One thing we've discovered is that something like

    ceilings []byte

looks deceptively "small", but if the median number of transitions is really something like 10, you're paying 24 bytes overhead to "save memory". Depending on your priorities, something like

    ceilings [16]byte
    ceilingsExtend *[]byte

will be the same size with better locality and GC interaction for the common case, or even two separate structures with the same interface (above 24 it sounds like you'd probably want to switch to map anyway). Or just make a hard limit of e.g. 24 transitions per.