So one of the things I was working on as part of DeviceAtlas (but which ultimately didn’t get used) was a cache-conscious hash table in Java. It’s not unique in design – in fact it comes right out of a research paper written for the SPIRE 2005 conference by Nikolas Askitis and Justin Zobel – but the implementation was interesting to me as I’d not done optimisation work in Java in a while, and some things had changed quite a bit since I last wrote Java code. And it was a bit of an ego boost that I got it to outperform java.util.HashMap:

Of course, there are the minor criticisms that it’s nowhere near as general-purpose as the HashMap class and that HashMap is arguably exhibiting an intelligent design choice rather than cheating per se, but I like my ego so I’m going to ignore those arguments!

Looking at the more interesting bits, there’s the hash function. This came from more work by Zobel, this time with Ramakrishna for a database conference in Melbourne in 1997, and it’s a fairly simple design: for each character in the input string, left shift the interim hashcode, add the character, then right shift the interim hashcode and add that to the mix, then xor the whole lot with the interim hashcode and that’s your interim hashcode for the next iteration. Which, as a description intended to convey simplicity, probably fails. Oh well, the code is clearer: