Star Battle スターバトル

Star Battle is a placement puzzle played on a square grid that thick borders divide into as many regions as the grid has rows. You place stars so that every row, every column and every region holds the same number of them, and no two stars touch, not even at a corner. There are no numbers to work with: the shapes of the regions are the only clues, and a well made grid has exactly one way to place the stars.

Star Battle is not one of the Nikoli puzzles. It is credited to Hans Eendebak, who created it for the 2003 World Puzzle Championship, and it has since become a fixture of puzzle championships and online collections such as puzzle-star-battle.com, Krazydad and Grandmaster Puzzles. LinkedIn’s daily Queens game uses the same one star rules with coloured regions.

On LogicPuzzles.io you get a new daily 8×8 one star puzzle that is the same for every player, plus unlimited 6×6 and 8×8 one star grids and 10×10 two star grids. Every puzzle is generated, re-solved by an independent exact solver to confirm a single solution, and checked by a step by step logic solver so it never needs guessing. Right click marks a cell with ×, and undo, a timer and a Check button are built in.

Begin with the smallest regions. A region that lies entirely inside one row takes all of that row’s stars, so every other cell of the row gets an ×; the same goes for columns. A region of two or three cells in an L shape is almost as good: find the cells that would touch every one of its cells, because a star there would leave the region empty, and mark them ×.

Every star rules out its eight neighbours, so mark them as soon as you place it. In the two star 10×10 grids this becomes a counting tool: any 2×2 block can hold at most one star, so two neighbouring rows form a strip of five 2×2 blocks that must hold four stars, one in each of four blocks. A region whose remaining cells would all fit inside one 2×2 block cannot hold its two stars, which often forces a star into the cell that keeps it open.

The strongest general technique is counting regions against lines. If two regions are confined to the same two rows, those regions use up all the stars of those rows, so every cell of those rows outside the two regions is empty. It works for columns and in reverse, when a set of rows lies completely inside the same number of regions, and with two stars per unit the numbers simply double.

The most common mistakes are forgetting that diagonal neighbours also touch, counting × marks as if they were stars, and placing a star because it looks right rather than because every alternative fails. When you get stuck, pick a cell and ask what a star there would leave for each neighbouring region and line.

Star Battle is credited to Hans Eendebak and first appeared at the 2003 World Puzzle Championship. It is not a Nikoli type, and its Japanese name スターバトル is simply the English name written in katakana. The genre spread through puzzle championships and then online: puzzle-star-battle.com offers grids from one to six stars per unit, Krazydad publishes one star 8×8, two star 10×10 and three star 14×14 books, and Grandmaster Puzzles treats 8×8 with one star and 10×10 with two stars as the smallest standard sizes. The New York Times has printed two star grids under the name Two Not Touch, and LinkedIn’s Queens brought the one star version to a large daily audience.