+ // There are no enemy pawns in the pawn's path
+ Bitboard b2 = squares_in_front_of(Us, s);
+
+ assert((b2 & pos.pieces(PAWN, Them)) == EmptyBoardBB);
+
+ // Squares attacked by us
+ Bitboard b4 = b2 & ei.attacked_by(Us);
+
+ // Squares attacked or occupied by enemy pieces
+ Bitboard b3 = b2 & (ei.attacked_by(Them) | pos.pieces_of_color(Them));
+
+ // If there is an enemy rook or queen attacking the pawn from behind,
+ // add all X-ray attacks by the rook or queen.
+ if ( (squares_behind(Us, s) & pos.pieces(ROOK, QUEEN, Them))
+ && (squares_behind(Us, s) & pos.pieces(ROOK, QUEEN, Them) & pos.attacks_from<QUEEN>(s)))
+ b3 = b2;
+
+ // Are any of the squares in the pawn's path attacked or occupied by the enemy?
+ if (b3 == EmptyBoardBB)
+ // No enemy attacks or pieces, huge bonus!
+ // Even bigger if we protect the pawn's path
+ ebonus += Value(tr * (b2 == b4 ? 17 : 15));
+ else
+ // OK, there are enemy attacks or pieces (but not pawns). Are those
+ // squares which are attacked by the enemy also attacked by us ?
+ // If yes, big bonus (but smaller than when there are no enemy attacks),
+ // if no, somewhat smaller bonus.
+ ebonus += Value(tr * ((b3 & b4) == b3 ? 13 : 8));
+
+ // At last, add a small bonus when there are no *friendly* pieces
+ // in the pawn's path.
+ if ((b2 & pos.pieces_of_color(Us)) == EmptyBoardBB)
+ ebonus += Value(tr);
+ }
+ } // tr != 0
+
+ // If the pawn is supported by a friendly pawn, increase bonus
+ Bitboard b1 = pos.pieces(PAWN, Us) & neighboring_files_bb(s);
+ if (b1 & rank_bb(s))
+ ebonus += Value(r * 20);
+ else if (pos.attacks_from<PAWN>(s, Them) & b1)
+ ebonus += Value(r * 12);
+
+ // Rook pawns are a special case: They are sometimes worse, and
+ // sometimes better than other passed pawns. It is difficult to find
+ // good rules for determining whether they are good or bad. For now,
+ // we try the following: Increase the value for rook pawns if the
+ // other side has no pieces apart from a knight, and decrease the
+ // value if the other side has a rook or queen.
+ if (square_file(s) == FILE_A || square_file(s) == FILE_H)
+ {
+ if ( pos.non_pawn_material(Them) <= KnightValueMidgame
+ && pos.piece_count(Them, KNIGHT) <= 1)
+ ebonus += ebonus / 4;
+ else if (pos.pieces(ROOK, QUEEN, Them))
+ ebonus -= ebonus / 4;
+ }