The theme is various fast ways to do large gravity simulations, but the main
build artifact turned into kind of a "star simulator" (adding pressure
alongside gravity) that renders videos like these in real-time:
https://www.youtube.com/watch?v=DqYFIrdAAio
https://mastodon.social/@sqx/113239969000196032

I also played around with the Barnes-Hut algorithm, and a Walter Dehnen "O(n)"
algorithm (https://arxiv.org/abs/astro-ph/0202512) which I struggled to
understand :-)

I ended up with a grid-based system, where acceleration from gravity is
calculated at each cell using convolution, which is optimized with FFTs.