knight-owl

A chess library in lisp. Defines a data structure for a board, validates and performs moves, and reads portable game notation.

Documentation

Board

In knight-owl, a chess board is stored as a two-dimensional array of piece symbols in the form [bw]-[prnbqk]. For example, ‘b-n would indicate a black knight. Empty squares are denoted with nil.

new-board

• Description: Evaluate to a fresh 2D array with initial the chess position.

duplicate-board board

• Arguments:
  • board
    • A two-dimensional array of piece symbols.
• Description: Create a new two-dimensional array identical to the input board.

print-board board &key stream

• Arguments:
  • board
    • A two-dimensional array of piece symbols.
  • stream
    • An input stream designator. Defaults to t.
• Description: Write a 2D chess board array to a stream in a pretty fashion.

get-square board file rank

• Arguments:
  • board
    • A two-dimensional array of piece symbols.
  • file
    • An integer.
  • rank
    • An integer.
• Description: Access a chess board x/y location. Note that (setf get-square) also exists.

Movement

make-move move board whitep

• Arguments:
  • move
    • A string chess action in standard algebraic notation.
  • board
    • A two-dimensional array of piece symbols.
  • whitep
    • A generalized boolean. Truth value for white, nil for black.
• Description: Destructively modify the board for the given move.

valid-move-p move board whitep

• Arguments:
  • move
    • A string chess action in standard algebraic notation.
  • board
    • A two-dimensional array of piece symbols.
  • whitep
    • A generalized boolean. Truth value for white, nil for black.
• Description: Determine if a move can legally be performed on a given board.

check-p board whitep

• Arguments:
  • board
    • A two-dimensional array of piece symbols.
  • whitep
    • A generalized boolean. Truth value for white, nil for black.
• Description: Determines if the king is being attacked.

checkmate-p board whitep

• Arguments:
  • board
    • A two-dimensional array of piece symbols.
  • whitep
    • A generalized boolean. Truth value for white, nil for black.
• Description: Determine if the king is both being attacked and cannot escape.

+all-moves+

• Description: Constant that’s defined as a list of all possible string algebraic notation moves.

Portable Game Notation

pgn-parse stream

• Arguments:
  • stream
    • An input stream designator
• Description: Parse a single chess game depicted in portable game notation.

Move Notation

For general moves, the first character (or lack thereof) indicates the piece: R for Rook, N for kNight, B for Bishop, Q for Queen, K for King, and the empty string for pawn. Optionally a file or rank (or both) may be given after the piece letter to remove ambiguity. An x comes next if a capture takes place. The next two characters always represent the destination file [a,z] and rank [1,8]. If the move includes a pawn promotion, an equals sign and the letter of the replacement piece are added. Lastly, a plus sign describes check and the hash symbol checkmate. For castling, O-O and O-O-O label kingside and queenside castles, respectively.

Context-Free Grammar

<move>             ::= <castle><checking>?
                       | <piece><file>?<rank>?<capture>?<file>
                         <rank><promotion>?<checking>?
<piece>            ::= R | N | B | Q | K | <empty string>
<empty string>     ::= 
<file>             ::= [a-h]
<rank>             ::= [1-8]
<capture>          ::= x
<promotion>        ::= =<piece>
<checking>         ::= [#+]
<castle>           ::= <queenside castle> | <kingside castle>
<kingside castle>  ::= O-O
<queenside castle> ::= O-O-O