Rectangular Spiral Galaxies are still hard
Erik D. Demaine, Maarten Löffler, Christiane Schmidt · Computational Geometry · 2022
Spiral Galaxies is a pencil-and-paper puzzle played on a grid of unit squares: given a set of points called centers , the goal is to partition the grid into polyominoes such that each polyomino contains exactly one center and is 180 ∘ rotationally symmetric about its center. We show that this puzzle is NP-complete, ASP-complete, and #P-complete even if (a) all solutions to the puzzle have rectangles for polyominoes; or (b) the polyominoes are required to be rectangles and all solutions to the puzzle have just 1 × 1 , 1 × 3 , and 3 × 1 rectangles. The proof for the latter variant also implies NP/ASP/#P-completeness of finding a noncrossing perfect matching in distance-2 grid graphs where edges connect vertices of Euclidean distance 2. Moreover, we prove NP-completeness of the design problem of minimizing the number of centers such that there exists a set of galaxies that exactly cover a given shape.