feat: Refactor king movement logic to improve handling of forbidden squares during captures

Author: miskibin

draughts/boards/standard.py

@@ -177,11 +177,8 @@ def _get_king_legal_moves_from(
         Args:
             square: The current square of the king
             is_capture_mandatory: Whether we're in a capture sequence
-            forbidden_squares: Squares of previously captured pieces that cannot be crossed
+            forbidden_squares: Mutable set of previously captured piece squares (modified in-place)
         """
-        if forbidden_squares is None:
-            forbidden_squares = set()
-        
         moves = []
         pos = self._pos  # Local reference for faster access
         turn_val = self.turn.value
@@ -191,7 +188,7 @@ def _get_king_legal_moves_from(
             dir_len = len(direction)
             for idx, target in enumerate(direction):
                 # Check if path crosses a forbidden square (previously captured piece)
-                if target in forbidden_squares:
+                if forbidden_squares and target in forbidden_squares:
                     break
 
                 target_val = pos[target]
@@ -203,17 +200,23 @@ def _get_king_legal_moves_from(
                     i = idx + 1
                     while i < dir_len and pos[direction[i]] == EMPTY:
                         # Check landing square is not forbidden
-                        if direction[i] in forbidden_squares:
+                        if forbidden_squares and direction[i] in forbidden_squares:
                             i += 1
                             continue
 
                         move = Move(
                             [square, direction[i]], [target], [target_val]
                         )
                         self.push(move, False)
-                        # Add the captured square to forbidden set for subsequent captures
-                        new_forbidden = forbidden_squares | {target}
-                        sub_moves = self._get_king_legal_moves_from(direction[i], True, new_forbidden)
+                        # Add captured square to forbidden set (backtracking pattern)
+                        if forbidden_squares is None:
+                            forbidden_squares = {target}
+                            sub_moves = self._get_king_legal_moves_from(direction[i], True, forbidden_squares)
+                            forbidden_squares = None
+                        else:
+                            forbidden_squares.add(target)
+                            sub_moves = self._get_king_legal_moves_from(direction[i], True, forbidden_squares)
+                            forbidden_squares.discard(target)
                         self.pop(False)
 
                         if sub_moves: