[math-fun] hilbertish curve via gray code in radix R

View X as an unsigned integer with <=D digits written in radix R, and also as the D-vector of those digits. The function uint gray(uint X){ return VectorSubtract(X, RightShift(X)) mod R; } converts X to a different such integer. This is a radix-R generalization of the usual binary gray code. This maps the integers which walk from 0 to R^D - 1, to D-dimensional vectors within the hypercube grid {0,1,2,...,R-1}^D which walk in hamilton-tour style via nearest neighbor steps, visiting each grid point exactly once. Extremely trivial implementation of a Hilbertish space filling curve... but it treats the different coordinates very differently, which is annoying.