What it is
Sudoku is the pencil puzzle from the back of the newspaper: nine rows, nine columns, every digit once per row, column and box, and no guessing needed if you read the board properly. I am building a version of it in Flutter that runs on iOS, Android, macOS and Windows. It is aimed at people who play one or two puzzles a day and want the puzzle itself — nothing to sign up for, nothing to wait on.
What it does
You pick a difficulty and a puzzle appears. Five levels run from Easy up to Master, and the label is a promise about how the puzzle solves, not just how many digits it starts with.
Beyond the classic grid there are three other kinds of board: Odd-Even, where cells are marked for odd or even digits; X-Sudoku, which adds the two diagonals as extra constraints; and Killer, where outlined cages carry a sum you have to hit.
While you solve, you can write pencil marks into a cell before settling on a digit, and undo steps back through every move you have made since the puzzle started. There is a daily challenge tied to the date if you want one board a day rather than an endless supply.
The hint is the part I care most about. Ask for one and it does not simply drop the right digit into the cell. It points out the evidence already on the board, highlights the row, column or box that does the work, and then names the placement — so you finish the hint knowing a technique you can use again. Thirteen techniques are covered, from a cell with one candidate left through X-Wing and Swordfish, plus the cage arithmetic that Killer boards turn on. Only when none of them applies to the cell you picked does it fall back to revealing the answer.
Puzzles are made on the device and progress is stored there, so the whole game works on a plane with the radio off.
How it's built
Flutter and Dart, one codebase covering both phones and desktop, with the puzzle logic kept as plain Dart that knows nothing about the screen.
The design decision worth explaining is how a puzzle earns its difficulty. Every board is built so that exactly one solution exists — a starting digit is only taken away if what remains can still be filled in one way and no other. Then, before the puzzle is handed to you, the app solves it the way a person would, using only the techniques a human actually applies, and records which ones were needed. A board is called Master because solving it genuinely demands the harder deductions, not because it happens to start with few digits on screen. That check is why an Easy puzzle stays gentle all the way to the last cell.
Where it is now
In development, and not released on the App Store or Play Store yet. The single-player game is built and running on device: generation, the four variants, pencil marks, undo, the daily board and the teaching hints.
The online side — a leaderboard and a head-to-head match mode — is written but not yet switched on for players. That, plus the store listings for iOS and Android, is what stands between the current build and a first release. Work on it is paused while other projects run.