chess

Introduction

Chess is a go library which provides common chess utilities such as move generation, turn management, checkmate detection, PGN encoding, and others. It is well tested and optimized for performance and has no dependencies outside the standard library.

Installation

The package can be installed using "go get".

go get -u github.com/notnil/chess

Usage

Movement

Chess exposes two ways of moving: valid move generation and notation parsing. Valid moves are calculated from the current position and are returned from the ValidMoves method. Even if the client isn't a go program (e.g. a web app) the list of moves can be serialized into their string representation and supplied to the client. Once a move is selected the MoveStr method can be used to parse the selected move's string.

Valid Moves

Valid moves generated from the game's current position:

game := chess.NewGame()
moves := game.ValidMoves()
game.Move(moves[0])
fmt.Println(moves[0]) // b1a3

Parse Notation

Game's MoveStr method accepts string input using the default Algebraic Notation:

game := chess.NewGame()
if err := game.MoveStr("e4"); err != nil {
	// handle error
}

Outcome

The outcome of the match is calculated automatically from the inputted moves if possible. Draw agreements, resignations, and other human initiated outcomes can be inputted as well.

Checkmate

Black wins by checkmate (Fool's Mate):

game := chess.NewGame()
game.MoveStr("f3")
game.MoveStr("e6")
game.MoveStr("g4")
game.MoveStr("Qh4")
fmt.Println(game.Outcome()) // 0-1
fmt.Println(game.Method()) // Checkmate
/*
 A B C D E F G H
8♜ ♞ ♝ - ♚ ♝ ♞ ♜
7♟ ♟ ♟ ♟ - ♟ ♟ ♟
6- - - - ♟ - - -
5- - - - - - - -
4- - - - - - ♙ ♛
3- - - - - ♙ - -
2♙ ♙ ♙ ♙ ♙ - - ♙
1♖ ♘ ♗ ♕ ♔ ♗ ♘ ♖
*/

Stalemate

Black king has no safe move:

fenStr := "k1K5/8/8/8/8/8/8/1Q6 w - - 0 1"
fen, _ := chess.FEN(fenStr)
game := chess.NewGame(fen)
game.MoveStr("Qb6")
fmt.Println(game.Outcome()) // 1/2-1/2
fmt.Println(game.Method())  // Stalemate
/*
 A B C D E F G H
8♚ - ♔ - - - - -
7- - - - - - - -
6- ♕ - - - - - -
5- - - - - - - -
4- - - - - - - -
3- - - - - - - -
2- - - - - - - -
1- - - - - - - -
*/

Resignation

Black resigns and white wins:

game := chess.NewGame()
game.MoveStr("f3")
game.Resign(chess.Black)
fmt.Println(game.Outcome()) // 1-0
fmt.Println(game.Method()) // Resignation

Draw Offer

Draw by mutual agreement:

game := chess.NewGame()
game.Draw(chess.DrawOffer)
fmt.Println(game.Outcome()) // 1/2-1/2
fmt.Println(game.Method())  // DrawOffer

Threefold Repetition

Threefold repetition occurs when the position repeats three times (not necessarily in a row). If this occurs both players have the option of taking a draw, but aren't required until Fivefold Repetition.

game := chess.NewGame()
moves := []string{"Nf3", "Nf6", "Ng1", "Ng8", "Nf3", "Nf6", "Ng1", "Ng8"}
for _, m := range moves {
	game.MoveStr(m)
}
fmt.Println(game.EligibleDraws()) //  [DrawOffer ThreefoldRepetition]

Fivefold Repetition

According to the FIDE Laws of Chess if a position repeats five times then the game is drawn automatically.

game := chess.NewGame()
moves := []string{
	"Nf3", "Nf6", "Ng1", "Ng8",
	"Nf3", "Nf6", "Ng1", "Ng8",
	"Nf3", "Nf6", "Ng1", "Ng8",
	"Nf3", "Nf6", "Ng1", "Ng8",
}
for _, m := range moves {
	game.MoveStr(m)
}
fmt.Println(game.Outcome()) // 1/2-1/2
fmt.Println(game.Method()) // FivefoldRepetition

Fifty Move Rule

Fifty-move rule allows either player to claim a draw if no capture has been made and no pawn has been moved in the last fifty moves.

fen, _ := chess.FEN("2r3k1/1q1nbppp/r3p3/3pP3/pPpP4/P1Q2N2/2RN1PPP/2R4K b - b3 100 23")
game := chess.NewGame(fen)
game.Draw(chess.FiftyMoveRule)
fmt.Println(game.Outcome()) // 1/2-1/2
fmt.Println(game.Method()) // FiftyMoveRule

Seventy Five Move Rule

According to FIDE Laws of Chess Rule 9.6b if 75 consecutive moves have been made without movement of any pawn or any capture, the game is drawn unless the last move was checkmate.

fen, _ := chess.FEN("2r3k1/1q1nbppp/r3p3/3pP3/pPpP4/P1Q2N2/2RN1PPP/2R4K b - b3 149 23")
game := chess.NewGame(fen)
game.MoveStr("Kf8")
fmt.Println(game.Outcome()) // 1/2-1/2
fmt.Println(game.Method()) // SeventyFiveMoveRule

Insufficient Material

Impossibility of checkmate, or insufficient material, results when neither white or black has the pieces remaining to checkmate the opponent.

fen, _ := chess.FEN("8/2k5/8/8/8/3K4/8/8 w - - 1 1")
game := chess.NewGame(fen)
fmt.Println(game.Outcome()) // 1/2-1/2
fmt.Println(game.Method()) // InsufficientMaterial

PGN

PGN, or Portable Game Notation, is the most common serialization format for chess matches. PGNs include move history and metadata about the match. Chess includes the ability to read and write the PGN format.

Example PGN

[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 {This opening is called the Ruy Lopez.}
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

Read PGN

PGN supplied as an optional parameter to the NewGame constructor:

pgn, err := chess.PGN(pgnReader)
if err != nil {
	// handle error
}
game := chess.NewGame(pgn)

Write PGN

Moves and tag pairs added to the PGN output:

game := chess.NewGame()
game.AddTagPair("Event", "F/S Return Match")
game.MoveStr("e4")
game.MoveStr("e5")
fmt.Println(game)
/*
[Event "F/S Return Match"]

1.e4 e5  *
*/

FEN

FEN, or Forsyth–Edwards Notation, is the standard notation for describing a board position. FENs include piece positions, turn, castle rights, en passant square, half move counter (for 50 move rule), and full move counter.

Read FEN

FEN supplied as an optional parameter to the NewGame constructor:

fen, err := chess.FEN("rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1")
if err != nil {
	// handle error
}
game := chess.NewGame(fen)

Write FEN

Game's current position outputted in FEN notation:

game := chess.NewGame()
pos := game.Position()
fmt.Println(pos.String()) // rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1

Notations

Chess Notation define how moves are encoded in a serialized format. Chess uses a notation when converting to and from PGN and for accepting move text.

Algebraic Notation

Algebraic Notation (or Standard Algebraic Notation) is the official chess notation used by FIDE. Examples: e2, e5, O-O (short castling), e8=Q (promotion)

game := chess.NewGame(chess.UseNotation(chess.AlgebraicNotation{}))
game.MoveStr("e4")
game.MoveStr("e5")
fmt.Println(game) // 1.e4 e5  *

Long AlgebraicNotation Notation

LongAlgebraicNotation is a more computer friendly alternative to algebraic notation. This notation uses the same format as the UCI (Universal Chess Interface). Examples: e2e4, e7e5, e1g1 (white short castling), e7e8q (for promotion)

game := chess.NewGame(chess.UseNotation(chess.LongAlgebraicNotation{}))
game.MoveStr("e2e4")
game.MoveStr("e7e5")
fmt.Println(game) // 1.e2e4 e7e5  *

Visualization

Chess has multiple ways to visualize board positions for debugging and presentation.

Text Representation

Board's Draw() method can be used to visualize a position using unicode chess symbols.

game := chess.NewGame()
fmt.Println(game.Position().Board().Draw())
/*
 A B C D E F G H
8♜ ♞ ♝ ♛ ♚ ♝ ♞ ♜
7♟ ♟ ♟ ♟ ♟ ♟ ♟ ♟
6- - - - - - - -
5- - - - - - - -
4- - - - - - - -
3- - - - - - - -
2♙ ♙ ♙ ♙ ♙ ♙ ♙ ♙
1♖ ♘ ♗ ♕ ♔ ♗ ♘ ♖
*/

SVG Representation

chessimg is an image utility designed to work with the chess package to render an SVG of a chess position. It presents configuration options such as dark and white square colors and squares to highlight.

// create file
f, err := os.Create("example.svg")
if err != nil {
	// handle error
}
defer f.Close()

// create board position
fenStr := "rnbqkbnr/pppppppp/8/8/3P4/8/PPP1PPPP/RNBQKBNR b KQkq - 0 1"
pos := &chess.Position{}
if err := pos.UnmarshalText([]byte(fenStr)); err != nil {
	// handle error
}

// write board SVG to file
yellow := color.RGBA{255, 255, 0, 1}
mark := chessimg.MarkSquares(yellow, chess.D2, chess.D4)
if err := chessimg.SVG(f, pos.Board(), mark); err != nil {
	// handle error
}

Example Program

Valid moves are randomly selected until the game is over:

package main

import (
	"fmt"
	"math/rand"

	"github.com/notnil/chess"
)

func main() {
	game := chess.NewGame()
	// generate moves until game is over
	for game.Outcome() == chess.NoOutcome {
		// select a random move
		moves := game.ValidMoves()
		move := moves[rand.Intn(len(moves))]
		game.Move(move)
	}
	// print outcome and game PGN
	fmt.Println(game.Position().Board().Draw())
	fmt.Printf("Game completed. %s by %s.\n", game.Outcome(), game.Method())
	fmt.Println(game.String())    
}

Performance

Chess has been performance tuned, using pprof, with the goal of being fast enough for use by chess bots. The original map based board representation was replaced by bitboards resulting in a large performance increase.

Benchmarks

The benchmarks can be run with the following command:

go test -bench=.

Results from the baseline 2015 MBP:

BenchmarkBitboardReverse-4              2000000000               1.01 ns/op
BenchmarkStalemateStatus-4                500000              3116 ns/op
BenchmarkInvalidStalemateStatus-4         500000              2290 ns/op
BenchmarkPositionHash-4                  1000000              1864 ns/op
BenchmarkValidMoves-4                     100000             13445 ns/op
BenchmarkPGN-4                               300           5549192 ns/op