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Simple Kotlin Chess Library

This project is a Kotlin Multiplatform fork of the chesslib library by bhlangonijr.

Kchesslib is a simple kotlin chess library for generating legal chess moves given a chessboard position, parse a chess game stored in PGN or FEN format and many other things.

Table of Contents

  1. Installing
  2. Usage
  3. Create a chessboard and make a move
  4. Undo a move
  5. Get FEN string from chessboard
  6. Load a chessboard position from FEN notation
  7. MoveList
  8. Generate all chess legal-moves for the current position
  9. Checking chessboard situation
  10. Comparing boards
  11. Load a chess game collection from a file
  12. Advanced usage
  13. Sanity checking of chesslib move generation with Perft
  14. Creating a full-fledged chess engine
  15. Capturing and reacting to events

Installing

This library is available on Maven Central. To use it, add the following to your build.gradle.kts file:

dependencies {
    implementation("io.github.cvb941:kchesslib:1.0.1")
}

Usage

Create a chessboard and make a move

// Creates a new chessboard in the standard initial position
val board = Board()

//Make a move from E2 to E4 squares
board.doMove(Move(Square.E2, Square.E4))

//print the chessboard in a human-readable form
println(board.toString())

Alternatively one could just specify the move using SAN, e.g.:

val board = Board()
board.doMove("e4")

Result:

rnbqkbnr
pppppppp


    P

PPPP PPP
RNBQKBNR
Side: BLACK

Undo a move

// Undo the last move from the stack and return it
val move = board.undoMove()

Get FEN string from chessboard

println(board.fen)

Load a chessboard position from FEN notation

// Load a FEN position into the chessboard
val fen = "rnbqkbnr/pppppppp/8/8/4P3/8/PPPP1PPP/RNBQKBNR b KQkq e3 0 1"
val board = Board()
board.loadFromFen(fen)

//Find the square locations of black bishops
val blackBishopSquares = board.getPieceLocation(Piece.BLACK_BISHOP)

//Get the piece at A1 square...
val piece = board.getPiece(Square.A1)

MoveList

MoveList stores a list of moves played in the chessboard. When created it assumes the initial position of a regular chess game. Arbitrary moves from a chess game can be loaded using SAN or LAN string:

val san = "e4 Nc6 d4 Nf6 d5 Ne5 Nf3 d6 Nxe5 dxe5 Bb5+ Bd7 Bxd7+ Qxd7 Nc3 e6 O-O exd5"
val list = MoveList()
list.loadFromSan(san)

println("FEN of final position: ${list.fen}")

Generate all chess legal-moves for the current position

// Generate legal chess moves for the current position
val board = Board()
val moves = board.legalMoves()
println("Legal moves: $moves")

Result:

a2a3 a2a4 b2b3 b2b4 c2c3 c2c4 d2d3 d2d4 e2e3 e2e4 f2f3 f2f4 g2g3 g2g4 h2h3 h2h4 b1a3 b1c3 g1f3 g1h3

Relaying the legal moves to the chessboard:

for (move in moves) {
    board.doMove(move)
    //do something
    board.undoMove()
}
println("Legal moves: $moves")

Checking chessboard situation

Chessboard situation can be checked using the methods:

  • board.isDraw
  • board.isInsufficientMaterial
  • board.isStaleMate
  • board.isKingAttacked
  • board.isMated
  • board.getSideToMove
  • ...

Comparing boards

There are two methods for comparing boards:

  • board.equals(board2): Compares ignoring the board history
  • board.strictEquals(board2): Compares the board and its history

Load a chess game collection from a PGN file

val pgn = PgnHolder("/opt/games/linares_2002.pgn")
pgn.loadPgn()
for (game in pgn.games) {
    game.loadMoveText()
    val moves = game.halfMoves
    val board = Board()
    // Replay all the moves from the game and print the final position in FEN format
    for (move in moves) {
        board.doMove(move)
    }
    println("FEN: ${board.fen}")
}

You could achieve the same by loading the move list final FEN position:

board.loadFromFen(moves.getFen());

Iterating over a PGN file games using the PgnIterator:

val games = PgnIterator("/opt/games/linares_2002.pgn")
for (game in games) {
    println("Game: $game")
}

Note: The iterator is highly recommended for processing large PGN files as it is not retaining in the memory intermediate objects loaded during the process of each iteration.

Capturing the comments from each move:

val games = PgnIterator("src/test/resources/rav_alternative.pgn")
for (game in games) {
    val moves = game.halfMoves.toSanArray()
    val comments = game.comments
    for (i in moves.indices) {
        val ply = ((i + 2) / 2).toString() + if (i % 2 != 0) ".." else " "
        val move = moves[i]
        val comment = comments[i + 1] ?: ""
        println("$ply$move $comment".trim())
    }
}

The output should be something like:

1 e4 Ponomariov plays 1. e4 in much the same way as any of the other top-level GMs.
1..e6 Now, along with Pe4 there is an indication Black will place pawns on light-color squares to prevent Bf1 from ever being dangerous. White will probably have to meet 2...d5 with e4-e5 to open the d3-h7 diagonal. So, White needs a Pd4 to support Pe5.

Advanced usage

Sanity checking of chesslib move generation with Perft

Perft, (performance test, move path enumeration) is a debugging function to walk the move generation tree of strictly legal moves to count all the leaf nodes of a certain depth. Example of a perft function using chesslib:

private fun perft(board: Board, depth: Int, ply: Int): Long {
    if (depth == 0) {
        return 1
    }
    var nodes: Long = 0
    val moves = board.legalMoves()
    for (move in moves) {
        board.doMove(move)
        nodes += perft(board, depth - 1, ply + 1)
        board.undoMove()
    }
    return nodes
}

There are plenty of known results for Perft tests on a given set of chess positions. It can be tested against the library to check if it's reliably generating moves and while keeping the Board in a consistent state, e.g.:

@Test
fun testPerftInitialPosition() {
    val board = Board()
    board.setEnableEvents(false)
    board.loadFromFen("rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1")

    val nodes = perft(board, 5, 1)
    assertEquals(4865609, nodes)
}

It's known that from the initial standard chess position, there should have exactly 4865609 positions for depth 5. Deviation from this number would imply a bug in move generation or keeping the board state.

Creating a full-fledged chess engine

kengine is a minimalistic chess engine built on top of kotlin and chesslib to showcase a more advanced use case.

Capturing and reacting to events

Actions occurring in the chessboard or when loading a PGN file are emitted as events by the library so that it can be captured by a GUI, for example:

Listening to PGN loading progress

Create your listener:

class MyListener : PgnLoadListener {

    private var games = 0

    override fun notifyProgress(games: Int) {
        println("Loaded $games games...")
    }
}

Add the listener to PgnHolder object and load the games:

val pgn = PgnHolder(".../games.pgn")
// add your listener
pgn.listener.add(myListener)
pgn.loadPgn()

Example implementing a SwingWorker to update a Swing ProgressBarDialog with PGN loading status:

import javax.swing.SwingWorker
import javax.swing.JOptionPane
import java.beans.PropertyChangeEvent
import java.beans.PropertyChangeListener

class LoadPGNWorker : SwingWorker<Int, Int>(), PgnLoadListener {

    private val progress = ProgressBarDialog("PGN Loader", frame)

    init {
        addPropertyChangeListener(object : PropertyChangeListener {
            override fun propertyChange(e: PropertyChangeEvent) {
                progress.progressBar.isIndeterminate = true
                progress.progressBar.value = e.newValue as Int
            }
        })
    }

    override fun doInBackground(): Int? {
        return try {
            getPgnHolder().listener.add(this)
            loadPGN()
            getPgnHolder().size
        } catch (e: Exception) {
            JOptionPane.showMessageDialog(owner, "Error message from bundle: ${e.message}", "Error", JOptionPane.ERROR_MESSAGE)
            log.error("Error loading pgn", e)
            progress.dispose()
            null
        }
    }

    override fun done() {
        progress = 100
    }

    override fun notifyProgress(games: Int) {
        progress = minOf(90, games)
        progress.label.text = "Loading games..."
    }
}

Listening to chessboard events

Moves played and game statuses are emitted by the Board whenever these actions happen.

Implement your Board listener:

class MyBoardListener : BoardEventListener {

    override fun onEvent(event: BoardEvent) {
        if (event.type == BoardEventType.ON_MOVE) {
            val move = event as Move
            println("Move $move was played")
        }
    }
}

Add your listener to Board and listen to played moves events:

val board = Board()
board.addEventListener(BoardEventType.ON_MOVE, MyBoardListener())

handToGui(board)
  • Beware that listeners are executed using the calling thread that updated the Board and depending on your listener processing requirements you'd want to hand the execution off to a separate thread like in a threadpool:
fun onEvent(event: BoardEvent) {
    executors.submit(myListenerRunnable)
}