rc_alloc

Claims a cache slot for every patch of world the frame is looking at, walking the frame at a coarse stride. A patch already in the table is stamped with this frame and with the pixel that can see it; one that is not takes the first slot that is empty or whose occupant has aged out, through an atomic exchange so two pixels racing for the same slot produce one winner.

Stamping is half the eviction policy: what no pixel asks for stops being stamped, and rc_gather releases it.