Fox and Hens

Goal of the game

A hungry fox takes on thirteen hens. The hens want to reach their henhouse, the nine points of the top arm, or trap the fox; the fox wants to eat enough of them so that they can no longer manage it.

Setup

• The cross-shaped board has 33 points joined by horizontal and vertical lines.
• The 13 hens occupy the three bottom rows (7 + 3 + 3 points) and the fox starts at the centre of the board.
• The hens move first, then the sides alternate.

Moves

• A hen moves one point along a line, forward (towards the henhouse) or sideways, to a free point. It never moves backwards and never captures.
• The fox moves one point in the four directions: forward, backward, left or right.
• The fox captures a hen by jumping over it in a straight line to the free point just behind it; the hen is removed immediately. It may chain several jumps in the same turn (multiple capture) and stop whenever it likes. Capturing is never compulsory.

End of the game

• The hens win if the fox can no longer move, or if they occupy all nine points of the henhouse.
• The fox wins if only eight hens remain: at least nine are needed to fill the henhouse.
• The hens must make progress: if the same position occurs three times, or if the hens can no longer move, the fox wins.
• A game that goes beyond 150 moves for each side is declared a draw.

Controls

• Tap a piece then its destination, or drag it. The possible destinations light up; the fox's jumps are shown in orange.
• At the Beginner to Advanced levels and in two-player mode, hens threatened with a jump are circled in red.
• The notation labels the columns a to g (left to right) and the rows 1 to 7 (bottom to top): d3-d4 for a step, d5×d3×b3 for a multiple capture.

About the AI

The engine, written for Ludo Genie, plays both sides. It explores the move tree with principal-variation negamax alpha-beta, with iterative deepening, a transposition table with Zobrist keys, killer moves, history, late move reductions and repetition detection. The fox's multiple captures are extended at the end of the search (quiescence) so that a capture is never underestimated.

The evaluation combines the remaining hens, their advancement, how full the henhouse is, immediate threats, the fox's mobility and, above all, the area it can still roam, computed by flood fill. At Master level the AI thinks for about two and a half seconds and often sees more than fifteen plies ahead: with the hens it almost always wins; with the fox it exploits the slightest gap.