✨ FEN

pkg/fen/fen.go

@@ -0,0 +1,124 @@
+package fen
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+
+	"code.c-base.org/gochess/libchess/pkg/board"
+)
+
+// DefaultSetup holds the FEN to the default board setup.
+const DefaultSetup = "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"
+
+// Import returns
+func Import(fen string) (*board.Board, error) {
+	var (
+		parts       = strings.Split(fen, " ")
+		pieces      = map[board.Square]board.Piece{}
+		fullMove    uint64
+		halfMove    uint64
+		ep          *board.Square
+		cr          *board.CastleRights
+		turn        board.Color
+		position    string
+		expandedFEN string
+		err         error
+	)
+	if len(parts) != 6 {
+		return nil, fmt.Errorf("invalid fields in FEN: %s", fen)
+	}
+	if fullMove, err = strconv.ParseUint(parts[5], 10, 64); err != nil {
+		return nil, err
+	}
+	if halfMove, err = strconv.ParseUint(parts[4], 10, 64); err != nil {
+		return nil, err
+	}
+	if parts[3] != "-" {
+		if square, ok := board.StrToSquareMap[strings.ToLower(parts[3])]; ok {
+			ep = &square
+		} else {
+			return nil, fmt.Errorf("unable to parse e.p. square: %s", parts[3])
+		}
+	}
+	if parts[2] != "-" {
+		cr, err = board.NewCastleRights(parts[2])
+		if err != nil {
+			return nil, err
+		}
+	}
+	switch strings.ToLower(parts[1]) {
+	case board.White.String():
+		turn = board.White
+	case board.Black.String():
+		turn = board.Black
+	default:
+		return nil, fmt.Errorf("unable to parse turn: %s", parts[2])
+	}
+
+	position = parts[0]
+	expandedFEN = expandFEN(position)
+	parts = strings.Split(expandedFEN, "/")
+	if len(parts) != 8 {
+		return nil, fmt.Errorf("unable to parse position: %s", position)
+	}
+	for r := len(parts) - 1; r >= 0; r-- {
+		for f := 0; f < 8; f++ {
+			square := board.NewSquare(board.File(f), board.Rank(7-r))
+			piece := board.NewPieceByFEN(rune(parts[r][f]))
+			if piece != board.NoPiece {
+				pieces[square] = piece
+			}
+		}
+	}
+
+	b, err := board.NewBoard(pieces, turn, *cr, ep, halfMove, fullMove)
+	if err != nil {
+		return nil, err
+	}
+	return b, nil
+}
+
+func expandFEN(s string) string {
+	out := ""
+	for _, c := range s {
+		switch c {
+		case 'K', 'Q', 'R', 'B', 'N', 'P', 'k', 'q', 'r', 'b', 'n', 'p', '/':
+			out += string(c)
+		case '1':
+			out += "1"
+		case '2':
+			out += "11"
+		case '3':
+			out += "111"
+		case '4':
+			out += "1111"
+		case '5':
+			out += "11111"
+		case '6':
+			out += "111111"
+		case '7':
+			out += "1111111"
+		case '8':
+			out += "11111111"
+		}
+	}
+	return out
+}
+
+// Export returns the FEN string of the given *board.Board{}.
+func Export(b *board.Board) string {
+	ep := "-"
+	if b.EnPassent != nil {
+		ep = b.EnPassent.String()
+	}
+	return fmt.Sprintf(
+		"%s %s %s %s %d %d",
+		b.String(),
+		b.Turn.String(),
+		b.CastleRights.String(),
+		ep,
+		b.HalfMove,
+		b.FullMove,
+	)
+}