Chess

Object of the game

Checkmate the opponent’s king: attack it in such a way that no legal move can save it from capture.

Movement

• King (K): one square in any direction. It can never move into check.
• Queen (Q): as many squares as you like in a straight line or diagonally.
• Rook (R): in a straight line; Bishop (B): diagonally; Knight (N): in an “L” shape, jumping over pieces.
• Pawn: one square forward (two from its starting square), captures diagonally. On reaching the last rank, it is promoted to a queen, rook, bishop or knight.

Special moves

• Castling: the king slides two squares towards a rook, which jumps over it, provided neither has moved, the squares between them are empty and the king is not in check and does not cross an attacked square.
• En passant: a pawn that has just advanced two squares can be captured, on the next move only, by an enemy pawn as if it had advanced just one square.

End of the game

• Checkmate: victory. Stalemate (no legal move while not in check): draw.
• A draw also results from threefold repetition of the position, fifty moves without a capture or pawn move, or insufficient material to checkmate.

Controls

• Tap a piece then its destination square, or drag it directly. Legal squares are shown; the last move and a king in check are highlighted.
• To castle, move the king two squares (or drop it on the rook).
• “Undo” takes back your last move; “Hint” asks for the engine’s suggestion.
• The side panel shows the move list in algebraic notation, the captured pieces, an evaluation bar and PGN export.

About the AI

A home-grown engine written for Ludo Genie: 0x88 representation with piece lists, principal variation search (PVS) with iterative deepening and aspiration windows, a transposition table with Zobrist keys, quiescence search ordered by MVV-LVA and static exchange evaluation (SEE), null move, late move reductions, killer moves and history, check extensions. The evaluation combines tapered piece-square tables (opening and endgame), mobility, pawn structure (isolated, doubled, passed), king safety and elementary endgames. A small in-house opening book varies its openings. The Master level thinks for about 3 to 4 seconds per move and aims for a strength above 2000 Elo; the lower levels reduce the depth, add noise to the evaluation and make increasingly rare inaccuracies.