This area of CS (one pass analysis over data streams with tight memory bounds) is pretty well-studied. They are known as "data stream algorithms." This 2002 p

This area of CS (one pass analysis over data streams with tight memory bounds) is pretty well-studied. They are known as "data stream algorithms." This 2002 paper describes a data structure for this that I suspect is superior to what is described in the article: https://www.cs.rutgers.edu/~farach/pubs/FrequentStream.pdf https://www.cs.rutgers.edu/~farach/pubs/FrequentStream.pdf