Every player gets an identical board without us storing a single puzzle. Here is the trick, and what it means for fairness.
Everybody who plays today's puzzle plays the exact same board. That sounds like it requires us to generate one puzzle and ship it to everyone, but we do not do that at all.
The board is generated independently on each device, and it comes out identical because of how we seed the random number generator.
The seed is derived, not stored
A seed is just a number. We compute it from the date, the game and the level, so the same inputs always produce the same seed — and therefore the same sequence of "random" choices.
Because the generator is deterministic, the same seed produces the same board on your phone, on our server and on a machine ten years from now.
The server can reproduce your run
This is the part that matters for the leaderboard. When you submit a score, the server does not trust your board — it regenerates the puzzle from your seed and checks whether your result is achievable.
A client that claims an impossible score simply fails verification. We never have to trust the browser, because we can always redo the work.
Why not just store the puzzles?
We could, and for some games that would be simpler. But derived seeds mean the archive is unbounded — any date, including dates in the future, generates a valid board on demand with nothing to store.
It also means there is no database to get out of sync. The date is the source of truth.
FAQ
Could someone work out the seed and generate the answer?
Yes, in principle — the seed derivation is not secret. That is why the server re-verifies submissions rather than relying on obscurity, and why runs that lose focus are marked unranked.