Add position.XFENString() (#126)

When working with 3rd party chess APIs these may require positions to
be expressed in X-FEN notation rather than FEN. e.g. Notably while
some of the lichess.org APIs work with either FEN or X-FEN, some of
them require X-FEN. This commit adds position.XFENString() for this
purpose. The key difference between X-FEN and FEN is the encoding of
the en passant square. X-FEN will only specify it when an opposing
pawn is in position to capture, while FEN will always specify it.

fen_test.go

@@ -17,6 +17,30 @@ var (
 		"3r1rk1/p3qppp/2bb4/2p5/3p4/1P2P3/PBQN1PPP/2R2RK1 w - - 0 1",
 		"4r1k1/1b3p1p/ppq3p1/2p5/8/1P3R1Q/PBP3PP/7K w - - 0 1",
 		"5k2/ppp5/4P3/3R3p/6P1/1K2Nr2/PP3P2/8 b - - 1 32",
+		"rnbqkbnr/pp1ppppp/8/8/1Pp1PP2/8/P1PP2PP/RNBQKBNR b KQkq b3 0 3",
+		"rnbqkbnr/p1ppppp1/7p/Pp6/8/8/1PPPPPPP/RNBQKBNR w KQkq b6 0 3",
+		"rnbqkbnr/1pppppp1/7p/pP6/8/8/P1PPPPPP/RNBQKBNR w KQkq a6 0 3",
+		"rnbqkbnr/1pppppp1/7p/pP6/4P3/8/P1PP1PPP/RNBQKBNR b KQkq e3 0 3",
+	}
+
+	validXFENs = []string{
+		"rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1",
+		"rnbqkbnr/pppppppp/8/8/4P3/8/PPPP1PPP/RNBQKBNR b KQkq - 0 1",
+		"rnbqkbnr/pp1ppppp/8/2p5/4P3/8/PPPP1PPP/RNBQKBNR w KQkq - 0 2",
+		"rnbqkbnr/pp1ppppp/8/2p5/4P3/5N2/PPPP1PPP/RNBQKB1R b KQkq - 1 2",
+		"7k/8/8/8/8/8/8/R6K w - - 0 1",
+		"7k/8/8/8/8/8/8/2B1KB2 w - - 0 1",
+		"8/8/8/4k3/8/8/8/R3K2R w KQ - 0 1",
+		"8/8/8/8/4k3/8/3KP3/8 w - - 0 1",
+		"8/8/5k2/8/5K2/8/4P3/8 w - - 0 1",
+		"r4rk1/1b2bppp/ppq1p3/2pp3n/5P2/1P1BP3/PBPPQ1PP/R4RK1 w - - 0 1",
+		"3r1rk1/p3qppp/2bb4/2p5/3p4/1P2P3/PBQN1PPP/2R2RK1 w - - 0 1",
+		"4r1k1/1b3p1p/ppq3p1/2p5/8/1P3R1Q/PBP3PP/7K w - - 0 1",
+		"5k2/ppp5/4P3/3R3p/6P1/1K2Nr2/PP3P2/8 b - - 1 32",
+		"rnbqkbnr/pp1ppppp/8/8/1Pp1PP2/8/P1PP2PP/RNBQKBNR b KQkq b3 0 3",
+		"rnbqkbnr/p1ppppp1/7p/Pp6/8/8/1PPPPPPP/RNBQKBNR w KQkq b6 0 3",
+		"rnbqkbnr/1pppppp1/7p/pP6/8/8/P1PPPPPP/RNBQKBNR w KQkq a6 0 3",
+		"rnbqkbnr/1pppppp1/7p/pP6/4P3/8/P1PP1PPP/RNBQKBNR b KQkq - 0 3",
 	}
 
 	invalidFENs = []string{
@@ -34,14 +58,19 @@ var (
 )
 
 func TestValidFENs(t *testing.T) {
-	for _, f := range validFENs {
+	for idx, f := range validFENs {
 		state, err := decodeFEN(f)
 		if err != nil {
 			t.Fatal("recieved unexpected error", err)
 		}
 		if f != state.String() {
 			t.Fatalf("fen expected board string %s but got %s", f, state.String())
 		}
+		xfen := state.XFENString()
+		if xfen != validXFENs[idx] {
+			t.Fatalf("xfen for fen %v (%v) was %v but expected %v", idx, f,
+				xfen, validXFENs[idx])
+		}
 	}
 }
 

position.go

@@ -148,6 +148,47 @@ func (pos *Position) String() string {
 	return fmt.Sprintf("%s %s %s %s %d %d", b, t, c, sq, pos.halfMoveClock, pos.moveCount)
 }
 
+// XFENString() is similar to String() except that it returns a string with
+// the X-FEN format
+func (pos *Position) XFENString() string {
+	b := pos.board.String()
+	t := pos.turn.String()
+	c := pos.castleRights.String()
+	sq := "-"
+	if pos.enPassantSquare != NoSquare {
+		// Check if there is a pawn in a position to capture en passant
+		var rank Rank
+		if pos.turn == White {
+			rank = Rank5
+		} else {
+			rank = Rank4
+		}
+		// The en passant target square will always be on the rank opposite the current turn's pawns
+		file := pos.enPassantSquare.File()
+		potentialPawnFiles := []File{file - 1, file + 1} // Pawns that could capture en passant will be on an adjacent file
+
+		for _, f := range potentialPawnFiles {
+			if f < FileA || f > FileH { // Ensure file is within bounds
+				continue
+			}
+
+			potentialPawnSquare := NewSquare(f, rank)
+			potentialPawn := pos.board.Piece(potentialPawnSquare)
+			if potentialPawn == NoPiece {
+				continue
+			}
+			if potentialPawn.Type() != Pawn {
+				continue
+			}
+			if potentialPawn.Color() == pos.turn {
+				sq = pos.enPassantSquare.String()
+				break
+			}
+		}
+	}
+	return fmt.Sprintf("%s %s %s %s %d %d", b, t, c, sq, pos.halfMoveClock, pos.moveCount)
+}
+
 // Hash returns a unique hash of the position
 func (pos *Position) Hash() [16]byte {
 	b, _ := pos.MarshalBinary()