Simplify and speed up previous patch

[stockfish] / src / position.cpp

diff --git a/src/position.cpp b/src/position.cpp
index ad699cc0e93423ef4112093b442da41c0466cb72..5193d20e85c89c8207935559d3a0d60d6ab3e780 100644 (file)
--- a/src/position.cpp
+++ b/src/position.cpp
@@ -1129,10 +1129,10 @@ void Position::undo_null_move() {
 
 
 /// Position::see() is a static exchange evaluator: It tries to estimate the
-/// material gain or loss resulting from a move. There are three versions of
-/// this function: One which takes a destination square as input, one takes a
-/// move, and one which takes a 'from' and a 'to' square. The function does
-/// not yet understand promotions captures.
+/// material gain or loss resulting from a move. Parameter 'asymmThreshold' takes
+/// tempi into account. If the side who initiated the capturing sequence does the
+/// last capture, he loses a tempo and if the result is below 'asymmThreshold'
+/// the capturing sequence is considered bad.
 
 int Position::see_sign(Move m) const {
 
@@ -1147,7 +1147,7 @@ int Position::see_sign(Move m) const {
   return see(m);
 }
 
-int Position::see(Move m) const {
+int Position::see(Move m, int asymmThreshold) const {
 
   Square from, to;
   Bitboard occupied, attackers, stmAttackers;
@@ -1224,6 +1224,15 @@ int Position::see(Move m) const {
 
   } while (stmAttackers);
 
+  // If we are doing asymmetric SEE evaluation and the same side does the first
+  // and the last capture, he loses a tempo and gain must be at least worth
+  // 'asymmThreshold', otherwise we replace the score with a very low value,
+  // before negamaxing.
+  if (asymmThreshold)
+      for (int i = 0; i < slIndex; i += 2)
+          if (swapList[i] < asymmThreshold)
+              swapList[i] = - QueenValueMg * 16;
+
   // Having built the swap list, we negamax through it to find the best
   // achievable score from the point of view of the side to move.
   while (--slIndex)