Puzzle Cottage
How the rating works
The rater tries to solve the puzzle using progressively harder techniques and reports the hardest one needed:
- Easy: solves with naked singles only. Every empty cell that can take only one digit gets filled, recursively.
- Medium: requires hidden singles — a digit that can only legally go in one cell of a row, column, or box.
- Hard: requires naked pairs/triples or pointing pairs — intermediate constraint-propagation techniques.
- Expert: requires X-wing, swordfish, or jellyfish — multi-row pattern recognition.
- Brute-force: can’t be solved by these techniques; backtracking required. Some “diabolical” puzzles fall here.
- Invalid: the puzzle has no solution or multiple solutions. Reports which.
The rating reflects what a logical solver would need. Computer-solver-only puzzles (those that always require backtracking) are reported as “brute-force needed,” which is a fairness marker: human-grade Sudoku always has a logical solution path.
Why clue count alone isn’t difficulty
A 30-clue puzzle can be Easy if all the empty cells reduce to naked singles; another 30-clue puzzle can be Expert if it requires X-wings to make progress. Common heuristics like “more blanks = harder” are loosely correlated but not deterministic.
Real difficulty depends on:
- Which techniques are needed — the central rating factor.
- How many times each technique is needed — one X-wing is harder to spot than three.
- Whether techniques can be applied in any order or require a specific sequence.
- Whether trial-and-error is required at any step.
This rater focuses on the first factor (which techniques are required at all). Commercial Sudoku solvers often use a more elaborate scoring system that weights the second and third factors too.
Related
- Free daily Sudoku — play three difficulty levels
- Sudoku tips when stuck — techniques explained with worked examples
- The math behind Sudoku — algorithms, NP-completeness, generation
- Easy Sudoku printables · Medium · Hard