FCC cube generator

I could have used this a week ago.

def fccstack():
	newlimit = 0
	while True:
		oldlimit = newlimit
		newlimit += 1
		# extend z
		for x in xrange(oldlimit):
			for y in xrange((x+oldlimit)&1, oldlimit, 2):
				yield (x,y,oldlimit)

		# extend y
		for x in xrange(oldlimit):
			for z in xrange((x+oldlimit)&1, newlimit, 2):
				yield (x,oldlimit,z)

		# extend x
		for y in xrange(newlimit):
			for z in xrange((y+oldlimit)&1, newlimit, 2):
				yield (oldlimit,y,z)

g = fccstack()
for dummy in xrange(512):
	p,q,r = g.next()
	print "%d %d %d\t%d" % (p,q,r, p+q+r)

This lists coordinates of sites in a face-centred cubic lattice, filling the smallest cube that contains the number of sites required.