Proven Sudoku
Techniques / Brute force and theory
Difficulty Rating Engines
Also called Sudoku Explainer rating, SE rating, HoDoKu score, Puzzle rating.
New to this shorthand? How to read a sudoku grid explains r4c7, houses, candidates and the rest.
What it means
Programs give a puzzle a score. They solve it, then watch how hard they had to work. Each program uses its own scale. So the same board can be easy in one app and hard in another. That is why labels rarely match.
The exact rule, for stronger players
The rule, stated exactly: A rating engine solves a puzzle with a fixed ladder of techniques and reports a number derived from the hardest technique required, or from a weighted sum of the steps taken.
What it removes: None.
Why it works: Not a proof. A rating is a property of the rating engine as much as of the puzzle, which is why two engines disagree on the same grid.
Worked examples
None yet. This pattern does not turn up in our daily boards, so there is no real example to show. We would rather wait than draw a fake one.
Learn these first
- Exact Cover and Dancing LinksA sudoku can be rewritten as a tidy tick box puzzle.
Related brute force
- Minimal Puzzles and the 17-Clue ResultA puzzle is minimal when you cannot take away any clue without giving it a second answer.
- SAT and Constraint Programming EncodingsThe puzzle is turned into a long list of yes or no questions.
- Unavoidable Sets and Puzzle GenerationAn unavoidable set is a group of squares you could rearrange to get a second answer.
How often does it come up?
Our rough estimate: this page exists because it explains what a difficulty label on a book puzzle actually means, which is the site's whole positioning.