fully lag compensated on both state level and bounding box level by checking neighbours) There's also a mostly "working" area based simulation engine which has quite support for mostly everything in 1.8.9
Some parts are written quite poorly and need a complete rewrite (eg InventoryTracker), some parts are purely just a proof of concept. Another limitation is that the start area is only one area (optimally this should support multiple areas to have less leniency). Theres also a half baked cloud based system I was working on, the cloud server is not provided in source. Theres probs more bad stuff but you'll see it in the code!
The reach and timer part is mostly based on Dusk, I would recommend taking a look.
If you have any questions feel free to contact me on discord @1.7.10 (or join the Minecraft Anticheat Community). I also have telegram @Beaness.