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Game State

In order to process a log, a model of the chess board's current position is essential. In addition to the basic 64 squares containing the pieces, several additional facts are necessary to capture the game state. The current state of a chess game is a 6-tuple of the following items:

Piece Placement. An 8-tuple showing pieces in each rank from 8 down to 1. Pieces are shown as single letters, upper case for white (PRNBQK), lower case for black (prnbqk). Pieces are coded P for pawn, R for rook, N for knight, B for bishop, Q for queen and K for king. Empty spaces are shown as the number of contiguous spaces. The entire rank can be coded as a 1-8 character string. 8 means no pieces in this rank. 4p3 means four empty spaces (a-d), a black pawn in file e, and 3 empty spaces (f-h). The entire 8-tuple of strings can be joined to make a string delimited by /'s. For example rnbqkbnr/pp1ppppp/8/2p5/4P3/5N2/PPPP1PPP/RNBQKB1R.

Active Color. A value (w or b) showing who's turn to move is next. This color is active because this player is contemplating their move. The starting position, for instance, has an active color of w, because white moves first.

Castling Availability. A string with 1 to 4 characters showing which castling moves are still allowed. If none are allowed, a - is shown. White codes are capital letters, black are lower case. When king side castling is possible, a K (white) or k (black) is included. When queen side caslting is possible a Q (white) or q (black) is included. At the start of the game, there are four characters: KQkq. As the game progress and kings castle or are moved for other reason, or rooks are moved, the string reduces in size to -.

En Passant target. Either - or a square in rank 6 or 3. When a pawn's first move advances two spaces (from 7 to 5 for black or 2 to 4 for white), the skipped-over space is named here on the next turn only. If an opposing pawn moves to this space, an En Passant capture has occured. If no En Passant vulnerability, a - is given.

Half Move Count. How many 1/2 moves since a pawn was advanced or a piece captures. This is zero after a pawn moves or a piece is captured. This is incremented after each 1/2 move (white or black) where no pawn moves and no piece is captured. When this reaches 50, the game is technically a draw.

Turn. This is the turn count, it increments from 1, by 1, after black's move.

  Published under the terms of the Open Publication License Design by Interspire