What kind of a tree does a HISM build

A HISM builds a tree representation of its instances every so often via FClusterBuilder::BuildTree()

What I’d like to know is what kind of a tree this is: a k-d tree, an R-tree, an octree…

I’ve been looking at BuildTree’s source code and have even made some optimizations to it here and there, but still don’t get the bigger picture of what type of tree I am optimizing.

Please provide as much detail as you can, it will go a long way to making everyone understand how it all works. I’ll do a PR with docs & comments once I understand the inner mechanics.