Hitori

Goal

Shade some cells so that:

• no number is repeated among the white cells of a row or column;
• two shaded cells never touch along a side (diagonals are allowed);
• all the white cells stay connected to one another through their sides;
• every shaded cell repeats a number that stays white in its row or column (you only ever shade duplicates).

Controls

• Tap a cell to shade it, tap it a second time to circle it (a cell that is certainly white), and a third time to reset it to neutral.
• Circle mode: each tap circles the cell directly. Drag to apply the same state to several cells.
• Keyboard: arrow keys, Space or X to shade, O to circle, Backspace to erase.

Techniques

• Triple: in “3 3 3”, both ends are shaded.
• Sandwich: in “5 2 5”, the middle cell is white (one of the two 5s will be shaded, and its neighbours stay white).
• Adjacent pair: two identical numbers side by side force you to shade the other copies in the row.
• Connectivity: a cell whose shading would cut off part of the grid is white.

About the AI

The generator first draws a valid pattern of shaded cells on a Latin square, then gives the shaded cells numbers that create duplicates. An exhaustive search checks that the solution is unique, and a logical solver grades the grid by the techniques it requires (up to trials refuted by connectivity). The same solver provides the explained hints and the Solve button.