Document asymmetric SEE pruning trick

[stockfish] / src / search.cpp

diff --git a/src/search.cpp b/src/search.cpp
index 40a887c27821654b7f26cb2f2a0bb3131080a1db..5abbafbcbfcd8b756dbf481efdffff9187e78016 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -440,6 +440,7 @@ namespace {
             if (    depth >= 12
                 && !stop
                 &&  PVSize == 1
+                &&  bestValue > VALUE_MATED_IN_MAX_PLY
                 && (   RootMoves.size() == 1
                     || Time::now() - SearchTime > (TimeMgr.available_time() * 20) / 100))
             {
@@ -678,7 +679,7 @@ namespace {
             if (nullValue >= VALUE_MATE_IN_MAX_PLY)
                 nullValue = beta;
 
-            if (depth < 6 * ONE_PLY)
+            if (depth < 12 * ONE_PLY)
                 return nullValue;
 
             // Do verification search at high depths
@@ -745,7 +746,7 @@ namespace {
         && ttMove == MOVE_NONE
         && (PvNode || (!inCheck && ss->staticEval + Value(256) >= beta)))
     {
-        Depth d = (PvNode ? depth - 2 * ONE_PLY : depth / 2);
+        Depth d = depth - 2 * ONE_PLY - (PvNode ? DEPTH_ZERO : depth / 4);
 
         ss->skipNullMove = true;
         search<PvNode ? PV : NonPV>(pos, ss, alpha, beta, d);
@@ -856,7 +857,8 @@ split_point_start: // At split points actual search starts from here
           && !captureOrPromotion
           && !inCheck
           && !dangerous
-          &&  move != ttMove)
+          &&  move != ttMove
+          &&  bestValue > VALUE_MATED_IN_MAX_PLY)
       {
           // Move count based pruning
           if (   depth < 16 * ONE_PLY
@@ -878,14 +880,19 @@ split_point_start: // At split points actual search starts from here
 
           if (futilityValue < beta)
           {
+              bestValue = std::max(bestValue, futilityValue);
+
               if (SpNode)
+              {
                   splitPoint->mutex.lock();
-
+                  if (bestValue > splitPoint->bestValue)
+                      splitPoint->bestValue = bestValue;
+              }
               continue;
           }
 
           // Prune moves with negative SEE at low depths
-          if (   predictedDepth < 3 * ONE_PLY
+          if (   predictedDepth < 4 * ONE_PLY
               && pos.see_sign(move) < 0)
           {
               if (SpNode)
@@ -1125,6 +1132,12 @@ split_point_start: // At split points actual search starts from here
     if (pos.is_draw<true>() || ss->ply > MAX_PLY)
         return DrawValue[pos.side_to_move()];
 
+    // Decide whether or not to include checks, this fixes also the type of
+    // TT entry depth that we are going to use. Note that in qsearch we use
+    // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
+    ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
+                                                  : DEPTH_QS_NO_CHECKS;
+
     // Transposition table lookup. At PV nodes, we don't use the TT for
     // pruning, but only for move ordering.
     posKey = pos.key();
@@ -1132,11 +1145,6 @@ split_point_start: // At split points actual search starts from here
     ttMove = tte ? tte->move() : MOVE_NONE;
     ttValue = tte ? value_from_tt(tte->value(),ss->ply) : VALUE_NONE;
 
-    // Decide whether or not to include checks, this fixes also the type of
-    // TT entry depth that we are going to use. Note that in qsearch we use
-    // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
-    ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
-                                                  : DEPTH_QS_NO_CHECKS;
     if (   tte
         && tte->depth() >= ttDepth
         && ttValue != VALUE_NONE // Only in case of TT access race
@@ -1217,10 +1225,11 @@ split_point_start: // At split points actual search starts from here
               continue;
           }
 
-          // Prune moves with negative or equal SEE
+          // Prune moves with negative or equal SEE.
+          // Also prune moves with positive SEE where capturing loses a tempo and SEE < beta - futilityBase.
           if (   futilityBase < beta
               && depth < DEPTH_ZERO
-              && pos.see(move) <= 0)
+              && pos.see_asymm(move, beta - futilityBase) <= 0)
           {
               bestValue = std::max(bestValue, futilityBase);
               continue;