Allow fifty-move claim ahead of a checkmating or stalemating move

can_claim_fifty_moves() pushed each non-zeroing legal move and tested the
result with is_fifty_moves(), which is False when the resulting position has
no legal moves. A move that completes the 100 halfmoves but also gives
checkmate or stalemate was therefore rejected.

Under FIDE 9.3 the claim is made ahead of the move: the player only declares
a non-pawn, non-capture move that completes the count; it is not played, so
its being game-ending does not invalidate the claim. With the clock at 99 any
non-zeroing legal move completes the count, so the push/is_fifty_moves() test
becomes a direct check. Positions that are already checkmate or stalemate
still return False (no legal moves to iterate).

Closes #1188.

Author: Booyaka101

chess/__init__.py

@@ -2297,14 +2297,11 @@ def can_claim_fifty_moves(self) -> bool:
             return True
 
         if self.halfmove_clock >= 99:
+            # The claim can also be made ahead of a move that would itself give
+            # checkmate or stalemate (FIDE 9.3): it is declared, not played.
             for move in self.generate_legal_moves():
                 if not self.is_zeroing(move):
-                    self.push(move)
-                    try:
-                        if self.is_fifty_moves():
-                            return True
-                    finally:
-                        self.pop()
+                    return True
 
         return False
 

test.py

@@ -1376,6 +1376,15 @@ def test_fifty_moves(self):
         self.assertFalse(board.can_claim_fifty_moves())
         self.assertFalse(board.can_claim_draw())
 
+        # Claiming ahead of a move that would itself give checkmate is valid
+        # (FIDE 9.3): the move is only declared, not played.
+        board = chess.Board("6rk/6pp/7N/5p2/6p1/8/2q5/K7 w - - 99 60")
+        self.assertFalse(board.is_game_over())
+        self.assertTrue(board.gives_checkmate(chess.Move.from_uci("h6f7")))
+        self.assertFalse(board.is_fifty_moves())
+        self.assertTrue(board.can_claim_fifty_moves())
+        self.assertTrue(board.can_claim_draw())
+
     def test_promoted_comparison(self):
         board = chess.Board()
         board.set_fen("5R2/3P4/8/8/7r/7r/7k/K7 w - - 0 1")