Don't update bestValue when pruning

[stockfish] / src / search.cpp

diff --git a/src/search.cpp b/src/search.cpp
index b781bb770864aa63d52f1050b316f9c052ec86c2..4c69a0522f90310e2eeb4fcf1d70197cab8954a9 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -366,7 +366,7 @@ void Search::think() {
   // Do we have to play with skill handicap? In this case enable MultiPV that
   // we will use behind the scenes to retrieve a set of possible moves.
   SkillLevelEnabled = (SkillLevel < 20);
-  MultiPV = (SkillLevelEnabled ? std::max(UCIMultiPV, 4U) : UCIMultiPV);
+  MultiPV = (SkillLevelEnabled ? std::max(UCIMultiPV, (size_t)4) : UCIMultiPV);
 
   // Write current search header to log file
   if (Options["Use Search Log"].value<bool>())
@@ -948,6 +948,9 @@ split_point_start: // At split points actual search starts from here
     {
         lock_grab(&(sp->lock));
         bestValue = sp->bestValue;
+        moveCount = sp->moveCount;
+
+        assert(bestValue > -VALUE_INFINITE && moveCount > 0);
     }
 
     // Step 11. Loop through moves
@@ -1065,13 +1068,7 @@ split_point_start: // At split points actual search starts from here
           if (futilityValue < beta)
           {
               if (SpNode)
-              {
                   lock_grab(&(sp->lock));
-                  if (futilityValue > sp->bestValue)
-                      sp->bestValue = bestValue = futilityValue;
-              }
-              else if (futilityValue > bestValue)
-                  bestValue = futilityValue;
 
               continue;
           }
@@ -1220,9 +1217,17 @@ split_point_start: // At split points actual search starts from here
     // case of StopRequest or thread.cutoff_occurred() are set, but this is
     // harmless because return value is discarded anyhow in the parent nodes.
     // If we are in a singular extension search then return a fail low score.
-    if (!SpNode && !moveCount)
+    if (!moveCount)
         return excludedMove ? oldAlpha : inCheck ? value_mated_in(ss->ply) : VALUE_DRAW;
 
+    // We have pruned all the moves, so return a fail-low score
+    if (bestValue == -VALUE_INFINITE)
+    {
+        assert(!playedMoveCount);
+
+        bestValue = alpha;
+    }
+
     // Step 21. Update tables
     // If the search is not aborted, update the transposition table,
     // history counters, and killer moves.