Object Oriented Design for a Chess game

I am trying to get a feel of how to design and think in an Object Oriented manner and want to get some feedback from the community on this topic. The following is an example of a chess game that I wish to design in an OO manner. This is a very broad design and my focus at this stage is just to identify who is responsible for what messages and how the objects interact each other to simulate the game. Please point out if there are elements of bad design (high coupling, bad cohesion etc.) and how to improve on them.

The Chess game has the following classes

• Board
• Player
• Piece
• Square
• ChessGame

The Board is made up of squares and so Board can be made responsible for creating and managing Square objects. Each piece also is on a square so each piece also has a reference to the square it is on. (Does this make sense?). Each piece then is responsible to move itself from one square to another. Player class holds references to all pieces he owns and is also responsible for their creation (Should player create Pieces?) . Player has a method takeTurn which in turn calls a method movePiece which belongs to the piece Class which changes the location of the piece from its current location to another location. Now I am confused on what exactly the Board class must be responsible for. I assumed it was needed to determine the current state of the game and know when the game is over. But when a piece changes it's location how should the board get updated? should it maintain a seperate array of squares on which pieces exist and that gets updates as pieces move?

Also, ChessGame intially creates the Board and player objects who in turn create squares and pieces respectively and start the simulation. Briefly, this might be what the code in ChessGame may look like

Player p1 =new Player();
Player p2 = new Player();

Board b = new Board();

while(b.isGameOver())
{
  p1.takeTurn(); // calls movePiece on the Piece object
  p2.takeTurn();

}

I am unclear on how the state of the board will get updated. Should piece have a reference to board? Where should be the responsibility lie? Who holds what references? Please help me with your inputs and point out problems in this design. I am deliberately not focusing on any algorithms or further details of game play as I am only interested in the design aspect. I hope this community can provide valuable insights.

I actually just wrote a full C# implementation of a chess board, pieces, rules, etc. Here's roughly how I modeled it (actual implementation removed since I don't want to take all the fun out of your coding):

public enum PieceType {
    None, Pawn, Knight, Bishop, Rook, Queen, King
}

public enum PieceColor {
    White, Black
}

public struct Piece {
    public PieceType Type { get; set; }
    public PieceColor Color { get; set; }
}

public struct Square {
    public int X { get; set; }
    public int Y { get; set; }

    public static implicit operator Square(string str) {
        // Parses strings like "a1" so you can write "a1" in code instead
        // of new Square(0, 0)
    }
}

public class Board {
    private Piece[,] board;

    public Piece this[Square square] { get; set; }

    public Board Clone() { ... }
}

public class Move {
    public Square From { get; }
    public Square To { get; }
    public Piece PieceMoved { get; }
    public Piece PieceCaptured { get; }
    public PieceType Promotion { get; }
    public string AlgebraicNotation { get; }
}

public class Game {
    public Board Board { get; }
    public IList<Move> Movelist { get; }
    public PieceType Turn { get; set; }
    public Square? DoublePawnPush { get; set; } // Used for tracking valid en passant captures
    public int Halfmoves { get; set; }

    public bool CanWhiteCastleA { get; set; }
    public bool CanWhiteCastleH { get; set; }
    public bool CanBlackCastleA { get; set; }
    public bool CanBlackCastleH { get; set; }
}

public interface IGameRules {
    // ....
}

The basic idea is that Game/Board/etc simply store the state of the game. You can manipulate them to e.g. set up a position, if that's what you want. I have a class that implements my IGameRules interface that is responsible for:

• Determining what moves are valid, including castling and en passant.
• Determining if a specific move is valid.
• Determining when players are in check/checkmate/stalemate.
• Executing moves.

Separating the rules from the game/board classes also means you can implement variants relatively easily. All methods of the rules interface take a Game object which they can inspect to determine which moves are valid.

Note that I do not store player information on Game. I have a separate class Table that is responsible for storing game metadata such as who was playing, when the game took place, etc.

EDIT: Note that the purpose of this answer isn't really to give you template code you can fill out -- my code actually has a bit more information stored on each item, more methods, etc. The purpose is to guide you towards the goal you're trying to achieve.

Here is my idea, for a fairly basic chess game :

class GameBoard {
 IPiece config[8][8];  

 init {
  createAndPlacePieces("Black");
  createAndPlacePieces("White");
  setTurn("Black");
 
 }

 createAndPlacePieces(color) {
   //generate pieces using a factory method
   //for e.g. config[1][0] = PieceFactory("Pawn",color);
 }

 setTurn(color) {
   turn = color;
 }

 move(fromPt,toPt) {
  if(getPcAt(fromPt).color == turn) {
    toPtHasOppositeColorPiece = getPcAt(toPt) != null && getPcAt(toPt).color != turn;
    possiblePath = getPcAt(fromPt).generatePossiblePath(fromPt,toPt,toPtHasOppositeColorPiece);
   if(possiblePath != NULL) {
      traversePath();
      changeTurn();
   }
  }
 } 

}

Interface IPiece {
  function generatePossiblePath(fromPt,toPt,toPtHasEnemy);
}

class PawnPiece implements IPiece{
  function generatePossiblePath(fromPt,toPt,toPtHasEnemy) {
    return an array of points if such a path is possible
    else return null;
  }
}

class ElephantPiece implements IPiece {....}

I recently created a chess program in PHP (website click here, source click here) and I made it object oriented. Here are the classes I used.

• ChessRulebook (static) - I put all my generate_legal_moves() code in here. That method is given a board, whose turn it is, and some variables to set the level of detail of the output, and it generates all the legal moves for that position. It returns a list of ChessMoves.
• ChessMove - Stores everything needed to create algebraic notation, including starting square, ending square, color, piece type, capture, check, checkmate, promotion piece type, and en passant. Optional additional variables include disambiguation (for moves like Rae4), castling, and board.
• ChessBoard - Stores the same information as a Chess FEN, including an 8x8 array representing the squares and storing the ChessPieces, whose turn it is, en passant target square, castling rights, halfmove clock, and fullmove clock.
• ChessPiece - Stores piece type, color, square, and piece value (for example, pawn = 1, knight = 3, rook = 5, etc.)
• ChessSquare - Stores the rank and file, as ints.

I am currently trying to turn this code into a chess A.I., so it needs to be FAST. I've optimized the generate_legal_moves() function from 1500ms to 8ms, and am still working on it. Lessons I learned from that are...

• Do not store an entire ChessBoard in every ChessMove by default. Only store the board in the move when needed.
• Use primitive types such as int when possible. That is why ChessSquare stores rank and file as int, rather than also storing an alphanumeric string with human readable chess square notation such as "a4".
• The program creates tens of thousands of ChessSquares when searching the move tree. I will probably refactor the program to not use ChessSquares, which should give a speed boost.
• Do not calculate any unnecessary variables in your classes. Originally, calculating the FEN in each of my ChessBoards was really killing the program's speed. I had to find this out with a profiler.

I know this is old, but hopefully it helps somebody. Good luck!