How to Solve Kurodoko: Sight-Line Logic for 7×7 Boards

Updated October 2026

Kurodoko is a shading puzzle in which every number is a lookout post. It reports how many white cells it can see in a straight line up, down, left and right, itself included, before a black cell or the border blocks the view. You paint the black cells; the numbers check your work. The rules fit in three sentences, yet boards can feel stubborn until you learn to treat each number as a budget that has to be shared between four directions. This guide covers the exact rules used here, the deductions that open most boards, and how to keep your time down.

The rules as this version checks them

A board is accepted the instant three things are true together. Every number sees exactly its value. No two black cells share a side; touching at a corner is allowed. All white cells form one connected group, joined through shared sides. Numbered cells are always white and cannot be painted at all, so you never have to wonder whether a clue might be hidden under a black square.

Every board is a 7 by 7 grid built fresh when you press Start or New. The generator places between six and twelve black cells, then strips numbers away one by one, keeping a number if removing it would allow a second solution or would leave the board unsolvable without guessing. A solver that uses only forced deductions, the same kinds described below, has to finish every board before you see it. At least thirteen numbers always remain. Before a board is shown, a full search confirms it has exactly one answer, so if you reach a contradiction, the mistake is in your reasoning, not in the puzzle.

On a 7 by 7 grid the largest possible clue is 13: the cell itself plus six cells across its row and six down its column. The smallest is 2. A 1 would need a black cell or the border on all four sides, and that would cut its cell off from every other white cell, which the connection rule forbids.

Think in budgets: the maximum-reach test

For any number, imagine extending its view in each direction until you reach the border or a cell you already know is black. Add those four reaches together and add one for the cell itself. That total is the most the number could ever see. Two deductions come straight out of it.

• No slack. If the maximum equals the clue, nothing in view may be painted. Every cell in those four arms is white, and you can dot them all at once.
• Forced minimum per arm. Work out what the other three arms could supply at most, plus one for the number itself. If that is still less than the clue, the remaining arm must make up the difference. A 9 whose other three arms can add at most 5 cells must see at least 3 cells along the fourth arm, so the first three cells of that arm are white.

Run this test on every big number first. Large clues near the middle of the board often have only one or two cells of slack, and a single test can settle half a row.

The cap: when a number already sees enough

The reverse situation matters just as much. Count the white cells a number is already sure to see, meaning the unbroken run of known white cells in each direction. If that count equals the clue, the view must stop right there, so the first undecided cell at the end of each run has to be black. A small number next to a few dotted cells can therefore force several black squares at once.

The classic case is the 2. It sees exactly one other cell. If you can tell which neighbour that is, every other neighbour of the 2 that is not the border is black. In the middle of the board that is three black cells from one clue; on an edge it is two, and in a corner it is one. Until you know which neighbour is the white one, you cannot paint any of them, but you can note that whichever neighbour stays white, the cell beyond it must be black.

Spreading from each black cell

Every time you paint a cell black, its four side neighbours become white immediately, because black cells may not touch. Mark them with dots. Those new whites usually extend somebody's sight line, which can push a nearby number over its value and force another black at the end of its view, and the chain continues. Most boards in this game are solved by alternating between these two steps: paint, dot the neighbours, recount the numbers whose lines passed through the new dots.

The tap cycle is empty, black, dot, empty. A tap only counts if your finger lifts on the same cell it pressed, so slide off a cell to cancel a mistaken tap. Dots are only a note for you: the board is judged on black cells alone, and an undotted white is just as valid as a dotted one.

The connection rule is a weapon, not just a check

Because the white area must stay in one piece, a chain of black cells touching corner to corner may never run from one border to another or close into a ring. Either shape would cut the white cells into two groups. Use this actively. When a cell would complete such a diagonal chain, it must be white, even if no number has said so yet.

Corners deserve particular care. A white corner cell has only two neighbours, so if one of them is black, the other must be white to keep the corner connected. Two black cells diagonally beside a corner would wall it off completely, which is never allowed.

Reading the colours

The numbers recolour as you work. A number turns green when it currently sees exactly its value. It turns red when it sees fewer cells than its value, which means you have blocked too much and must remove a black cell somewhere along its lines; adding more black can never fix a red number. A white number simply needs more blocking or is still undecided. Two black cells that touch are both shaded red so you can spot the clash at a glance.

Green is a statement about the moment, not a proof. A green number can turn wrong if you later paint a cell inside its view, so dot its visible cells once you are sure the line is final.

Solving faster

Your score is the time from the moment the board appears to the moment the last required cell is correct, measured to a tenth of a second. The board is accepted automatically, so there is no button to press at the end. The clock runs on real time and keeps going if you switch apps, so finish a board before you look away.

1. Scan for the biggest and smallest numbers and run the reach test on each before tapping anything.
2. After each black cell, dot its neighbours straight away; it costs a second and prevents adjacent-black errors later.
3. When progress stalls, look for clues whose line crosses a cell you just changed. Rechecking those few is far quicker than rescanning the whole grid.
4. Keep an eye on diagonal black chains near the borders, which is where accidental cuts usually happen.

Your fastest time is stored on your device, and every finished board can be submitted to the Hall of Fame from the solved card.