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RDChess GUI [1] [2]

a freeware open source chess program by Rudolf Posch, written in Object Pascal with x86/MMX inline assembly. RDChess comes with an own Windows GUI, written in Delphi, and also has a WinBoard mode.



Board Representation

The board is represented by a 12x12 mailbox array in conjunction with piece-lists. An attack table utilizes square control and a cheap SEE, and is calculated from scratch once per node - one byte per square and color in "ppbbrrqk" order, indicating which pieces control that square including bishop/rook x-rays through queen and king. The two-bit bb counter includes knight and bishop attacks. Strictly legal move generation requires determination of pinned pieces, also used in evaluation.


The search applies NegaScout with TT, AEL-pruning, quiescence and various extensions inside an iterative deepening framework with aspiration windows. Move ordering considers hash move, re-captures, two killers, captures with positive SEE, quiet moves with priority from attacked squares to none attacked or defended squares, pawn moves, losing captures, and remaining.


Evaluation focuses on material balance with trade down bonus if ahead. Positional feature terms, considering pawn structure, piece development, king safety, hanging pieces, etc., rarely exceed the value of one pawn, except unstoppable passers in pawn endgames due to implementation of the rule of the square.

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