Daily Killer Sudoku

The date is the seed

Searches for killer sudoku daily nearly always mean one thing: a board that is the same for everyone, so that a time is worth comparing. That normally implies a server with a puzzle stored against each date. This one has no stored puzzle and makes no request for one.

The generator is fed a single number built out of today’s date in Coordinated Universal Time, the year followed by the month followed by the day, run together into one figure. From that number it produces the same cage layout every time it is asked. Nothing about that arrangement is shared or sent. Two machines arrive at the same cage layout because they were both handed the same number and both did the same arithmetic with it, which is a different thing entirely from being served the same file.

The practical consequence is the changeover. The day turns at midnight UTC rather than at your local midnight, so depending on where you are the new puzzle may arrive in the middle of your afternoon, or overnight while you sleep.

It is a question worth asking of any daily killer sudoku online, because the phrase is used loosely. A puzzle picked at random when you arrive is not a daily puzzle, and the only way to tell from outside is to open the same page twice on the same day.

New puzzle does not give you the day’s board

The one button on this page that behaves differently from everywhere else on the site. New puzzle builds a random board at the same difficulty and does not re-derive the date, so once you have pressed it today’s puzzle is no longer on the screen. Reloading the page brings it back, because the seed is computed again from the date on load. The way back to the daily board is a refresh, not the button.

Comparing times, honestly

The timer starts the moment the finished board appears, which is a fraction of a second after the page opens, while the puzzle is being built and checked. It stops when every cell matches the answer.

Two things make a time less comparable than it looks. Hint fills a cell for you and the board still counts as solved when it does, so a time set with hints is not the same measurement as one set without. Check, by contrast, corrects nothing at all; it only reports how many of the digits currently on the grid are wrong, so it costs you nothing except the seconds you spend pressing it.

What difficulty today’s board is

It asks for a medium puzzle, which on a caged grid means the reasoning goes past cage arithmetic and single candidates into hidden singles, where a digit has only one cell left in a row, column or box, but stops short of adding whole regions up. The label above the grid reports what the board turned out to be rather than what was requested, so on a day when the generator could not land a medium inside its time budget it will say so plainly rather than mislabel the puzzle.