Chess

This is a browser based multiplayer chess engine written in Node.

This project made into the top 5 projects in TinkerHub HireHack.

Table Of Contents

• How does it work?
  • Basic Workflow
• Chess Logic
  • Storing Board State
  • The Piece Classes
  • The Board Class
  • Move Validation
  • Pawn Promotion
• Socket Events

How does it work?

• Express hosts the server
• Using Sockets.io, real time communication between server and clients could be achieved through events.
• The server stores the details of each room in a MongoDB.

Basic Workflow

• If the player creates a room, a new document is created in DB and a new room is allocated to the player.
• Joining or creating a room will lead to the getGameId event, which sends the gameId to the client.
• After 2 players are in a room, the game countdown starts by emitting the startCountDown event.
• If a player quit during the countdown, an endCountDownDisconnect event is emitted and the countdown stops.
• After the countdown ends, if the room still has 2 players, the game will begin by emitting startGame event.
• Each move made by the player is sent to the server using makeMove event.
• The board state is read from the db and the move is validated. The board is sent to the listening clients through updateBoard event.
• This goes on until the game is over through victory or draw.
• After the game is over, the game's details are removed from DB.

Chess Logic

The chess logic run by the server is stored in /server/chess/. The client side does not support exports and require, so it is separately maintained in /static/chessBrowser.js.

Storing the board state

FEN notation is used to store the board state in db. The only disadvantage is it can't store pawn promotion data, so it has to be separately maintained.

The Piece Classes

All the pieces extend the Base Class Piece.

The Piece Class

Member	Overriden	Usage
type	yes	The string representation
player	no	The owner of this piece
nextNormalMoves	yes	The directions for the next moves
nextCaptureMoves	yes	The directions for the next capture moves(Pawn Only)
multipleMoves	yes	Whether this piece can move multiple columns
getSpecialMoves()	yes	Return the absoule positions of this piece's special moves
getAllMoves()	no	Return the next moves based on the attributes
getPlayer()	no	Get the owner of the provided piece

getSpecialMoves(board, position, args)

Takes a board Object, the position(Integer), and additional args(dictionary) required for special Moves. This should be implemented by Pieces having special moves like Pawn(En Passant, Double First Move) and King(Castling).

getAllMoves(board, position, splMoveArgs)

Takes a board Object, the position(Integer), and additional args(dictionary) required for special Moves. This generates the next moves of a particular piece based on nextNormalMoves, nextCaptureMoves, getSpecialMoves(), and multipleMoves.

The Board Class

This Class is mainly used to represent the board state, to perform move generation, make moves and check game over.

Representing positions of the pieces

The positions of the pieces is represented in an array of length 64. Empty columns are marked null.

Move generation using getNextMoves(checkForCheck=true, player=this.player) function.

1. Get all pieces of the player from the board and store them by mapping to their position.
2. For each piece in pieces call piece.getAllMoves(), and store them to validMoves by mapping to their position.
3. If checkForCheck is false, return validMoves
4. Else check if move is legal by making the move and checking if this player is in check.
5. If not in check add to legalMoves[position].
6. return legalMoves.

isCheck(player=this.player) function.

1. Get the position of player's king.
2. Get all moves of the enemy by this.getNextMoves(false, changePlayer(player)). checkForCheck is set to false to avoid infinite recursion.
3. If any enemy piece can move to the king's location, the player is in check.

makeMove(from, to) function

This function makes the actual changes to the array property. It also changes the properties like enPassantSquare, castleData, etc according to the move

Game Over Conditions

1. A Player has won -> Enemy has no legal moves and is in check(checkVictory function)
2. The game is drawn(checkDraw function)
   • Stalemate: The enemy has no legal moves but isn't in check.(checkStalemate function)
   • 50 move rule: Game is automatically drawn after 50 consecutive halfmoves.

Move Validation

Currently move is validated outside the chess core files.

Server Side -> socketEvents

Client Side -> chessClient

The Logic

1. Generate NextMoves
2. Check if this move in NextMoves.

Pawn Promotion

1. The makeMove function will set the Board.pawnPromotion attribute if eligible for pawn promotion. The player won't be changed if eligible.

2. The server will send a startPawnPromotion event to the room. The eligible player has to pick a piece for promotion. No other moves are allowed. The promotionData would be written to the db.

3. On choosing a piece the client would send a promotePawn event to the server. Which would in turn send a successPawnPromotion or errorPawnPromotion event back based on the promotionData stored by the db.

4. On successPawnPromotion, the game will resume and the other player gets their turn.

Socket Events

All server side socket handling is located in socketEvents.js

All client side socket handling is located in index.html

Event	Emitter	Data	Functionality
roomCreate	Client	null	Indicate to the server a new room has been created.
roomJoin	Client	null	Indicate to the server a new player has joined the room.
getGameId	Server	gameId	Emit the game Id to the client.
startCountDown	Server	null	Emit to the room to start the countdown.
coutdownFinished	Client	gameId	Emit to the server that coutdown has finished.
endCountDownDisconnect	Server	null	Emit to the room that countdown must be stopped due to a disconnecting player
startGame	Server	initialBoard	Emit to the room to start the game.
playerColor	Server	color	Emit the color of the client to the client.
invalidRoomId	Server	null	Emit that the room id is invalid to the client.
makeMove	Client	gameId, from, to	Emit the move made by the player to server.
boardUpdated	Server	fen	Emit the current board state to the room.
invalidMove	Server	null	Emit to indicate the proposed move is invalid.
victory	Server	player	Emit to the room, that player is the winner.
draw	Server	null	Emit to the room that the game has been drawn.
startPawnPromotion	Server	player	Emit to the room that player can promote a pawn.
successPawnPromotion	Server	null	Emit to the client that Pawn Promotion is successfull.
errorPawnPromotion	Server	null	Emit to the client that Pawn Promotion Failed.
promotePawn	Client	gameId, piece	Emit to the server that the client requests to promote the pawn to piece
playerDisconnect	Server	null	Emit to the room that a player has disconnected.
endGameDisconnect	Server	null	Emit to the room that the current game has ended due to a player being disconnected.