Rolling Fingerprint: Mental Model

Start with the idea

Begin with the concrete question: Using base 10 and modulus 101, return the hash of digits 3,1,4. For the visible input digits=[3,1,4], the stated result is 11. Before reading code, name the smallest piece of information that must survive from one step to the next. A key-derived representation lets later work reuse an earlier fact. It may be a set, a map from keys to state, compressed ranks, or a rolling fingerprint; name which one this pattern uses. That is the mental model for Rolling Fingerprint; it gives you a reason for each update instead of a line of code to memorise.

The core mechanism is this: Update a window hash by removing the outgoing contribution and adding the incoming one. Treat it as a claim to test after every operation. Write down the state before the first operation, update it once, and ask which part of the claim is still true. If an item is skipped, inserted, or removed, explain why that action cannot destroy an answer that should be retained. A useful trace has columns for the current input position, the state before the step, the action, and the state afterward. Use the visible sample until the expected result becomes inevitable rather than merely plausible.

Now inspect the Rust sketch, not just its final expression. The supplied Rust sketch is a starting point.