+ // After castling, the rook and king final positions are the same in Chess960
+ // as they would be in standard chess.
+ Square kfrom = pos.king_square(us);
+ Square rfrom = pos.castle_rook_square(CR[us]);
+ Square kto = relative_square(us, Side == KING_SIDE ? SQ_G1 : SQ_C1);
+ Square rto = relative_square(us, Side == KING_SIDE ? SQ_F1 : SQ_D1);
+ Bitboard enemies = pos.pieces(flip(us));
+
+ assert(!pos.in_check());
+ assert(pos.piece_on(kfrom) == make_piece(us, KING));
+ assert(pos.piece_on(rfrom) == make_piece(us, ROOK));
+
+ // Unimpeded rule: All the squares between the king's initial and final squares
+ // (including the final square), and all the squares between the rook's initial
+ // and final squares (including the final square), must be vacant except for
+ // the king and castling rook.
+ for (Square s = std::min(rfrom, rto), e = std::max(rfrom, rto); s <= e; s++)
+ if (s != kfrom && s != rfrom && !pos.square_is_empty(s))
+ return mlist;
+
+ for (Square s = std::min(kfrom, kto), e = std::max(kfrom, kto); s <= e; s++)
+ if ( (s != kfrom && s != rfrom && !pos.square_is_empty(s))
+ ||(pos.attackers_to(s) & enemies))
+ return mlist;
+
+ // Because we generate only legal castling moves we need to verify that
+ // when moving the castling rook we do not discover some hidden checker.
+ // For instance an enemy queen in SQ_A1 when castling rook is in SQ_B1.
+ if (pos.is_chess960())
+ {
+ Bitboard occ = pos.occupied_squares();
+ clear_bit(&occ, rfrom);
+ if (pos.attackers_to(kto, occ) & enemies)
+ return mlist;
+ }