Checkers

Object of the game

Capture all of your opponent’s pieces, or block them so that your opponent can no longer move. The board has 64 squares; you play only on the dark squares, and each side starts with 12 men.

English / American rules (checkers)

• Black moves first. Men move one square diagonally and capture only forwards.
• Capture is mandatory, but you are free to choose which one if several are possible. A multi-jump that has begun must be carried through to the end.
• A man that reaches the last row becomes a king and its move ends there.
• The king moves or captures one square at a time, in all four directions.
• Draw after three repetitions of the same position, or after 40 moves by each side without a capture or a man move.

International rules on 8 × 8

• White moves first. Men move one square but also capture backwards.
• Capture is mandatory and the sequence that takes the most pieces must be played (majority capture, a king counting as one man).
• The king is flying: it travels along a free diagonal for its whole length and, when capturing, lands on any free square beyond the captured piece.
• Captured pieces are removed only at the end of the sequence and cannot be jumped twice (Turkish strike).
• A man is promoted only if it ends its move on the last row: if it merely passes through it during a multi-jump, it stays a man.
• Draw after three repetitions, after 25 moves by each side played only with kings without a capture, or when a lone king holds out for 16 moves against three pieces (5 moves against two pieces or a single king).

Controls

• Tap a piece then its destination square, or drag it.
• When a capture is mandatory, the pieces concerned are circled in red and only legal capture sequences are offered.
• If several capture sequences lead to the same square, then tap one of the pieces circled in purple to specify your choice.
• Playable squares carry the official numbering. The move list uses official notation: 32-28 for a move, 19x28 for a capture.
• Esc cancels the selection. The Undo and Hint buttons are in the top bar.

About the AI

A dedicated engine, written for Ludo Genie, runs in the background: complete generation of capture sequences (majority capture, Turkish strike), negamax alpha-beta search with a principal-variation window and iterative deepening, a transposition table with Zobrist keys, extension of captures and threats (quiescence), cautious reductions of late moves, killer moves and history for move ordering.

The evaluation weighs material (a king is worth about three men), the advancement and tempo of the men, control of the centre, the solidity of formations, the defence of the back row, mastery of the main diagonal and the handling of endgames. At Master level, the AI thinks for about three seconds per move and often sees more than ten plies ahead, much further in forced captures. The lower levels limit the depth and sometimes pick a weaker move, the way a human player would.