Cyclic barrier synchronization rounds

N threads repeatedly synchronize at a cyclic barrier of party size N: a barrier 'trips' (releases) exactly when N threads have called arrive() since the last trip, and the counter then resets for the next round. You are given a list arrivals of thread ids in the order they call arrive(). Each arrival increments the count; every time the count reaches N the barrier trips (count a completed round) and resets to 0. Return a pair [rounds, waiting] where rounds is the number of completed barrier trips and waiting is the number of threads currently blocked at the barrier (arrived but not yet released) at the end.