Clue
The digit is still there
Engraved into the stone, deliberately low in contrast. It is not there to be noticed; it is there to be taken away.
Inverted sudoku
You do not fill the grid. You hollow it out.
The grid arrives already solved. You are not looking for the missing digits — you are taking away the ones that are there, one at a time, for as long as the grid still works without them. The day one of them is genuinely needed, it comes back and the cell is gone.
5
4
2
5
9
7
2
1
8
4
9
9
6
5
7
2
1
7
3
1
6
1
9
4
4
8
5
The board
Three states, and nothing else
There is no progress bar and no counter: what you have done to the grid shows on the grid.
Open
The light comes through
You took that digit out and the grid still solves without it. The score goes up by one, and the board is a little brighter.
Sealed
The cell is gone
Without that digit, the grid had two solutions. It comes back, the cell turns black, and it stays that way until the end of the game.
The rule
Exactly one solution
A removal goes through if the grid that remains still lets you work out every empty cell. If there are two ways to fill it in, it is refused. That is the only condition, and it applies to every move.
7
9
1
2
3
1
9
5
7
8
3
2
5
7
1
8
6
7
5
2
8
7
8
4
9
9
7
1
2
5
9
8
8
2
The removal stands
There was still enough to deduce it
The other digits are enough to force this one. The cell opens.
7
9
1
2
3
1
9
5
7
8
3
2
5
7
1
8
6
7
5
6
2
8
7
8
4
9
9
7
1
2
5
9
8
8
2
The removal seals
Two grids would have held
That digit was the last thing telling two solutions apart. It comes back, and the cell is sealed.
These two boards come out of the game’s solver, not out of a drawing: it is the same grid, and both cells were picked while this page was being built, according to what it said about them.
The horizon
The floor is seventeen
A 9×9 sudoku with a single solution cannot go below seventeen clues — that has been proved. The game shows the limit without ever promising it: in practice you stop well before.
The lit part is not an estimate: the grid at the top was hollowed out until no removal would pass. A different order might have got one step lower — which is also what happens to you while playing.
Difficulty
It sets itself
There are no levels and no timer. The progression comes from the rule, and nobody had to balance it.
Every removal makes the next one harder
There is one fewer clue holding the deduction up. The first moves come quickly, the last ones much less so.
The obvious moves are already played
Before handing you the grid, the generator takes out everything a glance would have found. The first move left to you already needs thinking about.
Every grid is new
It is drawn from a seed, filled and then hollowed on the spot. There is no catalogue of grids, in the app or anywhere else.
Stopping
Nothing ends the game
You do not lose at Karadoku. A sealed cell does not throw you out: it lowers what you can still reach, and leaves the next move to your judgement.
Bank whenever you like
The score is yours the moment you put it down. Carrying on means playing one more cell for one more point.
The ceiling drops, the score stays
A sealed cell takes nothing back from what you have opened. It removes one of the cells you had left to open.
The right moment is not displayed
It is the one thing the game gives you no indication about, and that is deliberate.
Every day
The daily grid, without a server
Everyone gets the same grid each day, and nobody downloads it: the date is enough to work it out.
The date is the seed
The game carries its own random-number generator, written to draw the same values on a phone and in a browser, this year and in ten years.
You share a result, not a grid
The pattern of hollowed cells is the solution to the problem. What gets shared follows the order of your moves, never their position on the board.
It works offline
In flight mode as on the first launch: there is nothing to go and fetch.
In short
The numbers
81cells on the board
35clues to start with
17the proved floor
5languages
0network requests
0accounts, ads, purchases
A grid is waiting
Take one out, see whether the light comes through.
The game opens in the browser. There is nothing to install and nothing to sign up for.