Star Battle

Place a star in every row, every column and every region — no two stars touching, not even diagonally.

What it is

A square grid divided into as many regions as it has rows. The regions are the entire clue: there are no numbers anywhere. Exactly one star goes in each row, each column and each region, and stars never touch, even at a corner.

How to play

Start with the counting arguments: a region squeezed into one row means that row’s star is spoken for, so everything else in the row is empty. When a star lands, its eight neighbours are empty too — which often collapses a neighbouring region to a single candidate. Harder boards need placement reasoning: enumerate where a region’s star could go and keep only what every possibility agrees on.

Purpose

The most-requested modern logic genre the catalogue was missing — a staple of Puzzle Baron, GMPuzzles and championship sets. It also exercises a constraint shape the engine did not have: pure placement with an adjacency veto and no numeric clue at all, where the shape of the regions carries all the information.

History

Invented as Sternenschlacht by Hans Eendebak for the 2003 World Puzzle Championship, and popularised online as Star Battle. Published boards range from gentle 1★ 6×6s to the tournament-standard 2★ 10×10.

This implementation

• Spec knobs: size (5–9; one star per line), difficulty, cell, line.
• Generation: place a valid star arrangement first (depth-first, no two touching), grow one region around each star by seeded random flood, then accept only boards the shared solver proves unique.
• Solving: four propagators — line/region counting, exact placement enumeration (the no-touch rule folded in), star-neighbour elimination, and the genre’s signature confinement move (a region pinned to one line empties the rest of that line, and vice versa), rated at sudoku’s pointing-pair rung.
• Guarantees: deterministic per seed; exactly one solution, proven by exhaustive count; solvable by the technique ladder without guessing, and rated by the hardest technique actually used with board size as the tie-break (rating_basis names this).