yeah, unfortunate "hard studying student who eats algorithms for breakfast" compared to "boring reality". I'm sure there's a fancy data structure for this. In reality, I'd make three buckets (one for each day, and a "isLoyal" byte buffer), and update them as I scan along. O(N) time, O(N) space.

"they don’t know the size of the data upfront", okay. I spend a scan finding the highest customer number and probably make some 10MB index-assossiated buffer. If I'm fancy I find the range and use offset indices to reduce the overall size. You already said it fits in memory and I'm not a distsrubuted programmer. Space is cheap in boring reality

I guess it's one of those cool brain teasers that gets you excited to use your skills from college. Not many get to in reality. Or they prefer other domain-specific skills.