Adjacent Pair Scan: Mental Model

Start with the idea

Begin with the concrete question: Count positions where a reading is lower than the reading immediately before it. For the visible input [3, 5, 4, 4, 1], the stated result is 2. Before reading code, name the smallest piece of information that must survive from one step to the next. A scan turns a sequence into a small piece of state. Say what the state means after the first i elements, before you describe a loop body. That is the mental model for Adjacent Pair Scan; it gives you a reason for each update instead of a line of code to memorise.

The core mechanism is this: Compare each item with its predecessor while keeping only local context. 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.