I'm currently working on fast routines for the multiplication of arbitrary long integers. This sounds like it's been done a lot of times before, but I find the maths behind it very interesting and it allows me to study that and certain aspects of the implementation in great detail. Stimulating.

The current status of affairs is that my Schönhage-Strassen FFT multiplication code works, yeah! There's still a lot to be refined, but right now it beats the Toom-Cook mul above ~2200 64-bit words.