Detect impossible check due to en passant

Author: niklasf

chess/__init__.py

@@ -3356,12 +3356,21 @@ def status(self) -> Status:
         # More than the maximum number of possible checkers in the variant.
         checkers = self.checkers_mask()
         our_kings = self.kings & self.occupied_co[self.turn] & ~self.promoted
-        if popcount(checkers) > 2:
-            errors |= STATUS_TOO_MANY_CHECKERS
-        elif popcount(checkers) == 2 and ray(lsb(checkers), msb(checkers)) & our_kings:
-            errors |= STATUS_IMPOSSIBLE_CHECK
-        elif valid_ep_square is not None and any(ray(checker, valid_ep_square) & our_kings for checker in scan_reversed(checkers)):
-            errors |= STATUS_IMPOSSIBLE_CHECK
+        if checkers:
+            if popcount(checkers) > 2:
+                errors |= STATUS_TOO_MANY_CHECKERS
+
+            if valid_ep_square is not None:
+                pushed_to = valid_ep_square ^ A2
+                pushed_from = valid_ep_square ^ A4
+                occupied_before = (self.occupied & ~BB_SQUARES[pushed_to]) | BB_SQUARES[pushed_from]
+                if popcount(checkers) > 1 or (
+                        msb(checkers) != pushed_to and
+                        self._attacked_for_king(our_kings, occupied_before)):
+                    errors |= STATUS_IMPOSSIBLE_CHECK
+            else:
+                if popcount(checkers) > 2 or (popcount(checkers) == 2 and ray(lsb(checkers), msb(checkers)) & our_kings):
+                    errors |= STATUS_IMPOSSIBLE_CHECK
 
         return errors
 

chess/variant.py

@@ -320,7 +320,8 @@ def status(self) -> chess.Status:
             status &= ~chess.STATUS_NO_BLACK_KING
         if chess.popcount(self.checkers_mask()) <= 14:
             status &= ~chess.STATUS_TOO_MANY_CHECKERS
-        status &= ~chess.STATUS_IMPOSSIBLE_CHECK
+        if self._valid_ep_square() is None:
+            status &= ~chess.STATUS_IMPOSSIBLE_CHECK
         return status
 
 
@@ -409,7 +410,7 @@ def has_insufficient_material(self, color: chess.Color) -> bool:
     def status(self) -> chess.Status:
         status = super().status()
         if self.is_check():
-            status |= chess.STATUS_RACE_CHECK | chess.STATUS_TOO_MANY_CHECKERS
+            status |= chess.STATUS_RACE_CHECK | chess.STATUS_TOO_MANY_CHECKERS | chess.STATUS_IMPOSSIBLE_CHECK
         if self.turn == chess.BLACK and all(self.occupied_co[co] & self.kings & chess.BB_RANK_8 for co in chess.COLORS):
             status |= chess.STATUS_RACE_OVER
         if self.pawns:

test.py

@@ -994,7 +994,7 @@ def test_status(self):
 
         # Triple check.
         board = chess.Board("4k3/5P2/3N4/8/8/8/4R3/4K3 b - - 0 1")
-        self.assertEqual(board.status(), chess.STATUS_TOO_MANY_CHECKERS)
+        self.assertEqual(board.status(), chess.STATUS_TOO_MANY_CHECKERS | chess.STATUS_IMPOSSIBLE_CHECK)
 
         # Impossible checker alignment.
         board = chess.Board("3R4/8/q4k2/2B5/1NK5/3b4/8/8 w - - 0 1")
@@ -1675,6 +1675,14 @@ def test_king_captures_unmoved_rook(self):
         board.push(move)
         self.assertEqual(board.fen(), "8/8/8/B2p3Q/2qPp1P1/b7/2P2P1P/4K2k w - - 0 2")
 
+    def test_impossible_check_due_to_en_passant(self):
+        board = chess.Board("rnbqk1nr/bb3p1p/1q2r3/2pPp3/3P4/7P/1PP1NpPP/R1BQKBNR w KQkq c6")
+        self.assertEqual(board.status(), chess.STATUS_IMPOSSIBLE_CHECK)
+        self.assertEqual(board.ep_square, chess.C6)
+        self.assertTrue(board.has_pseudo_legal_en_passant())
+        self.assertFalse(board.has_legal_en_passant())
+        self.assertEqual(len(list(board.legal_moves)), 2)
+
 
 class LegalMoveGeneratorTestCase(unittest.TestCase):
 
@@ -4279,7 +4287,7 @@ def test_legal_moves_after_end(self):
     def test_racing_kings_status_with_check(self):
         board = chess.variant.RacingKingsBoard("8/8/8/8/R7/8/krbnNB1K/qrbnNBRQ b - - 1 1")
         self.assertFalse(board.is_valid())
-        self.assertEqual(board.status(), chess.STATUS_RACE_CHECK | chess.STATUS_TOO_MANY_CHECKERS)
+        self.assertEqual(board.status(), chess.STATUS_RACE_CHECK | chess.STATUS_TOO_MANY_CHECKERS | chess.STATUS_IMPOSSIBLE_CHECK)
 
 
 class HordeTestCase(unittest.TestCase):