How to test a hash function?

Question

Is there a way to test the quality of a hash function? I want to have a good spread when used in the hash table, and it would be great if this is verifyable in a unit test.

Answer 1

First I think you have to define what you mean by a good spread to yourself. Do you mean a good spread for all possible input, or just a good spread for likely input?

For example, if you're hashing strings that represent proper full (first+last) names, you're not going to likely care about how things with the numerical ASCII characters hash.

As for testing, your best bet is to probably get a huge or random input set of data you expect, and push it through the hash function and see how the spread ends up. There's not likely going to be a magic program that can say "Yes, this is a good hash function for your use case.". However, if you can programatically generate the input data, you should easily be able to create a unit test that generates a significant amount of it and then verify that the spread is within your definition of good.

Edit: In your case with a 64 bit long, is there even really a reason to use a hash map? Why not just use a balanced tree directly, and use the long as the key directly rather than rehashing it? You pay a little penalty in overall node size (2x the size for the key value), but may end up saving it in performance.