Why the puzzle of the day is the same for everyone
Diverday doesn’t download its puzzles: each phone makes its own, even with no connection. So how can a phone in Madrid and another in London end up with exactly the same board?
A number that is always the same board
Every puzzle starts from a number, its seed. Give the generator the same seed and it builds the same board, square by square, every time. So two phones don’t need to talk to each other: they only need to agree on the number.
The number comes from the date
For the puzzle of the day, the app counts the days since 1 January 2020 and puts that count together with the game and the level. The result is a single number, and then it is scrambled, spreading neighbouring days across the whole range of numbers.
The scramble has a property that matters: two different combinations of day, game and level never get the same number. No two puzzles of the day are ever the same by accident.
Counted in UTC, on purpose
The day is the date on your phone, but the days between that date and 1 January 2020 are counted in UTC, not in local time. In local time, the day the clocks change lasts 23 or 25 hours, and counting that way once gave two different dates the same number, and so the same board. A test caught it.
Same game, same day, same level
The level is part of the number, so anyone playing at Gentle and anyone playing at Hard get different boards. And when we change how a game is built, the app tells anyone with an older version to update: an old version would make a different board from the same number.
