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The Castling rights specify whether both sides are principally able to castle king- or queen side, now or later during the game - whether the involved pieces have already moved or in case of the rooks, were captured. Castling rights do not specify, whether castling is actually possible, but a pre-condition for both wing castlings.

Two bits per side are appropriate to store the castling rights, often one uses one nibble to store the rights for both sides inside a position object, a kind a array of four booleans. In make move one has to consider that king-moves reset both castling bits per side.

Rook-moves from their original square, or captures of rooks on their original squares reset the appropriate castling bits per wing and side. Changed castling rights should be considered in the zobrist key of the position, to reflect the irreversibility of the otherwise reversible move, concerning repetitions. On the other hand, changed castling rights don't necessarily reset the halfmove clock regarding the fifty-move rule.

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