Kuromasu: rules and how to play

Goal of the game

Choose black cells so that each numbered white cell sees exactly its stated number of white cells along its row and column, including itself.

Rules

• Numbered cells always stay white. Count from each clue in the four straight directions and add the clue cell once.
• Sight stops at the first black cell or the board edge. White numbered cells do not block sight; diagonals and turns do not count.
• No two black cells may share a side, though diagonal contact is allowed.
• All white cells must form one area connected through sides. Every cell is either white or black in the finished grid; visibility is a count of cells, not a sum of printed numbers.

A small example, step by step

1. Consider a clue 4 with exactly one white cell visible to its left, one to its right and one above.
2. Count the clue itself once: 1 + 1 + 1 + 1 = 4.
3. If those three white cells are already certain, every further sightline must stop: the cell below the clue and the cells immediately beyond those three visible cells must be black wherever they exist.
4. Check the black-adjacency rule and white connectivity too. Meeting one clue’s count does not by itself complete a valid board.

Beginner deductions

• Count the whites already certain in each clue’s sightlines, then the maximum it could still see before known black cells or edges.
• If the certain count already reaches the clue, block further visibility. If even the maximum only just reaches it, all cells needed to reach that maximum must be white.
• Every black cell forces its side-neighbours white. Preserve the only possible white connection between confirmed areas.

Common mistakes

• Do not forget the clue cell or count it separately in each direction.
• Do not stop sight at another number, count diagonals or add clue values.
• Do not leave white regions connected only at a corner, or borrow Nurikabe’s island-size rule.