Do not lose info about invalid castling rights

Author: niklasf

chess/__init__.py

@@ -1889,17 +1889,27 @@ def set_fen(self, fen):
             flag = flag.lower()
             backrank = BB_RANK_1 if color == WHITE else BB_RANK_8
             rooks = self.occupied_co[color] & self.rooks & backrank
+            king = self.occupied_co[color] & self.kings & backrank
 
             if flag == "q":
                 # Select the leftmost rook.
-                self.castling_rights |= (rooks & -rooks)
+                mask = rooks & -rooks
+
+                if king and bit_scan(mask) < bit_scan(king):
+                    self.castling_rights |= mask
+                else:
+                    self.castling_rights |= BB_FILE_A & backrank
             elif flag == "k":
                 # Select the rightmost rook.
                 mask = BB_VOID
                 while rooks:
-                    mask = (rooks & -rooks)
+                    mask = rooks & -rooks
                     rooks = rooks & (rooks - 1)
-                self.castling_rights |= mask
+
+                if king and bit_scan(king) < bit_scan(mask):
+                    self.castling_rights |= mask
+                else:
+                    self.castling_rights |= BB_FILE_H & backrank
             else:
                 self.castling_rights |= BB_FILES[FILE_NAMES.index(flag)] & backrank
 

test.py

@@ -333,6 +333,10 @@ def test_invalid_castling_rights(self):
         self.assertEqual(board.status(), chess.STATUS_BAD_CASTLING_RIGHTS)
         self.assertEqual(board.fen(), "4k2r/8/8/8/8/8/8/R1K5 w k - 0 1")
 
+        board = chess.Board("1r2k3/8/1p6/8/8/5P2/8/1R2KR2 w KQkq - 0 1", chess960=True)
+        self.assertEqual(board.status(), chess.STATUS_BAD_CASTLING_RIGHTS)
+        self.assertEqual(board.fen(), "1r2k3/8/1p6/8/8/5P2/8/1R2KR2 w KQq - 0 1")
+
     def test_insufficient_material(self):
         # Starting position.
         board = chess.Board()