How puzzles are made

Every puzzle here comes from our own code and is checked by a solver before it reaches you. The same date gives every player the same puzzle.

The daily puzzle

Each daily puzzle is picked by a seed made from the date, the game and the level. No one chooses it by hand, and because the steps are fixed, your browser builds the same puzzle as everyone else's. The date rolls over at midnight in New York.

Mahjong

Boards are dealt in reverse. Starting with the whole layout, the generator removes two tiles that are free at that moment and gives them a matching pair of faces, then repeats until the layout is empty. Playing that order forward clears the board, so every deal has at least one solution. We test this on a thousand deals for each layout. The tile art was drawn for this site.

Sudoku

We start from a full grid and remove numbers, keeping only removals that leave exactly one solution. A grader then solves the puzzle using human techniques, and the level comes from the hardest technique needed: singles for easy, up to X-Wing and Swordfish for expert. Puzzles that need something beyond that are thrown out. Each one you see is then relabeled and rearranged by its seed, in ways that keep its logic the same.

Nonograms

The pictures are original drawings. Each runs through a line solver, which works out the grid one row or column at a time, as you would. A drawing is used only if the solver can finish it this way, which means it has one solution and needs no guessing. Where a drawing fell short, a few squares were adjusted until it passed.

The guides

Worked examples in the guides come straight from real puzzles in our collection, and the code checks that each one shows the technique it claims.

Spotted a puzzle that seems wrong? Let us know.