Gareth D. Rees > Games-Chess > Games::Chess

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NAME ^

Games::Chess - represent chess positions and games

SYNOPSIS ^

    use Games::Chess qw(:constants);
    my $p = Games::Chess::Position->new;
    $p->at(0,0,BLACK,ROOK);
    $p->at(7,7,WHITE,ROOK);
    print $p->to_text;

DESCRIPTION ^

The Games::Chess package provides the class Games::Chess::Piece to represent chess pieces, and the class Games::Chess::Position to represent a position in a chess game. Objects can be instantiated from data in standard formats and exported to these formats.

NOTATION ^

See Games::Chess::PGN for full details of the notations.

SAN

Standard Algebraic Notation. The modern international notation for chess games. For example,

    1. e4 e5
    2. f4 exf4
    3. Nf3 g5
FEN

Forsythe-Edwards Notation. A compact representation for chess positions. FEN specifies the piece placement, the active color, the castling availability, the en passant target square, the halfmove clock, and the fullmove number as six fields separated by spaces. For example, the opening position is described in FEN as follows:

    rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1
PGN

Portable Game Notation. A notation for chess games, including the moves, commentary, variations, and metadata such as the players, the event, the round number, and the date of the match. For example,

    [Event "F/S Return Match"]
    [Site "Belgrade, Serbia JUG"]
    [Date "1992.11.04"]
    [Round "29"]
    [White "Fischer, Robert J."]
    [Black "Spassky, Boris V."]
    [Result "1/2-1/2"]

    1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 Nf6 5. O-O Be7 6. Re1
    b5 7. Bb3 d6 8. c3 O-O 9. h3 Nb8 10. d4 Nbd7 11. c4 c6
    12. cxb5 axb5 13. Nc3 Bb7 14. Bg5 b4 15. Nb1 h6 16. Bh4 c5
    17. dxe5 Nxe4 18. Bxe7 Qxe7 19. exd6 Qf6 20. Nbd2 Nxd6
    21. Nc4 Nxc4 22. Bxc4 Nb6 23. Ne5 Rae8 24. Bxf7+ Rxf7
    25. Nxf7 Rxe1+ 26. Qxe1 Kxf7 27. Qe3 Qg5 28. Qxg5 hxg5
    29. b3 Ke6 30. a3 Kd6 31. axb4 cxb4 32. Ra5 Nd5 33. f3 Bc8
    34. Kf2 Bf5 35. Ra7 g6 36. Ra6+ Kc5 37. Ke1 Nf4 38. g3 Nxh3
    39. Kd2 Kb5 40. Rd6 Kc5 41. Ra6 Nf2 42. g4 Bd3 43. Re6
    1/2-1/2
EPD

Extended Position Description. An extensible notation based on FEN. Intended for data interchange between chess-playing programs and for the construction of opening databases. Not used by Games::Chess.

ERROR HANDLING ^

All the Games::Chess::* packages share a common error-handling protocol, depending on the debugging level passed to the Games::Chess::debug function. (The debugging level is 0 by default.)

At debugging level 0, each function and method returns undefined if it is passed invalid arguments or if some other error happens. A description of the error is available by calling Games::Chess::errmsg.

At debugging level 1, each function and method returns undefined if it is passed invalid arguments or if some other error happens. A warning is also issued. A description of the error is available by calling Games::Chess::errmsg.

At debugging level 2, each function and method calls die if it is passed invalid arguments.

CONSTANTS ^

Piece colours are represented by the constants BLACK and WHITE. Piece values are represented by the constants PAWN, KNIGHT, BISHOP, ROOK, QUEEN, and KING.

To import these constants into your namespace, include the tags :colours (colours only), :pieces (piece values only), or :constants (all constants) in the use statement. For example:

    use Games::Chess qw(:colours);

(If you use strict;, you'll have to write these constants as function calls like WHITE() or &WHITE to satisfy the compiler.)

FUNCTIONS ^

To import all these functions into your namespace, include the tag :functions in the use statement, for example

    use Games::Chess qw(:functions);
algebraic_to_xy($square)

If $square represents a square in Standard Algebraic Notation (from a1 to h8), return a list of two elements ($x,$y) giving the coordinates of that square, from (0,0) to (7,7). Return undefined otherwise.

colour_valid($colour)

Return 1 if $colour is a valid colour value, WHITE or BLACK. Return undefined otherwise.

debug($level)

Set the debugging level. See "ERROR HANDLING".

errmsg

Return a description of the most recent error in any of the Games::Chess::* packages, or the empty string if no errors have occurred. See "ERROR HANDLING".

halfmove_valid($halfmove)

Return 1 if $halfmove is a valid value for the halfmove clock, which counts the number of ply (moves by either player) since the last pawn move or capture. Return undefined otherwise.

move_valid($move)

Return 1 if $move is a valid value for the full move count (the number of black moves since the start of the game, plus 1). Return undefined otherwise.

piece_valid($piece)

Return 1 if $piece is a valid piece value, from PAWN to KING. Return undefined otherwise.

xy_to_algebraic($x,$y)

If ($x,$y) is a valid board position, from (0,0) to (7,7), return the algebraic notation for that square, from a1 to h8. Return undefined otherwise.

xy_valid($x,$y)

Return 1 if ($x,$y) is a valid board position, from (0,0) to (7,7). Return undefined otherwise.

CHESS PIECES ^

A chess piece, or an empty square on a chess board, is represented as an object belonging to the Games::Chess::Piece class.

PIECE REPRESENTATION

A chess piece is represented as a reference to an 8-bit value where bits 0-2 are the piece type (0 = empty square, 1 = pawn, 2 = knight, 3 = bishop, 4 = rook, 5 = queen and 6 = king) and bits 3-4 are the piece color (0 = empty square, 1 = white, 2 = black).

Bits 5-7 are reserved for future use.

So for example, a black knight is represented as the binary value 00010010 (decimal 18).

PIECE CONSTRUCTORS

Piece->new

With no argument, return an object representing an empty square.

Piece->new($piece)

With a single argument that is a member of the Games::Chess::Piece class, return an object representing the same piece as $piece.

Piece->new($number)

With a numeric argument, return an object representing a piece with that encoding. Return undefined if $number is not an integer in the range 0 to 255.

Piece->new($character)

With a single character string as an argument, return an object representing a piece with that encoding in the standard Forsythe-Edwards Notation. A space represents an empty square, and the letters P, N, B, R, Q, and K represent pawn, knight, bishop, rook, queen, and king respectively. Upper-case letters represent white pieces and lower-case letters black pieces. Return undefined if $character does not represent a piece.

Piece->new($color,$piece)

Return an object representing the piece described. Return undefined if $color is not WHITE or BLACK, or $piece is not PAWN, KNIGHT, BISHOP, ROOK, QUEEN or KING.

PIECE METHODS

Piece->code

Return the FEN code for the piece as a single character (PNBRQKpnbrqk), or a space if the piece represents an empty square.

Piece->colour

Return EMPTY, WHITE or BLACK as appropriate.

Piece->colour_name

Return "empty", "white" or "black" as appropriate.

Piece->name

Return a string describing the piece, for example "black knight", or "white king", or "empty square".

Piece->piece

Return EMPTY, PAWN, KNIGHT, BISHOP, ROOK, QUEEN, or KING as appropriate.

Piece->piece_name

Return "square", "pawn", "knight", "bishop", "rook", "queen", or "king" as appropriate.

CHESS POSITIONS ^

A chess position represented as an object belonging to the Games::Chess::Position class.

POSITION REPRESENTATION

A chess position is represented as a reference to a hash, whose board member is a vector of 64 bytes, where the square at FILE,RANK is at byte (FILE*8)+RANK. For example, square c7 is at byte (2*8)+6 = 22. Each byte is in the format described for the Games::Chess::Piece class, above.

The representation is reasonably compact: a position is represented in 64 bytes. But more importantly, it is easy and efficient to manipulate in C; you can cast the vector to a 2-dimensional array using code like

     unsigned char (*position)[8][8]
       = (unsigned char(*)[8][8])vector;

and then position[x][y] refers to the square at file x, rank y.

POSITION CONSTRUCTORS

Position->new

With no argument, return an object representing a position with all 16 pieces in their initial positions.

Position->new($position)

With a single argument that is a member of the Games::Chess::Position class, return a copy of $position.

Position->new($FEN)

With one string argument, return an object representing the chess position described by the Forsythe-Edwards Notation string $FEN.

The Position constructor does not require $FEN to contain all six FEN fields; only the first (the piece placement) is required. The remaining five fields, if missing, take the values w - - 0 1.

POSITION METHODS

Position->at($x,$y)

If ($x,$y) is a valid board position, return an object of class Games::Chess::Piece representing the square at ($x,$y). Return undefined otherwise.

Position->at($x,$y,@piece)

If ($x,$y) is a valid board position, and @piece would be valid as arguments to the Games::Chess::Piece constructor (see "PIECE CONSTRUCTORS"), put the specified piece on the specified square and return 1. Return undefined otherwise.

Position->board

Return the board position as a vector of 64 bytes.

Position->can_castle($colour,$piece)

If $colour is a valid colour, and $piece is KING or QUEEN, return true if the player given by $colour can castle on the side given by $piece, false if they cannot. Return undefined otherwise.

Position->can_castle($colour,$piece,$can_castle)

If $colour is a valid colour, and $piece is KING or QUEEN, set the castling availability for the player given by $colour and the side given by $piece to the truth value of $can_castle, and return 1. Return undefined otherwise.

Position->clear($x,$y)

If ($x,$y) is a valid board position, clear the specified square and return 1. Return undefined otherwise. Equivalent to

   Position->at($x,$y,Piece->new);
Position->en_passant

Return the en passant target square as the list (FILE,RANK), or undefined if there is no en passant target square.

Position->en_passant($x,$y)

If ($x,$y) is a valid board position, set the en passant target square to ($x,$y) and return 1. Return undefined otherwise.

Position->halfmove_clock

Return the halfmove clock (the number of ply since the last pawn move or capture).

Position->halfmove_clock($halfmove)

If $halfmove is a valid halfmove clock value, set the halfmove clock to $halfmove and return 1. Return undefined otherwise.

Position->move_number

Return the move number (the number of full moves since the beginning of the game, plus 1).

Position->move_number($move)

If $move is a valid move number, set the move number to $move and return 1. Return undefined otherwise.

Position->player_to_move

Return WHITE if white is to move, BLACK otherwise.

Position->player_to_move($colour)

If $colour is WHITE or BLACK, set the player to move to $colour and return 1. Return undefined otherwise.

Position->to_FEN

Return a string representing the board position in Forsythe-Edwards notation. For example, the initial position is returned as the string

    rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1
Position->to_GIF(option => value, ...)

Return a string representing the board position as a GIF (an image in Graphics Interchange Format). The following options can be passed to control the image:

border

The width of the black border around the chess board, in pixels (defaults to 2).

letters

If true, draw a margin to the left of the board containing rank numbers, and a margin below the board containing file letters (defaults to true).

bmargin

The height of the margin to draw below the board (containing the file letters), in pixels (defaults to 20). Ignored if the letters option is false.

lmargin

The width of the margin to draw to the left of the board (containing the rank numbers), in pixels (defaults to 20). Ignored if the letters option is false.

font

A reference to a GD::Font object describing the font to use to draw the rank numbers and file letters (defaults to GD::Font::Giant). Ignored if the letters option is false.

Position->to_text

Return a string representing the board position as an ASCII diagram. For example, the initial position is returned as the string

    r n b q k b n r
    p p p p p p p p
      .   .   .   .
    .   .   .   .  
      .   .   .   .
    .   .   .   .  
    P P P P P P P P
    R N B Q K B N R
Position->validate

Apply some simple validation tests to the position. Return 1 if the position passes the tests, undefined otherwise. If the position fails to validate, the reason for failure can be found by calling Games::Chess::errmsg.

These tests are applied:

  1. The total of pawns plus obviously promoted pieces (for example, a second queen or a third rook) must be no more than 8 on each side.
  2. Each side must have exactly one king.
  3. There must be no pawns on ranks 1 and 8.
  4. The en passant target square, if specified, must be plausible. That is, if white is to move, the ep square must be on rank 6, with a black pawn on rank 5 and empty squares on ranks 6 and 7 in that file. If black is to move, the ep square must be on rank 3, with a white pawn on rank 4 and empty squares on ranks 2 and 3 in that file.
  5. The castling availability must be plausible. For example, if white can castle queenside, there must be a white rook on a1 and a white king on e1.
  6. The halfmove count must be between 0 and 50 (it can't be greater than 50 or the game would have been drawn).
  7. The full move number must be 1 or more.

BUGS ^

SEE ALSO ^

Games::Chess::PGN - a description of the Portable Game Notation (PGN) standard, incorporating Standard Algebraic Notation (SAN), Forsythe-Edwards Notation (FEN) and Extended Position Description (EPD). (Steven J. Edwards)

GD - a Perl interface to Tom Boutell's libgd graphics library. (Lincoln Stein)

AUTHOR ^

Gareth Rees <garethr@cre.canon.co.uk>.

COPYRIGHT ^

Copyright (c) 1999 Gareth Rees.

This module is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

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