Extra thinking before accepting draw PVs.

[stockfish] / src / search.cpp

diff --git a/src/search.cpp b/src/search.cpp
index f0224d8c72d3d9fdba241800fde5b3feac540b69..6ef74bea5e1a5c7b92951a89961117b37fd60763 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -149,6 +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.
@@ -475,11 +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;
 
@@ -1427,6 +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.
 
@@ -1588,6 +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)
     {