Is Rule 16 Impossible? the Engine Logic Behind the Password Game's Chess Puzzle
By Sarah Jenkins
Running a heavy, depth-30 Stockfish engine inside the browser client on every single keystroke would instantly freeze the user's browser tab. Instead, the verification architecture balances local string validation with predetermined position solutions.
When the puzzle loads, the application binds the displayed FEN string to a specific, verified engine evaluation. In competitive computer chess, Stockfish computes candidate moves down to specific search depths, evaluating terminal nodes using positional heuristics and alpha-beta pruning. For The Password Game, the required answer mirrors the move that Stockfish flags at depth 18 or higher.
The application verifies your text input through regular expressions designed around Standard Algebraic Notation (SAN). The input box continuously scans the string for the sequence matching the solution key. If the engine determines the optimal move is a queen sacrifice on the h7 square, your string must contain that precise move. Typing the square coordinates alone (`h8` to `h7`) will fail. Typing out the word `Queen` will fail. The JavaScript engine matches characters directly against standard chess notation conventions.
| Board State Scenario | Valid Input (SAN) | Common Rejected Input | Rejection Reason |
|---|---|---|---|
| Knight captures Pawn on f7 | Nxf7 |
nxf7 or Nf7 |
Case sensitivity failure or missing capture operator |
| Forced Checkmate on back rank | Rd8# |
Rd8+ or Rd8 |
Engine insists on mate identifier (#) over check (+) |
| Ambiguous Rook move to d1 | Rfd1 |
Rd1 |
Missing file disambiguation when both rooks can move to d1 |
| Pawn push without capture | e4 |
Pe4 |
Standard notation prohibits piece letters for pawns |
Sarah Jenkins is a veteran tech journalist with over 12 years of experience covering artificial intelligence, mobile innovations, and digital ethics. Her insights have appeared in leading technology publications worldwide.
