// be detected even when the weaker side has some pawns.
Bitboard pawns = pos.pieces(strongSide, PAWN);
- File pawnFile = file_of(pos.list<PAWN>(strongSide)[0]);
+ File pawnsFile = file_of(lsb(pawns));
// All pawns are on a single rook file ?
- if ( (pawnFile == FILE_A || pawnFile == FILE_H)
- && !(pawns & ~file_bb(pawnFile)))
+ if ( (pawnsFile == FILE_A || pawnsFile == FILE_H)
+ && !(pawns & ~file_bb(pawnsFile)))
{
Square bishopSq = pos.list<BISHOP>(strongSide)[0];
- Square queeningSq = relative_square(strongSide, make_square(pawnFile, RANK_8));
+ Square queeningSq = relative_square(strongSide, make_square(pawnsFile, RANK_8));
Square kingSq = pos.king_square(weakSide);
+ // If the bishop has the wrong color, and the defending king is on the file
+ // of the pawn(s) or the neighboring file, then it's potentially a draw.
if ( opposite_colors(queeningSq, bishopSq)
- && square_distance(queeningSq, kingSq) <= 1)
- return SCALE_FACTOR_DRAW;
- }
-
- // If all the pawns are on the same B or G file, then it's potentially a draw
- if ( (pawnFile == FILE_B || pawnFile == FILE_G)
- && !(pos.pieces(PAWN) & ~file_bb(pawnFile))
- && pos.non_pawn_material(weakSide) == 0
- && pos.count<PAWN>(weakSide) >= 1)
- {
- // Get weakSide pawn that is closest to the home rank
- Square weakPawnSq = backmost_sq(weakSide, pos.pieces(weakSide, PAWN));
-
- Square strongKingSq = pos.king_square(strongSide);
- Square weakKingSq = pos.king_square(weakSide);
- Square bishopSq = pos.list<BISHOP>(strongSide)[0];
-
- // There's potential for a draw if our pawn is blocked on the 7th rank,
- // the bishop cannot attack it or they only have one pawn left
- if ( relative_rank(strongSide, weakPawnSq) == RANK_7
- && (pos.pieces(strongSide, PAWN) & (weakPawnSq + pawn_push(weakSide)))
- && (opposite_colors(bishopSq, weakPawnSq) || pos.count<PAWN>(strongSide) == 1))
+ && file_distance(kingSq, lsb(pawns)) <= 1)
{
- int strongKingDist = square_distance(weakPawnSq, strongKingSq);
- int weakKingDist = square_distance(weakPawnSq, weakKingSq);
-
- // It's a draw if the weak king is on its back two ranks, within 2
- // squares of the blocking pawn and the strong king is not
- // closer. (I think this rule only fails in practically
- // unreachable positions such as 5k1K/6p1/6P1/8/8/3B4/8/8 w
- // and positions where qsearch will immediately correct the
- // problem such as 8/4k1p1/6P1/1K6/3B4/8/8/8 w)
- if ( relative_rank(strongSide, weakKingSq) >= RANK_7
- && weakKingDist <= 2
- && weakKingDist <= strongKingDist)
+ // If the defending king has distance <= 1 to the promotion square or
+ // is placed somewhere in front of the frontmost pawn, it's a draw.
+ Rank rank = relative_rank(strongSide, (frontmost_sq(strongSide, pawns)));
+
+ if ( square_distance(kingSq, queeningSq) <= 1
+ || relative_rank(strongSide, kingSq) >= rank)
return SCALE_FACTOR_DRAW;
}
}