Sudoku Constraint Search: Mental Model

Start with the idea

Begin with the concrete question: Return the missing digit in row 0 of the nearly solved grid. For the visible input row0=53.678912, the stated result is 4. Before reading code, name the smallest piece of information that must survive from one step to the next. Most patterns here explore a decision tree; meet-in-the-middle instead enumerates two halves and combines them. Define the partial choice or subset summary that the pattern retains. That is the mental model for Sudoku Constraint Search; it gives you a reason for each update instead of a line of code to memorise.

The core mechanism is this: Choose an empty cell, try allowed symbols, and undo; picking the fewest candidates prunes harder. 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.