Given a stream of pairs (id, weight), write a function that selects an id at random, with probability proportional to its weight (compared to the total weight sum when the stream has ended). The stream size is unknown in advance and you're allowed O(1) memory.
Example: if the stream was {(1,2),(2,2),(3,4),(4,10)}, then id 2 would be returned with probability 1/9.
