  I describe a fast algorithm for the identification of connected sets of
points where the point-wise connections are determined by a fixed spatial
distance - a task commonly referred to in the cosmological simulation community
as Friends-of-Friends (FOF) group finding. This technique sorts particles into
fine cells sufficiently compact to guarantee their cohabitants are linked, and
uses locality sensitive hashing to search for neighbouring (blocks of) cells.
Tests on N-body simulations of up to a billion particles exhibit speed
increases of factors up to 20x compared with FOF via trees (a factor around 8
is typical), and is consistently complete in less than the time of a k-d tree
construction, giving it an intrinsic advantage over tree-based methods. The
code is open-source and available online at https://github.com/pec27/hfof .
