I created a flexible collision engine for broad detection in C that's also pretty fast. Personally that's my best, so I wanted to show it here. Although... still not as fast as DragonEnergy's one that supposedly handled millions of agents bouncing off each other every frame on an old i3, haha. I hope I get better at this! He set the bar pretty high for me.

I was considering including some small function that tries finding the nearest entity, although I think it might be better to simply use query for that matter. Perhaps in the future I will get a good idea of how to do that more efficiently.

This is not a clickbait. Although generally you also call other functions in a tick, a collision check between 500,000 circles ("circles", not "squares", because distance is checked and forces are applied based on the angle between) is done below 5ms on not really that demanding hardware. I achieve lower times than that on my budget laptop, slightly more on my PC (yeah, weird). The total time spent in a tick is 3-4 times that.