Merge pull request #67 from Belzedar94/codex/fix-legal-move-detection-for-battlekings

Fix Battlekings gating legality on captures

Author: Belzedar94

src/position.cpp

@@ -1274,7 +1274,16 @@ bool Position::legal(Move m) const {
       Bitboard occ = occupied | gating_square(m);
       if (type_of(m) == EN_PASSANT)
           occ ^= capture_square(to);
-      if (attackers_to(gating_square(m), occ, ~us))
+
+      Bitboard attackers = attackers_to(gating_square(m), occ, ~us);
+
+      // Ignore attacks from enemy pieces that will be removed as part of the move
+      if (capture(m) && piece_on(to) != NO_PIECE)
+          attackers &= ~SquareBB[to];
+      if (type_of(m) == EN_PASSANT)
+          attackers &= ~SquareBB[capture_square(to)];
+
+      if (attackers)
           return false;
   }
 

test.py

@@ -435,7 +435,7 @@ def test_chess_promotion_does_not_gate(self):
         self.assertEqual(fen, "Q7/8/8/8/8/8/7p/7K b - - 0 1")
 
     def test_battlekings_king_spawn_blocked(self):
-        fen = "8/8/8/8/8/3p4/4Q3/8 w - - 0 1"
+        fen = "8/8/8/8/8/4r3/4Q3/4r3 w - - 0 1"
         moves = sf.legal_moves("battlekings", fen, [])
         self.assertFalse(moves)
 
@@ -466,6 +466,11 @@ def test_battlekings_first_commoner_capture_with_multiple(self):
         fen = "3qk3/pppp1ppp/2nkr3/4p1b1/P1N5/NPPP3P/NBNNPPPN/8 w - - 3 8"
         self._check_immediate_game_end("battlekings", fen, ["c4d6"], True, -sf.VALUE_MATE)
 
+    def test_battlekings_gating_capture_allows_commoner_spawn(self):
+        fen = "1Q1QRQ2/BqqqQQQQ/R2Rqq1Q/QQQQ2qQ/qB1QRRq1/rq1q1BNQ/qQQQNRNP/1R6 b - - 0 57"
+        moves = sf.legal_moves("battlekings", fen, [])
+        self.assertIn("a2b2", moves)
+
     def test_legal_moves(self):
         fen = "10/10/10/10/10/k9/10/K9 w - - 0 1"
         result = sf.legal_moves("capablanca", fen, [])