The friends-of-friends (FoF) algorithm has been used for over 40 years in extragalactic astronomy to identify galaxy-groups in redshift surveys. Whilst this simple percolation algorithm was sufficient in the era of 100s of thousands of galaxies, the next generation of redshift surveys (DESI, WAVES, 4HS) promise to take us well into the 10s of millions. And, since FoF scales quadratically, we have reached a point where the classic implementation is entirely impractical.

Nessie is a group finder with the explicit goal of modernizing the FoF algorithm for this next generation of redshift surveys. In this talk I will explain how Nessie implements tree data structures to reduce the complexity of the FoF algorithm; why this is different to other group finders and other FoF implementations; and finally, talk about how the rust ecosystem made Nessie possible, and attempt to convince other astronomers to adopt it for their next project.