OKtree
OKtree is a simple octree implementation in OCaml, intended to efficiently find the nearest match for an arbitrary point among a set of target points in 3D coordinate space.
An octree is a tree data structure in which each internal node has exactly eight children. Octrees are most often used to partition a three-dimensional space by recursively subdividing it into eight octants. Octrees are the three-dimensional analog of quadtrees.