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Fix x-ray attack from behind in evaluate_passed_pawns()

Fix a condition for x-ray attack of a queen or
a rook behind a pawn of us. Previous condition does
not check if the enemy slider behind our pawn is
really attacking the pawn.

Signed-off-by: Marco Costalba <>
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1 parent ab4d26f commit 2655f93c322c69c9ad85bd4be69d545c05e9260a @mcostalba committed Oct 11, 2009
Showing with 2 additions and 2 deletions.
  1. +2 −2 src/evaluate.cpp
@@ -940,8 +940,8 @@ namespace {
// If there is an enemy rook or queen attacking the pawn from behind,
// add all X-ray attacks by the rook or queen.
- if ( bit_is_set(ei.attacked_by(Them, ROOK) | ei.attacked_by(Them, QUEEN), s)
- && (squares_behind(Us, s) & pos.pieces(ROOK, QUEEN, Them)))
+ 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?

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