That was my conclusion too, this is a super easy problem with a fixed window length. The bit hashing is neat but I was too confused by what seemed to be an over-complication of the problem to really appreciate it. Did you and I both miss something here?
I don't think so. As much as I love using bit operations: in this case I'd actually prefer a table of character counts for a "true" O(n) solution, as bit counting isn't guaranteed to be a "native" operation.
