Enhanced verify search (#1338)

by disabling null-move-pruning for the side to move for first part of
the remaining search tree. This helps to better recognize zugzwang.

STC:
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 18220 W: 3379 L: 3253 D: 11588
http://tests.stockfishchess.org/tests/view/5a2fa6460ebc590ccbb8bc2f

LTC:
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 41899 W: 5359 L: 5265 D: 31275
http://tests.stockfishchess.org/tests/view/5a2fcf440ebc590ccbb8bc47

For further detail see commit notes and discussion at
https://github.com/pb00068/Stockfish/commit/6401a80ab91df5c54390ac357409fef2e51ff5bb

bench: 5776193

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;

diff --git a/src/thread.cpp b/src/thread.cpp
index 3f3f26cd63f4e6e6f354a1d26ff4fc25febc4640..58d693cf2a52547c4fc5f1140436293bd9fcda2d 100644 (file)
--- a/src/thread.cpp
+++ b/src/thread.cpp
@@ -187,6 +187,8 @@ void ThreadPool::start_thinking(Position& pos, StateListPtr& states,
       th->rootDepth = th->completedDepth = DEPTH_ZERO;
       th->rootMoves = rootMoves;
       th->rootPos.set(pos.fen(), pos.is_chess960(), &setupStates->back(), th);
+      th->nmp_ply = 0;
+      th->pair = -1;
   }
 
   setupStates->back() = tmp;

diff --git a/src/thread.h b/src/thread.h
index 1780abe1da85d821a51312457315b6aa0697e27d..e2009a7d78b98cd6411391c49c2f8c4b50f408e3 100644 (file)
--- a/src/thread.h
+++ b/src/thread.h
@@ -61,7 +61,7 @@ public:
   Material::Table materialTable;
   Endgames endgames;
   size_t PVIdx;
-  int selDepth;
+  int selDepth, nmp_ply, pair;
   std::atomic<uint64_t> nodes, tbHits;
 
   Position rootPos;