Change pinning logic in Static Exchange Evaluation (SEE)

[stockfish] / src / position.cpp

diff --git a/src/position.cpp b/src/position.cpp
index 21eff88c46898b46ca5e4fb86578f45bb895d8aa..812eabcaa5f625a786792be4d5cf0c09f7214087 100644 (file)
--- a/src/position.cpp
+++ b/src/position.cpp
@@ -498,14 +498,15 @@ Bitboard Position::slider_blockers(Bitboard sliders, Square s, Bitboard& pinners
   Bitboard blockers = 0;
   pinners = 0;
 
-  // Snipers are sliders that attack 's' when a piece is removed
+  // Snipers are sliders that attack 's' when a piece and other snipers are removed
   Bitboard snipers = (  (PseudoAttacks[  ROOK][s] & pieces(QUEEN, ROOK))
                       | (PseudoAttacks[BISHOP][s] & pieces(QUEEN, BISHOP))) & sliders;
+  Bitboard occupancy = pieces() & ~snipers;
 
   while (snipers)
   {
     Square sniperSq = pop_lsb(&snipers);
-    Bitboard b = between_bb(s, sniperSq) & pieces();
+    Bitboard b = between_bb(s, sniperSq) & occupancy;
 
     if (b && !more_than_one(b))
     {
@@ -1076,8 +1077,8 @@ bool Position::see_ge(Move m, Value threshold) const {
       stmAttackers = attackers & pieces(stm);
 
       // Don't allow pinned pieces to attack (except the king) as long as
-      // all pinners are on their original square.
-      if (!(st->pinners[~stm] & ~occupied))
+      // any pinners are on their original square.
+      if (st->pinners[~stm] & occupied)
           stmAttackers &= ~st->blockersForKing[stm];
 
       // If stm has no more attackers then give up: stm loses