Puzzle generation and grading

The catalog was generated for Sudoku Slate on September 6, 2026. No third-party puzzle collection was imported. “Original” describes that generation process; it does not claim that no other person or program could independently produce a matching puzzle.

Each accepted puzzle starts from its own generated complete grid. We also check clue-pattern differences under standard Sudoku symmetries, including relabeling digits, rearranging rows and columns within their allowed groups, and reflecting or rotating the grid.

1. 01

   Generate a completed grid

   A seeded generator fills an empty board using randomized search. The seed makes the catalog reproducible; it does not provide a uniform sample of every possible Sudoku.

2. 02

   Remove clues and check uniqueness

   The generator visits cells in a shuffled order. It removes a clue only when an exact search still finds one solution. The search stops after two solutions because that is enough to reject ambiguity.

3. 03

   Solve and rate difficulty

   A second solver works through the supported human techniques without guessing. The puzzle is accepted only if that solver finishes and reaches the same answer as the exact search.

4. 04

   Assign a stable puzzle ID

   Accepted puzzles receive a versioned ID. The same ID always refers to the same puzzle in this catalog. Pages use the precomputed puzzle and answer rather than generating a new board during each visit.

The grader repeatedly tries naked singles, hidden singles, locked candidates, naked pairs and hidden pairs, in that order. After each successful placement or elimination it starts again with singles. The most advanced technique used in that complete solving path sets the label.

These site-specific technique ratings do not measure every player's difficulty, predict solving time or prove the shortest solution. Clue count alone does not determine the level.

The grader does not use triples, fish, chains, uniqueness-based deductions or guessing. A puzzle it cannot finish with the five supported techniques is excluded. The game’s hints currently explain singles and candidates; the catalog rating does not mean the hint interface teaches every pair or elimination step.

The daily puzzle changes at 00:00 UTC. Everyone viewing the same UTC date receives the same board. Medium, Hard and Expert rotate in that order through a 384-day cycle with no repeated board inside the cycle.

After 384 days, the underlying boards recur. A daily ID still includes its date, so completion records distinguish different dates that reuse a board. Crossing midnight updates the entry point to the new daily puzzle; it does not overwrite a puzzle you are already playing.

Ordinary difficulty sequences also repeat after their catalog entries are used. Each level currently contains 128 puzzles. When a puzzle returns, it keeps its original ID. The daily game, ordinary game and printable sheets draw from the same catalog, so you can encounter a board in more than one place.

The Sudoku checker accepts standard 9 × 9 clues and first checks for duplicates. An exact search then distinguishes no solution, multiple solutions and one unique solution. Finding two completions is enough to report multiple solutions; it does not calculate their total number.

The checker runs in a browser worker and stops after a 15-second limit or when you cancel. An interrupted check is not proof that a puzzle has no solution. This tool returns a diagnosis and, when unique, a completed grid; it does not claim to provide a human-style solving explanation or a difficulty rating for pasted puzzles.

If a result seems wrong, send the puzzle ID or 81-cell grid and the result you expected to contact@sudokuslate.com.