Any tips on how to implement a hashmap in C with (close to) zero collisions while not allocating much more space than necessary?

I’m new to this sort of stuff, so pardon my ignorance. Here is what I have so far: https://github.com/dannyvankooten/c-hashmap

It distributes pretty evenly, but in my example using ~45.000 dates keys about 19% share a spot with at least one other nose.