Shakashaka

Shakashaka (シャカシャカ) is a logic puzzle developed by publisher Nikoli.^[1]

An example Shakashaka puzzle

The same puzzle with solution marked in teal

Rules

Shakashaka is played on a rectangular grid of white and black squares. Some black cells may contain a number.

The objective of the puzzle is to place triangles in some of the white cells. There are four kinds of triangles which can be put in squares:

In the resulting grid,

• The white parts of the grid (uncovered by black triangles) must form a rectangle or a square, not sharing an edge with other white squares/rectangles.
• Black cells with a number must be orthogonally adjacent to the specified number of black triangles.^[2]

Computational complexity

It is NP-complete to decide whether a given Shakashaka puzzle has a solution.^[3] Furthermore, counting the number of solutions to a given Shakashaka puzzle is #P-complete.^[3]