Relax TT condition for zugzwang verified null values

[stockfish] / src / search.cpp

diff --git a/src/search.cpp b/src/search.cpp
index 8b45130a4319b71e16c4815802d6d9ede427e5fa..e6ffcb19f42cf73abcbb082f7a4789442ba36a06 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -232,7 +232,7 @@ namespace {
   const Value EasyMoveMargin = Value(0x200);
 
   // Last seconds noise filtering (LSN)
-  const bool UseLSNFiltering = true;
+  const bool UseLSNFiltering = false;
   const int LSNTime = 4000; // In milliseconds
   const Value LSNValue = value_from_centipawns(200);
   bool loseOnTime = false;
@@ -294,7 +294,7 @@ namespace {
   Depth extension(const Position&, Move, bool, bool, bool, bool, bool, bool*);
   bool ok_to_do_nullmove(const Position& pos);
   bool ok_to_prune(const Position& pos, Move m, Move threat);
-  bool ok_to_use_TT(const TTEntry* tte, Depth depth, Value beta, int ply);
+  bool ok_to_use_TT(const TTEntry* tte, Depth depth, Value beta, int ply, bool allowNullmove);
   Value refine_eval(const TTEntry* tte, Value defaultEval, int ply);
   void update_history(const Position& pos, Move move, Depth depth, Move movesSearched[], int moveCount);
   void update_killers(Move m, SearchStack& ss);
@@ -1299,7 +1299,7 @@ namespace {
     tte = TT.retrieve(posKey);
     ttMove = (tte ? tte->move() : MOVE_NONE);
 
-    if (tte && ok_to_use_TT(tte, depth, beta, ply))
+    if (tte && ok_to_use_TT(tte, depth, beta, ply, allowNullmove))
     {
         ss[ply].currentMove = ttMove; // Can be MOVE_NONE
         return value_from_tt(tte->value(), ply);
@@ -1379,13 +1379,20 @@ namespace {
             if (nullValue >= value_mate_in(PLY_MAX))
                 nullValue = beta;
 
-            if (depth < 6 * OnePly)
-                return nullValue;
+            // Do zugzwang verification search for high depths, don't store in TT
+            // if search was stopped.
+            if (   (   depth < 6 * OnePly
+                    || search(pos, ss, beta, depth-5*OnePly, ply, false, threadID) >= beta)
+                && !AbortSearch
+                && !TM.thread_should_stop(threadID))
+            {
+                assert(value_to_tt(nullValue, ply) == nullValue);
 
-            // Do zugzwang verification search
-            Value v = search(pos, ss, beta, depth-5*OnePly, ply, false, threadID);
-            if (v >= beta)
+                // Special flag null values that are not zugzwang checked
+                ValueType vt = (depth < 6 * OnePly ? VALUE_TYPE_NS_LO : VALUE_TYPE_LOWER);
+                TT.store(posKey, nullValue, vt, depth, MOVE_NONE);
                 return nullValue;
+            }
         } else {
             // The null move failed low, which means that we may be faced with
             // some kind of threat. If the previous move was reduced, check if
@@ -1620,7 +1627,7 @@ namespace {
     tte = TT.retrieve(pos.get_key());
     ttMove = (tte ? tte->move() : MOVE_NONE);
 
-    if (!pvNode && tte && ok_to_use_TT(tte, depth, beta, ply))
+    if (!pvNode && tte && ok_to_use_TT(tte, depth, beta, ply, true))
     {
         assert(tte->type() != VALUE_TYPE_EVAL);
 
@@ -1661,7 +1668,7 @@ namespace {
         alpha = bestValue;
 
     // If we are near beta then try to get a cutoff pushing checks a bit further
-    bool deepChecks = depth == -OnePly && staticValue >= beta - PawnValueMidgame / 8;
+    bool deepChecks = (depth == -OnePly && staticValue >= beta - PawnValueMidgame / 8);
 
     // Initialize a MovePicker object for the current position, and prepare
     // to search the moves. Because the depth is <= 0 here, only captures,
@@ -2301,14 +2308,18 @@ namespace {
   }
 
 
-  // ok_to_use_TT() returns true if a transposition table score
-  // can be used at a given point in search.
+  // ok_to_use_TT() returns true if a transposition table score can be used at a
+  // given point in search. To avoid zugzwang issues TT cutoffs at the root node
+  // of a null move verification search are not allowed if the TT value was found
+  // by a null search, this is implemented testing allowNullmove and TT entry type.
 
-  bool ok_to_use_TT(const TTEntry* tte, Depth depth, Value beta, int ply) {
+  bool ok_to_use_TT(const TTEntry* tte, Depth depth, Value beta, int ply, bool allowNullmove) {
 
     Value v = value_from_tt(tte->value(), ply);
 
-    return   (   tte->depth() >= depth
+    return   (allowNullmove || !(tte->type() & VALUE_TYPE_NULL))
+
+          && (   tte->depth() >= depth
               || v >= Max(value_mate_in(PLY_MAX), beta)
               || v < Min(value_mated_in(PLY_MAX), beta))