position

Documentation

Overview

Package position represents data structures and functions for a chess board and its position. It uses a 8x8 piece board and bitboards, a stack for undo moves, zobrist keys for transposition tables, piece lists, material and positional value counter.

Create a new instance with NewPosition(...) with no parameters to get the chess start position.

Index

• Constants
• func NewRandom(seed uint64) random
• type Key
• type Position
  • func NewPosition(fen ...string) *Position
  • func NewPositionFen(fen string) (*Position, error)
  • func (p *Position) CastlingRights() CastlingRights
  • func (p *Position) CheckRepetitions(reps int) bool
  • func (p *Position) DoMove(m Move)
  • func (p *Position) DoNullMove()
  • func (p *Position) GamePhase() int
  • func (p *Position) GamePhaseFactor() float64
  • func (p *Position) GetEnPassantSquare() Square
  • func (p *Position) GetPiece(sq Square) Piece
  • func (p *Position) GivesCheck(move Move) bool
  • func (p *Position) HalfMoveClock() int
  • func (p *Position) HasCheck() bool
  • func (p *Position) HasInsufficientMaterial() bool
  • func (p *Position) IsAttacked(sq Square, by Color) bool
  • func (p *Position) IsCapturingMove(move Move) bool
  • func (p *Position) IsLegalMove(move Move) bool
  • func (p *Position) KingSquare(c Color) Square
  • func (p *Position) LastCapturedPiece() Piece
  • func (p *Position) LastMove() Move
  • func (p *Position) Material(c Color) Value
  • func (p *Position) MaterialNonPawn(c Color) Value
  • func (p *Position) NextPlayer() Color
  • func (p *Position) OccupiedAll() Bitboard
  • func (p *Position) OccupiedBb(c Color) Bitboard
  • func (p *Position) PiecesBb(c Color, pt PieceType) Bitboard
  • func (p *Position) PsqEndValue(c Color) Value
  • func (p *Position) PsqMidValue(c Color) Value
  • func (p *Position) String() string
  • func (p *Position) StringBoard() string
  • func (p *Position) StringFen() string
  • func (p *Position) UndoMove()
  • func (p *Position) UndoNullMove()
  • func (p *Position) WasCapturingMove() bool
  • func (p *Position) WasLegalMove() bool
  • func (p *Position) ZobristKey() Key

Constants

const (
	// StartFen is a string with the fen position for a standard chess game
	StartFen string = "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"
)

Functions

func NewRandom

func NewRandom(seed uint64) random

NewRandom creates a random object with a seed. Seed must not be negative or zero.

Types

type Key

type Key uint64

Key is used for zobrist keys in chess positions. Zobrist keys need all 64 bits for distribution

type Position

type Position struct {
	// contains filtered or unexported fields
}

Position This struct represents the chess board and its position. It uses a 8x8 piece board and bitboards, a stack for undo moves, zobrist keys for transposition tables, piece lists, material and positional value counter.

Needs to be created with NewPosition() or NewPosition(fen string)

func NewPosition

func NewPosition(fen ...string) *Position

NewPosition creates a new position. When called without an argument the position will have the start position When a fen string is given it will create a position with based on this fen. Additional fens/strings are ignored

func NewPositionFen

func NewPositionFen(fen string) (*Position, error)

NewPositionFen creates a new position with the given fen string as board position It returns nil and an error if the fen was invalid.

func (*Position) CastlingRights

func (p *Position) CastlingRights() CastlingRights

CastlingRights returns the castling rights instance of the position

func (*Position) CheckRepetitions

func (p *Position) CheckRepetitions(reps int) bool

CheckRepetitions Repetition of a position:. To detect a 3-fold repetition the given position must occur at least 2 times before:<br/> <code>position.checkRepetitions(2)</code> checks for 3 fold-repetition <p> 3-fold repetition: This most commonly occurs when neither side is able to avoid repeating moves without incurring a disadvantage. The three occurrences of the position need not occur on consecutive moves for a claim to be valid. FIDE rules make no mention of perpetual check; this is merely a specific type of draw by threefold repetition.

Return true if this position has been played reps times before

func (*Position) DoMove

func (p *Position) DoMove(m Move)

DoMove commits a move to the board. Due to performance there is no check if this move is legal on the current position. Legal check needs to be done beforehand or after in case of pseudo legal moves. Usually the move will be generated by a MoveGenerator and therefore the move will be assumed legal anyway.

func (*Position) DoNullMove

func (p *Position) DoNullMove()

DoNullMove is used in Null Move Pruning. The position is basically unchanged but the next player changes. The state before the null move will be stored to history. The history entry will be changed. So in effect after an UndoNullMove() the external view on the position is unchanged (e.g. fenBeforeNull == fenAfterNull and zobristBeforeNull == zobristAfterNull but positionBeforeNull != positionAfterNull.

func (*Position) GamePhase

func (p *Position) GamePhase() int

GamePhase returns the current game phase value of the position. GamePhase is 24 at the start of the game (24 is also the max). End games when no officers are left have a GamePhase value of 0.

func (*Position) GamePhaseFactor

func (p *Position) GamePhaseFactor() float64

GamePhaseFactor returns a factor between 0 and 1 which reflects the ratio between the actual game phase and the max game phase

func (*Position) GetEnPassantSquare

func (p *Position) GetEnPassantSquare() Square

GetEnPassantSquare returns the en passant square or SqNone if not set

func (*Position) GetPiece

func (p *Position) GetPiece(sq Square) Piece

GetPiece returns the piece on the given square. Empty squares are initialized with PieceNone and return the same.

func (*Position) GivesCheck

func (p *Position) GivesCheck(move Move) bool

GivesCheck determines if the given move will give check to the opponent of p.NextPlayer() and returns true if so.

func (*Position) HalfMoveClock

func (p *Position) HalfMoveClock() int

HalfMoveClock returns the positions half move clock

func (*Position) HasCheck

func (p *Position) HasCheck() bool

HasCheck returns true if the next player is threatened by a check (king is attacked). This is cached for the current position. Multiple calls to this on the same position are therefore very efficient.

func (*Position) HasInsufficientMaterial

func (p *Position) HasInsufficientMaterial() bool

HasInsufficientMaterial returns true if no side has enough material to force a mate (does not exclude combination where a helpmate would be possible, e.g. the opponent needs to support a mate by mistake)

func (*Position) IsAttacked

func (p *Position) IsAttacked(sq Square, by Color) bool

IsAttacked checks if the given square is attacked by a piece of the given color.

func (*Position) IsCapturingMove

func (p *Position) IsCapturingMove(move Move) bool

IsCapturingMove determines if a move on this position is a capturing move incl. en passant

func (*Position) IsLegalMove

func (p *Position) IsLegalMove(move Move) bool

IsLegalMove tests a move if it is legal on the current position. Basically tests if the king would be left in check after the move or if the king crosses an attacked square during castling.

func (*Position) KingSquare

func (p *Position) KingSquare(c Color) Square

KingSquare returns the current square of the king of color c

func (*Position) LastCapturedPiece

func (p *Position) LastCapturedPiece() Piece

LastCapturedPiece returns the captured piece of the the last move made on the position or MoveNone if the move was non-capturing or the position has no history of earlier moves.

func (*Position) LastMove

func (p *Position) LastMove() Move

LastMove returns the last move made on the position or MoveNone if the position has no history of earlier moves.

func (*Position) Material

func (p *Position) Material(c Color) Value

Material returns the material value for the given color on this position

func (*Position) MaterialNonPawn

func (p *Position) MaterialNonPawn(c Color) Value

MaterialNonPawn returns the non pawn material value for given color

func (*Position) NextPlayer

func (p *Position) NextPlayer() Color

NextPlayer returns the next player as Color for the position

func (*Position) OccupiedAll

func (p *Position) OccupiedAll() Bitboard

OccupiedAll returns a Bitboard of all pieces currently on the board

func (*Position) OccupiedBb

func (p *Position) OccupiedBb(c Color) Bitboard

OccupiedBb returns a Bitboard f all pieces of Color c

func (*Position) PiecesBb

func (p *Position) PiecesBb(c Color, pt PieceType) Bitboard

PiecesBb returns the Bitboard for the given piece type of the given color

func (*Position) PsqEndValue

func (p *Position) PsqEndValue(c Color) Value

PsqEndValue returns the positional value for the given color for later game phases. Best used together with a game phase factor

func (*Position) PsqMidValue

func (p *Position) PsqMidValue(c Color) Value

PsqMidValue returns the positional value for the given color for early game phases. Best used together with a game phase factor

func (*Position) String

func (p *Position) String() string

String returns a string representing the board instance. This includes the fen, a board matrix, game phase, material and pos values.

func (*Position) StringBoard

func (p *Position) StringBoard() string

StringBoard returns a visual matrix of the board and pieces

func (*Position) StringFen

func (p *Position) StringFen() string

StringFen returns a string with the FEN of the current position

func (*Position) UndoMove

func (p *Position) UndoMove()

UndoMove resets the position to a state before the last move has been made

func (*Position) UndoNullMove

func (p *Position) UndoNullMove()

UndoNullMove restores the state of the position to before the DoNullMove() call. The history entry will be changed but the history counter reset. So in effect the external view on the position is unchanged (e.g. fenBeforeNull == fenAfterNull and zobristBeforeNull == zobristAfterNull but positionBeforeNull != positionAfterNull If positionBeforeNull != positionAfterNull would be required this function would have to be changed to reset the history entry as well. Currently this is not necessary and therefore we spare the time to do this.

func (*Position) WasCapturingMove

func (p *Position) WasCapturingMove() bool

WasCapturingMove returns true if the last move was a capturing move.

func (*Position) WasLegalMove

func (p *Position) WasLegalMove() bool

WasLegalMove tests if the last move was legal. Basically tests if the king is now in check or if the king crossed an attacked square during castling or of there was a castling although in check. If the position does not have a last move (history empty) this will only check if the king of the opponent is attacked e.g. could now be captured by the next player.

func (*Position) ZobristKey

func (p *Position) ZobristKey() Key

ZobristKey returns the current zobrist key for this position