How 123doku makes sure every grid has exactly one solution

Every grid in 123doku has exactly one solution, and you can always reach it by logic alone. That promise sounds simple, but it shaped the whole project.

Generated offline, checked twice Grids are not created on your phone. They are generated in advance on my computer, then checked by two separate programs: an exhaustive solver that proves the solution is unique, and a deduction engine that solves the grid step by step, the way a player would, without ever guessing. A grid that is unique but needs a guess is thrown away.

Ranked by the hardest step Each grid gets a difficulty score based on the hardest deduction it requires, not just its size. That is how the game adapts to your level: you get grids that ask for the next technique you are ready to learn.

New in 1.5: exclusive grids Players who reach the maximum level now unlock exclusive grids with no starting stone at all, only the row and column sums. Finding grids like that is rare: out of hundreds of thousands of random 8x8 attempts, only a tiny fraction were both unique and solvable by pure logic.

Thanks to everyone who tried the game here this week!