how to play star battle
Star battle is a logic puzzle on a 10x10 grid divided into 10 irregular regions. Place 2 stars in every row, every column and every region. Two stars cannot touch one another, including diagonally.
Solving often requires ruling cells out before placing stars. Mark the cells that can't hold a star with an X to narrow down the possible star positions.
You shouldn't need to guess. Every board has exactly one solution that can be reached by applying the rules below.
the rules
Every row holds exactly 2 stars.
Every column holds exactly 2 stars.
Every outlined region holds exactly 2 stars, whatever its shape or size.
No two stars share an edge or a corner. All eight cells around a star can be ruled out.
a worked example
Let's walk through solving a 4x4 board with one star per row, column and region. The real puzzle is 10x10 with 2 stars, but the logic is the same.
Four regions, four rows and four columns. Each needs one star. We can start by ruling cells out.
The three-cell region at the top left sits entirely inside the top row, so the top row's star has to be that region's star. The fourth cell of the row is out.
That leaves the region on the right with cells only in the second row, so the second row's star is one of those two. The other two cells in that row are out.
The square region on the left has lost both of its second-row cells, so its star is in the third row. The two third-row cells outside it are out.
There must be a star in one of the empty cells in the third row. The two bottom-left cells touch both of these, so they can't be stars.
Look at row 2, column 3. A star there would rule out both of its diagonal neighbors on the left. But if those were ruled out, we'd be unable to put a star in column 2. So it can't have a star.
The second row is down to one cell, so its star goes there. That fills the rightmost column, ruling out the bottom cell of it.
The bottom row has one cell left, so its star goes there. The cell it touches diagonally is out.
The third row is down to one cell, which fills the leftmost column, and the top row is left with one cell of its own. All done!
controls
Tap a cell to mark it with an X. Tap again to mark it a star. Tap again to clear it.
You can tap and drag out from a tile to mark a bunch of cells at once.
If using a keyboard: arrow keys move between cells, "x" marks an X, "s" places a star, backspace clears a cell, and space cycles through the three.
tips
- Look for small regions. If a region is 3x1 tiles, you know where its stars must go.
- Reduce large shapes into smaller shapes. By ruling out cells, you can reduce a whole row, column, or large region to a 3x1 shape.
- Always rule out neighbors. A star rules out all eight cells around it at once. Turn on auto-x neighbors to stop doing that by hand.