Kurodoko (kuromasu) Puzzle

Kurodoko is a shading puzzle, also called kuromasu. Some cells show a number. You shade other cells so each number counts exactly the white cells it can see. You can play grids from 5×5 up to 10×10, each in easy, medium, and hard. Every puzzle here has exactly one shaded set.

Rules

The board is an n×n grid. Thin lines divide the cells. A thick frame sits around the outside.

A number counts every white cell it can see in a straight line up, down, left, and right, including its own cell. The count stops at a shaded cell or at the edge of the board. The numbered cell itself stays white.

Shaded cells never share a side. They may touch at a corner. Every white cell, including the numbered cells, must connect to the other white cells by sharing sides. A white cell cut off from the rest is not allowed.

A dot is a mark for a cell you already know is white. Dots are optional. The puzzle does not ask you to dot every white cell. A numbered cell can take a dot.

The puzzle is solved when the shaded cells are that unique set and nothing on the board breaks a rule. White cells may still be blank. You do not have to place a dot on every white cell.

Controls

Choose Shade or Dot under the board. Tap a cell, or drag across cells, to place that mark. Tap a cell that already has the mark to clear it. A cell that has the other mark takes the new one when you tap it. A drag follows the first cell: it places the mark on empty cells along the path, or it clears when that cell already has the mark. With a mouse, right-click places a dot, or clears a dot that is already there.

On a keyboard, the arrow keys move around the grid. Space or Enter places the selected mark, or clears it when the cell already has that mark. S or 1 shades the cell. X, D, E, or 2 places a dot. M switches Shade and Dot. Backspace, Delete, or 0 clears the cell.

A cell turns coral when a number is shaded, two shaded cells share a side, a number already sees too many white cells, a number can no longer see enough, or the white cells are split. Press Hint for one step the rules already force, with a sentence saying why. Press Check when you want the board checked against the rules. Undo and Redo step through your moves, and Reset clears the board. When the shaded cells are the unique set, the board turns teal.

Techniques

Easy grids follow from one number at a time, plus the rule that shaded cells stay apart:

• Beside a shade: a shaded cell cannot touch another shaded cell, so the cells beside it stay white.
• Sight max: a number can see only as far as its value. When the open cells already make exactly that count, those cells stay white.
• Already enough: a number already sees its count, so the next cell on each open line is shaded.
• One open line: only one direction is still open, so that line has to be exactly this long.
• Needed white: every way to make the number sees through this cell, so the cell stays white.

Medium grids look at the lengths that still add up, and at white cells that would split or get trapped. Hard grids try the other mark on one cell and keep the mark that does not break the board.

Beginner tips

Start with the largest numbers and the cells on the edge. A number near the edge has fewer places it can see, so a shaded cell is often close by.

When two shaded cells would share a side, dot the cells between them before you look for the next number. Then check that each number can still reach its count.

If you are new to Kurodoko, start with a 5×5 easy grid, or the daily 8×8 medium grid, and move up once the sight lines feel natural.