I think I understand how to build a hash table except for one thing: which hash function should you use? Is there a simple one that’s good enough for most things? How do you pick one?
You would write your hash table constructor to accept a hash function as a function pointer then pass in the appropriate hash function for whatever the key type is.
You can use the same hash function for any arbitrary struct as long as there's no pointers in it. If there are pointers, you'd have to implement one combining the hashes of all the members through something like xor. I used fnv1a_64 for my hash function in AOC 2020 which is a pretty good general choice for strings and other contiguous objects and is just 10 lines of code. Pseudocode on Wikipedia or I'm sure you can find C code on Stackoverflow. Hope that helps :)
if your hash table is a really good modern open addressed hash table, something like Google's Swiss Tables, you need a really good hash because you will be badly penalised otherwise, these modern structures demand a hash with proper behaviour for even halfway reasonable performance.
If it's just a My First Hash table with closed addressing, buckets etc. you needn't care too much. If you're hashing integers, just use the integer itself, that's a terrible "hash" but with this like 1980s data structure it's good enough.
