Update fivefold repetition rule (fixes niklasf#295)

niklasf · niklasf · commit 63ec55b83bb1 · 2018-07-03T10:15:43.000+02:00
diff --git a/chess/__init__.py b/chess/__init__.py
@@ -1731,32 +1731,28 @@ def is_seventyfive_moves(self):
     def is_fivefold_repetition(self):
         """
         Since the 1st of July 2014 a game is automatically drawn (without
-        a claim by one of the players) if a position occurs for the fifth time
-        on consecutive alternating moves.
+        a claim by one of the players) if a position occurs for the fifth time.
+        Originally this had to occur on consecutive alternating moves, but
+        this has since been revised.
         """
         transposition_key = self._transposition_key()
-
-        if len(self.move_stack) < 16:
-            return False
-
+        repetitions = 1
         switchyard = collections.deque()
 
-        for _ in range(4):
-            # Go back two full moves each.
-            for _ in range(4):
-                switchyard.append(self.pop())
+        while self.move_stack and repetitions < 5:
+            move = self.pop()
+            switchyard.append(move)
 
-            # Check if the position was the same as before.
-            if self._transposition_key() != transposition_key:
-                while switchyard:
-                    self.push(switchyard.pop())
+            if self.is_irreversible(move):
+                break
 
-                return False
+            if self._transposition_key() == transposition_key:
+                repetitions += 1
 
         while switchyard:
             self.push(switchyard.pop())
 
-        return True
+        return repetitions >= 5
 
     def can_claim_draw(self):
         """
diff --git a/test.py b/test.py
@@ -1089,10 +1089,10 @@ def test_fivefold_repetition(self):
         self.assertFalse(board.is_fivefold_repetition())
         board.push_san("Qd8")
 
-        # This is a threefold repetition but not a fivefold repetition, because
-        # consecutive moves are required for that.
+        # This is a threefold repetition, and also a fivefold repetition since
+        # it no longer has to occur on consecutive moves.
         self.assertTrue(board.can_claim_threefold_repetition())
-        self.assertFalse(board.is_fivefold_repetition())
+        self.assertTrue(board.is_fivefold_repetition())
         self.assertEqual(board.fen().split()[0], fen.split()[0])
 
     def test_fifty_moves(self):
