No guessing, and how we know

In most sudoku apps a hard board is a board where you eventually have to try a digit and see. Here a wrong digit costs a life, so that would just be a coin toss. The game promises something stricter.

The promise

Every board has exactly one solution, and you can reach it by reasoning alone. A harder layer or Depth asks for a better technique or fewer clues, never for luck.

How a board is made

1. The game builds a full, valid grid from the run’s seed.
2. It removes digits one at a time, and after each removal checks that exactly one solution is left.
3. It then plays the board itself, step by step, with three layers of technique and nothing else: a square that has only one digit left; a digit that fits only one square in its row, column or box; and pairs, hidden cages and thermometer limits, the more advanced steps that the later layers use.
4. If the board cannot be finished with those, it is thrown away and made again. If it can, the technique it needed sets how hard the board is.

This runs for every board, on your device, before you see it. There is no list of “approved” boards and no board that was only tested once.

The same holds for the extras

• Hidden cages (the Library bosses) are only hidden when the board stays unique without them, so a hidden cage can never leave you guessing.
• Guarded squares (the other four bosses) stay shut until the rest of their group is filled in. The game checks that the board can still be solved by reasoning while those squares are shut, and adds clues until it can.
• Relics depend only on the seed, the layer, the world and what you already hold, and everything on offer is shown. Nothing is rolled while you watch.
• The seed decides a whole run: the same seed gives the same map, the same boards and the same relics on offer.

A promise you can check

A normal run starts from a secret seed that the game makes at random on your device. The seed decides everything, so you may ask whether the game could have picked it with an eye on you. Before your first move the game therefore shows the SHA-256 fingerprint of the secret, and you can copy it. When the run is over the game shows the secret itself and checks that its fingerprint is the one it promised. You can check it yourself: put the secret in any SHA-256 tool and compare.

You may add a word of your own after the promise was made; it is mixed into the seed, so the game could not have planned around it. A daily run or a seed you type yourself is open from the start and has no promise. What this proves is that the seed was fixed before you began. It does not say anything about how hard a board feels.

Teaching it

Six short, optional lessons with Nikkel the gnome show the techniques on real boards: the only digit that fits a square, the only square a digit fits, cages, thermometers, diagonals and pairs. The Lantern relic points at the easiest next square.

See the worlds