Extra thinking before accepting draw PVs.

[stockfish] / src / search.cpp

diff --git a/src/search.cpp b/src/search.cpp
index 182d2d7f60493bc0069ec1bdc05ead0153609885..6ef74bea5e1a5c7b92951a89961117b37fd60763 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -87,10 +87,9 @@ namespace {
 
   // Skill structure is used to implement strength limit
   struct Skill {
-    Skill(int l) : level(l) {}
+    explicit Skill(int l) : level(l) {}
     bool enabled() const { return level < 20; }
     bool time_to_pick(Depth depth) const { return depth / ONE_PLY == 1 + level; }
-    Move best_move(size_t multiPV) { return best ? best : pick_best(multiPV); }
     Move pick_best(size_t multiPV);
 
     int level;
@@ -150,6 +149,7 @@ namespace {
   void update_pv(Move* pv, Move move, Move* childPv);
   void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
   void update_stats(const Position& pos, Stack* ss, Move move, Move* quiets, int quietsCnt, int bonus);
+  bool pv_is_draw(Position& pos);
 
   // perft() is our utility to verify move generation. All the leaf nodes up
   // to the given depth are generated and counted, and the sum is returned.
@@ -476,11 +476,17 @@ void Thread::search() {
               // from the previous search and just did a fast verification.
               const int F[] = { mainThread->failedLow,
                                 bestValue - mainThread->previousScore };
-
               int improvingFactor = std::max(229, std::min(715, 357 + 119 * F[0] - 6 * F[1]));
-              double unstablePvFactor = 1 + mainThread->bestMoveChanges;
+
+              Color us = rootPos.side_to_move();
+              bool thinkHard =    DrawValue[us] == bestValue
+                               && Limits.time[us] - Time.elapsed() > Limits.time[~us]
+                               && ::pv_is_draw(rootPos);
+
+              double unstablePvFactor = 1 + mainThread->bestMoveChanges + thinkHard;
 
               bool doEasyMove =   rootMoves[0].pv[0] == easyMove
+                               && !thinkHard
                                && mainThread->bestMoveChanges < 0.03
                                && Time.elapsed() > Time.optimum() * 5 / 44;
 
@@ -514,8 +520,8 @@ void Thread::search() {
 
   // If skill level is enabled, swap best PV line with the sub-optimal one
   if (skill.enabled())
-      std::swap(rootMoves[0], *std::find(rootMoves.begin(),
-                rootMoves.end(), skill.best_move(multiPV)));
+      std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
+                skill.best ? skill.best : skill.pick_best(multiPV)));
 }
 
 
@@ -543,7 +549,7 @@ namespace {
     Depth extension, newDepth;
     Value bestValue, value, ttValue, eval;
     bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
-    bool captureOrPromotion, doFullDepthSearch, moveCountPruning, skipQuiets, ttCapture;
+    bool captureOrPromotion, doFullDepthSearch, moveCountPruning, skipQuiets, ttCapture, pvExact;
     Piece movedPiece;
     int moveCount, quietCount;
 
@@ -813,6 +819,7 @@ moves_loop: // When in check search starts from here
                            &&  tte->depth() >= depth - 3 * ONE_PLY;
     skipQuiets = false;
     ttCapture = false;
+    pvExact = PvNode && ttHit && tte->bound() == BOUND_EXACT;
 
     // Step 11. Loop through moves
     // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
@@ -957,6 +964,10 @@ moves_loop: // When in check search starts from here
               if ((ss-1)->moveCount > 15)
                   r -= ONE_PLY;
 
+              // Decrease reduction for exact PV nodes
+              if (pvExact)
+                  r -= ONE_PLY;
+
               // Increase reduction if ttMove is a capture
               if (ttCapture)
                   r += ONE_PLY;
@@ -1423,6 +1434,24 @@ moves_loop: // When in check search starts from here
   }
 
 
+  // Is the PV leading to a draw position? Assumes all pv moves are legal
+  bool pv_is_draw(Position& pos) {
+
+    StateInfo st[MAX_PLY];
+    auto& pv = pos.this_thread()->rootMoves[0].pv;
+
+    for (size_t i = 0; i < pv.size(); ++i)
+        pos.do_move(pv[i], st[i]);
+
+    bool isDraw = pos.is_draw(pv.size());
+
+    for (size_t i = pv.size(); i > 0; --i)
+        pos.undo_move(pv[i-1]);
+
+    return isDraw;
+  }
+
+
   // When playing with strength handicap, choose best move among a set of RootMoves
   // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
 
@@ -1446,7 +1475,7 @@ moves_loop: // When in check search starts from here
         int push = (  weakness * int(topScore - rootMoves[i].score)
                     + delta * (rng.rand<unsigned>() % weakness)) / 128;
 
-        if (rootMoves[i].score + push > maxScore)
+        if (rootMoves[i].score + push >= maxScore)
         {
             maxScore = rootMoves[i].score + push;
             best = rootMoves[i].pv[0];
@@ -1584,6 +1613,10 @@ void Tablebases::filter_root_moves(Position& pos, Search::RootMoves& rootMoves)
     ProbeDepth = Options["SyzygyProbeDepth"] * ONE_PLY;
     Cardinality = Options["SyzygyProbeLimit"];
 
+    // Don't filter any moves if the user requested analysis on multiple
+    if (Options["MultiPV"] != 1)
+        return;
+
     // Skip TB probing when no TB found: !TBLargest -> !TB::Cardinality
     if (Cardinality > MaxCardinality)
     {