Enhanced verify search (#1338)

[stockfish] / src / search.cpp

diff --git a/src/search.cpp b/src/search.cpp
index ed01e0dee04c89d4f31dd29264ad576a441df89f..8b3b8fb4864436c17c6964a24151a85db92f873f 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -681,7 +681,8 @@ namespace {
     // Step 8. Null move search with verification search (is omitted in PV nodes)
     if (   !PvNode
         &&  eval >= beta
-        &&  ss->staticEval >= beta - 36 * depth / ONE_PLY + 225)
+        &&  ss->staticEval >= beta - 36 * depth / ONE_PLY + 225
+               && (ss->ply >= thisThread->nmp_ply || ss->ply % 2 == thisThread->pair))
     {
 
         assert(eval - beta >= 0);
@@ -707,8 +708,17 @@ namespace {
                 return nullValue;
 
             // Do verification search at high depths
+            R += ONE_PLY;
+            // disable null move pruning for side to move for the first part of the remaining search tree
+            int nmp_ply = thisThread->nmp_ply;
+            int pair = thisThread->pair;
+            thisThread->nmp_ply = ss->ply + 3 * (depth-R) / 4;
+            thisThread->pair = (ss->ply % 2) == 0;
+
             Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta)
                                         :  search<NonPV>(pos, ss, beta-1, beta, depth-R, false, true);
+            thisThread->pair = pair;
+            thisThread->nmp_ply = nmp_ply;
 
             if (v >= beta)
                 return nullValue;