How a puzzle gets checked

Short version: a program builds the puzzle, and a different program tries to break it.

The count that matters

Every sudoku, killer sudoku, calcudoku and word fill-in is handed to a solver with the answer stripped out. The solver doesn’t just find an answer — it counts how many exist. Exactly one, or the puzzle is destroyed and replaced. Every puzzle, not a sample.

Every puzzle published also gets a fingerprint recorded permanently, so a grid that appeared in one book can never appear in another one, or here.

Exactly where the guarantee ends

Codewords are built on a grid that has already passed the fill-in uniqueness check, so the number-to-letter key is genuinely one-to-one and the grid fits together exactly one way. I don’t claim a mathematical proof that there is only one route to deducing it. No commercial codeword publisher claims that either.

Crossword difficulty is my judgement, not a number I compute.

And puzzles are enjoyable. That’s all I’ll say about them. I make no claims about anybody’s brain, memory or health, and you should be suspicious of puzzle publishers who do.

Found something wrong?

Send me the book title, the page number and the puzzle number. I’ll re-run the checker on that exact puzzle and write back either way. If I’m wrong, I’ll fix the file and fix the printing. hello@bookatello.com