Added undo move method and optimized board string methods. Gone from …

board.go

@@ -3,6 +3,7 @@ package chess
 import (
 	"strconv"
 	"strings"
+	"bytes"
 )
 
 // A Board represents a chess board and its relationship between squares and pieces.
@@ -59,28 +60,30 @@ func (b *Board) Draw() string {
 
 // String implements the fmt.Stringer interface and returns
 // a string in the FEN board format: rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR
+// TODO use zobrist instead
 func (b *Board) String() string {
-	fen := ""
+	var fen bytes.Buffer
+	var retval string
 	for r := 7; r >= 0; r-- {
 		for f := 0; f < numOfSquaresInRow; f++ {
 			sq := getSquare(File(f), Rank(r))
 			p := b.Piece(sq)
 			if p != NoPiece {
-				fen += p.getFENChar()
+				fen.WriteString(p.getFENChar())
 			} else {
-				fen += "1"
+				fen.WriteString("1")
 			}
 		}
 		if r != 0 {
-			fen += "/"
+			fen.WriteString("/")
 		}
 	}
 	for i := 8; i > 1; i-- {
 		repeatStr := strings.Repeat("1", i)
 		countStr := strconv.Itoa(i)
-		fen = strings.Replace(fen, repeatStr, countStr, -1)
+		retval = strings.Replace(fen.String(), repeatStr, countStr, -1)
 	}
-	return fen
+	return retval
 }
 
 // Piece returns the piece for the given square.

game.go

@@ -170,6 +170,26 @@ func (g *Game) Move(m *Move) error {
 	return nil
 }
 
+// Unmove unmakes the last move
+// and sets the game state to the
+// one of the previous move
+func (g *Game) Unmove() error {
+	if (len(g.moves) < 1) {
+		return fmt.Errorf("chess: no previous moves to unmake")
+	}
+	// delete the last move
+	g.moves = g.moves[:len(g.moves)-1]
+	// delete the last position
+	g.positions = g.positions[:len(g.positions)-1]
+	// update the current position to the last one
+	g.pos = g.positions[len(g.positions)-1]
+	// reset outcome, etc.
+	g.method = NoMethod
+	g.outcome = NoOutcome
+	//g.updatePosition()
+	return nil
+}
+
 // MoveStr decodes the given string in game's notation
 // and calls the Move function.  An error is returned if
 // the move can't be decoded or the move is invalid.
@@ -335,6 +355,7 @@ func (g *Game) RemoveTagPair(k string) bool {
 
 func (g *Game) updatePosition() {
 	method := g.pos.Status()
+
 	if method == Stalemate {
 		g.method = Stalemate
 		g.outcome = Draw

piece.go

@@ -183,10 +183,40 @@ var (
 )
 
 func (p Piece) getFENChar() string {
+	switch p {
+	case WhiteKing:
+		return "K"
+	case WhiteQueen:
+		return "Q"
+	case WhiteRook:
+		return "R"
+	case WhiteBishop:
+		return "B"
+	case WhiteKnight:
+		return "N"
+	case WhitePawn:
+		return "P"
+	case BlackKing:
+		return "k"
+	case BlackQueen:
+		return "q"
+	case BlackRook:
+		return "r"
+	case BlackBishop:
+		return "b"
+	case BlackKnight:
+		return "n"
+	case BlackPawn:
+		return "p"
+	default:
+		return ""
+	}
+	/*
 	for key, piece := range fenPieceMap {
 		if piece == p {
 			return key
 		}
 	}
 	return ""
+	*/
 }

position.go

@@ -43,6 +43,8 @@ func (cr CastleRights) String() string {
 // to its outcome.  Position is translatable to FEN notation.
 type Position struct {
 	board           *Board
+	cached_board_string string
+	cached_castle_string string
 	turn            Color
 	castleRights    CastleRights
 	enPassantSquare Square
@@ -74,6 +76,8 @@ func (pos *Position) Update(m *Move) *Position {
 	b.update(m)
 	return &Position{
 		board:           b,
+		cached_board_string: "",
+		cached_castle_string: "",
 		turn:            pos.turn.Other(),
 		castleRights:    ncr,
 		enPassantSquare: pos.updateEnPassantSquare(m),
@@ -213,9 +217,23 @@ func (pos *Position) updateEnPassantSquare(m *Move) Square {
 	return NoSquare
 }
 
+func (pos *Position)boardString() string {
+	if (pos.cached_board_string == "") {
+		pos.cached_board_string = pos.board.String()
+	}
+	return pos.cached_board_string
+}
+
+func (pos *Position)castleString() string {
+	if (pos.cached_castle_string == "") {
+		pos.cached_castle_string = pos.castleRights.String()
+	}
+	return pos.cached_castle_string
+}
+
 func (pos *Position) samePosition(pos2 *Position) bool {
-	return pos.board.String() == pos2.board.String() &&
+	return pos.boardString() == pos2.boardString() &&
 		pos.turn == pos2.turn &&
-		pos.castleRights.String() == pos2.castleRights.String() &&
+		pos.castleString() == pos2.castleString() &&
 		pos.enPassantSquare == pos2.enPassantSquare
 }