Fibonacci Hashing: The Optimization that the World Forgot (or: a Better Alternative to Integer Modulo)

I recently posted a blog post about a new hash table, and whenever I do something like that, I learn at least one new thing from my comments. In my last comment section Rich Geldreich talks about his hash table which uses “Fibonacci Hashing”, which I hadn’t heard of before. I have worked a lot on hash tables, so I thought I have at least heard of all the big important tricks and techniques, but I also know that there are so many small tweaks and improvements that you can’t possibly know them all. I thought this might be another neat small trick to add to the collection.

Turns out I was wrong. This is a big one. And everyone should be using it. Hash tables should not be prime number sized and they should not use an integer modulo to map hashes into slots. Fibonacci hashing is just better. Yet somehow nobody is using it and lots of big hash tables (including all the big implementations of std::unordered_map) are much slower than they should be because they don’t use Fibonacci Hashing. So let’s figure this out.

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