I'm not a particularly good programmer, so I may be missing something here, but I'd think that moving the items should be its own function that happens in between gets and sets, no? Besides adding a decidedly non-read-only operation to what should be read-only, shouldn't the movement happen when you're not trying to do anything with the hash table?
typically rehashing is done during insert. if you were going to do it while not doing anything else with the hash table you'd have to use a background thread, and then you'd have to do some form of synchronization, so there would be significant additional complexity and overhead.
