> // This assumes keys is sorted (which I think is safe), but if not > you can always sort them yourself
This algorithm is what I was thinking of. Its basically the top N tags example but just counting uniques. There is a gotcha in that you need to sort the keys yourself because of a weird reduce thing I never tracked down completely. IIRC, it was something about how the final reduce runs that requires this. Otherwise, what Randall said.
