The insight is that the only way to get W bits set is for the last W letters to all have been unique. Duplicate letters toggle bits on and off, but never get yo

The insight is that the only way to get W bits set is for the last W letters to all have been unique. Duplicate letters toggle bits on and off, but never get you any extra 1 bits set. If you wanted to detect V unique characters from a window of W characters where V < W, this trick wouldn't work. But you could still have a rolling window, it would just be a map counting "how many of this character in the window", which you can increment the relevant character for as it enters the window, and decrement as it leaves.