Sudoku Master

Sudoku doesn't need an introduction. A 9x9 grid, nine 3x3 sub-grids, the digits 1 through 9, and the rule that none of them can repeat in any row, column, or sub-grid. It's the most-played pen-and-paper puzzle of the last twenty years, and there are good reasons for that — but there's also a reason most online versions feel worse than a newspaper. That's what we wanted to fix.

Online Sudoku usually goes wrong in two ways. First, the difficulty tiers are misnamed — what one site calls "Hard," another calls "Easy." Second, the input is painful. Click a cell, find a button, click a number, hope you got the right cell. We took both of these seriously.

Our difficulty levels actually mean something. Easy puzzles have at least 36 starting clues and never require techniques beyond single-candidate elimination. Medium drops to around 30 clues and adds hidden pairs. Hard requires X-wing or simple swordfish. Expert is at the boundary of what's solvable without guessing. We tested each tier against ourselves and against the kids — Josie can solve Easy in about three minutes, struggles through Medium, and finishes a Hard in twenty if she's patient.

The Notes feature matters. Toggling Notes mode and writing small candidates into each cell isn't an optional flair — it's how anyone solves Medium and above. Our Notes mode is fast: toggle it once, then every number you type goes into the cell as a pencil mark instead of a final answer. Press the number a second time to remove it. The whole thing is keyboard-friendly so you can solve a puzzle without your mouse hand ever moving.

The Solve button is honest about itself. If you get stuck on Expert and click Solve, the game fills in the rest and tells you it solved it — it doesn't pretend you finished. That's the right behaviour. We'd rather you use the solver to learn what the missing technique was than have you stare at a stuck board for twenty minutes and bounce off.

Build notes. Generating valid Sudoku puzzles at a target difficulty is harder than it sounds. The standard approach is: generate a fully-solved grid, then remove clues one at a time, checking after each removal that the puzzle still has exactly one solution. We use a simple constraint-satisfaction solver under the hood. The Expert puzzles are produced by removing the maximum number of clues that still allows a unique solution. They're hard because they're meant to be.