Null move driven internal iterative deepening

[stockfish] / src / search.cpp

diff --git a/src/search.cpp b/src/search.cpp
index a9953bb69e736c4b6c3eaf071bbbabd3de28869a..2a2e32f943fae435ef69719364504b7b2f58fedc 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -106,6 +106,9 @@ namespace {
   const bool UseIIDAtPVNodes = true;
   const bool UseIIDAtNonPVNodes = false;
 
+  // Use null move driven internal iterative deepening?
+  bool UseNullDrivenIID = true;
+
   // Internal iterative deepening margin.  At Non-PV moves, when
   // UseIIDAtNonPVNodes is true, we do an internal iterative deepening search
   // when the static evaluation is at most IIDMargin below beta.
@@ -188,7 +191,7 @@ namespace {
   // Time managment variables
   int SearchStartTime;
   int MaxNodes, MaxDepth;
-  int MaxSearchTime, AbsoluteMaxSearchTime, ExtraSearchTime, TimeAdvantage;
+  int MaxSearchTime, AbsoluteMaxSearchTime, ExtraSearchTime;
   Move BestRootMove, PonderMove, EasyMove;
   int RootMoveNumber;
   bool InfiniteSearch;
@@ -427,8 +430,6 @@ void think(const Position &pos, bool infinite, bool ponder, int side_to_move,
   int myIncrement = increment[side_to_move];
   int oppTime = time[1 - side_to_move];
 
-  TimeAdvantage = myTime - oppTime;
-
   if (!movesToGo) // Sudden death time control
   {
       if (myIncrement)
@@ -436,7 +437,7 @@ void think(const Position &pos, bool infinite, bool ponder, int side_to_move,
           MaxSearchTime = myTime / 30 + myIncrement;
           AbsoluteMaxSearchTime = Max(myTime / 4, myIncrement - 100);
       } else { // Blitz game without increment
-          MaxSearchTime = myTime / 40;
+          MaxSearchTime = myTime / 30;
           AbsoluteMaxSearchTime = myTime / 8;
       }
   }
@@ -680,10 +681,6 @@ namespace {
                 ExtraSearchTime = BestMoveChangesByIteration[Iteration]   * (MaxSearchTime / 2)
                                 + BestMoveChangesByIteration[Iteration-1] * (MaxSearchTime / 3);
 
-            // If we need some more and we are in time advantage take it
-            if (ExtraSearchTime > 0 && TimeAdvantage > 2 * MaxSearchTime)
-                ExtraSearchTime += MaxSearchTime / 2;
-
             // Try to guess if the current iteration is the last one or the last two
             LastIterations = (current_search_time() > ((MaxSearchTime + ExtraSearchTime)*58) / 128);
 
@@ -1130,12 +1127,13 @@ namespace {
 
     if (tte && ok_to_use_TT(tte, depth, beta, ply))
     {
-        ss[ply].currentMove = ttMove; // can be MOVE_NONE ?
+        ss[ply].currentMove = ttMove; // can be MOVE_NONE
         return value_from_tt(tte->value(), ply);
     }
 
     Value approximateEval = quick_evaluate(pos);
     bool mateThreat = false;
+    bool nullDrivenIID = false;
     bool isCheck = pos.is_check();
 
     // Null move search
@@ -1149,7 +1147,21 @@ namespace {
         UndoInfo u;
         pos.do_null_move(u);
         int R = (depth > 7 ? 4 : 3);
+
         Value nullValue = -search(pos, ss, -(beta-1), depth-R*OnePly, ply+1, false, threadID);
+
+        // Check for a null capture artifact, if the value without the null capture
+        // is above beta then there is a good possibility that this is a cut-node.
+        // We will do an IID later to find a ttMove.
+        if (   UseNullDrivenIID
+            && nullValue < beta
+            && depth > 6 * OnePly
+            && ttMove == MOVE_NONE
+            && ss[ply + 1].currentMove != MOVE_NONE
+            && pos.move_is_capture(ss[ply + 1].currentMove)
+            && pos.see(ss[ply + 1].currentMove) * PawnValueMidgame + nullValue > beta - IIDMargin)
+            nullDrivenIID = true;
+
         pos.undo_null_move(u);
 
         if (nullValue >= beta)
@@ -1169,8 +1181,10 @@ namespace {
             // low score (which will cause the reduced move to fail high in the
             // parent node, which will trigger a re-search with full depth).
             if (nullValue == value_mated_in(ply + 2))
+            {
                 mateThreat = true;
-
+                nullDrivenIID = false;
+            }
             ss[ply].threatMove = ss[ply + 1].currentMove;
             if (   depth < ThreatDepth
                 && ss[ply - 1].reduction
@@ -1194,6 +1208,19 @@ namespace {
         search(pos, ss, beta, Min(depth/2, depth-2*OnePly), ply, false, threadID);
         ttMove = ss[ply].pv[ply];
     }
+    else if (nullDrivenIID)
+    {
+        // The null move failed low due to a suspicious capture. Perhaps we
+        // are facing a null capture artifact due to the side to move change
+        // and this is a cut-node. So it's a good time to search for a ttMove.
+        Move tm = ss[ply].threatMove;
+
+        assert(tm != MOVE_NONE);
+
+        search(pos, ss, beta, Min(depth/2, depth-3*OnePly), ply, false, threadID);
+        ttMove = ss[ply].pv[ply];
+        ss[ply].threatMove = tm;
+    }
 
     // Initialize a MovePicker object for the current position, and prepare
     // to search all moves: