Simplify SEE Pruning (#2191)

[stockfish] / src / search.cpp

diff --git a/src/search.cpp b/src/search.cpp
index b31157e4406c3755f6789da25df8fd13c556e215..d9543899a0f7844e2b817b3e2095f321a8ca51fc 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -71,7 +71,7 @@ namespace {
   int Reductions[MAX_MOVES]; // [depth or moveNumber]
 
   Depth reduction(bool i, Depth d, int mn) {
-    int r = Reductions[d / ONE_PLY] * Reductions[mn] / 1024;
+    int r = Reductions[d / ONE_PLY] * Reductions[mn];
     return ((r + 512) / 1024 + (!i && r > 1024)) * ONE_PLY;
   }
 
@@ -149,7 +149,7 @@ namespace {
 void Search::init() {
 
   for (int i = 1; i < MAX_MOVES; ++i)
-     Reductions[i] = int(733.3 * std::log(i));
+      Reductions[i] = int(22.9 * std::log(i));
 }
 
 
@@ -239,19 +239,15 @@ void MainThread::search() {
       for (Thread* th: Threads)
           minScore = std::min(minScore, th->rootMoves[0].score);
 
-      // Vote according to score and depth
+      // Vote according to score and depth, and select the best thread
       for (Thread* th : Threads)
+      {
           votes[th->rootMoves[0].pv[0]] +=
-               (th->rootMoves[0].score - minScore + 14) * int(th->completedDepth);
+              (th->rootMoves[0].score - minScore + 14) * int(th->completedDepth);
 
-      // Select best thread
-      auto bestVote = votes[this->rootMoves[0].pv[0]];
-      for (Thread* th : Threads)
-          if (votes[th->rootMoves[0].pv[0]] > bestVote)
-          {
-              bestVote = votes[th->rootMoves[0].pv[0]];
+          if (votes[th->rootMoves[0].pv[0]] > votes[bestThread->rootMoves[0].pv[0]])
               bestThread = th;
-          }
+      }
   }
 
   previousScore = bestThread->rootMoves[0].score;
@@ -283,7 +279,7 @@ void Thread::search() {
   Move  pv[MAX_PLY+1];
   Value bestValue, alpha, beta, delta;
   Move  lastBestMove = MOVE_NONE;
-  Depth lastBestMoveDepth = DEPTH_ZERO, rootDepth = DEPTH_ZERO;
+  Depth lastBestMoveDepth = DEPTH_ZERO;
   MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
   double timeReduction = 1, totBestMoveChanges = 0;
   Color us = rootPos.side_to_move();
@@ -404,17 +400,14 @@ void Thread::search() {
                   beta = (alpha + beta) / 2;
                   alpha = std::max(bestValue - delta, -VALUE_INFINITE);
 
+                  failedHighCnt = 0;
                   if (mainThread)
-                  {
-                      failedHighCnt = 0;
                       mainThread->stopOnPonderhit = false;
-                  }
               }
               else if (bestValue >= beta)
               {
                   beta = std::min(bestValue + delta, VALUE_INFINITE);
-                  if (mainThread)
-                      ++failedHighCnt;
+                  ++failedHighCnt;
               }
               else
                   break;
@@ -541,13 +534,13 @@ namespace {
     bool ttHit, ttPv, inCheck, givesCheck, improving;
     bool captureOrPromotion, doFullDepthSearch, moveCountPruning, ttCapture;
     Piece movedPiece;
-    int moveCount, captureCount, quietCount;
+    int moveCount, captureCount, quietCount, singularLMR;
 
     // Step 1. Initialize node
     Thread* thisThread = pos.this_thread();
     inCheck = pos.checkers();
     Color us = pos.side_to_move();
-    moveCount = captureCount = quietCount = ss->moveCount = 0;
+    moveCount = captureCount = quietCount = singularLMR = ss->moveCount = 0;
     bestValue = -VALUE_INFINITE;
     maxValue = VALUE_INFINITE;
 
@@ -592,10 +585,10 @@ namespace {
     // starts with statScore = 0. Later grandchildren start with the last calculated
     // statScore of the previous grandchild. This influences the reduction rules in
     // LMR which are based on the statScore of parent position.
-       if (rootNode)
-               (ss + 4)->statScore = 0;
-       else
-               (ss + 2)->statScore = 0;
+    if (rootNode)
+        (ss + 4)->statScore = 0;
+    else
+        (ss + 2)->statScore = 0;
 
     // Step 4. Transposition table lookup. We don't want the score of a partial
     // search to overwrite a previous full search TT value, so we use a different
@@ -606,7 +599,7 @@ namespace {
     ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
     ttMove =  rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
             : ttHit    ? tte->move() : MOVE_NONE;
-    ttPv = (ttHit && tte->is_pv()) || (PvNode && depth > 4 * ONE_PLY);
+    ttPv = PvNode || (ttHit && tte->is_pv());
 
     // At non-PV nodes we check for an early TT cutoff
     if (  !PvNode
@@ -853,7 +846,6 @@ moves_loop: // When in check, search starts from here
     value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
     moveCountPruning = false;
     ttCapture = ttMove && pos.capture_or_promotion(ttMove);
-    int singularExtensionLMRmultiplier = 0;
 
     // Step 12. Loop through all pseudo-legal moves until no moves remain
     // or a beta cutoff occurs.
@@ -910,12 +902,13 @@ moves_loop: // When in check, search starts from here
           ss->excludedMove = MOVE_NONE;
 
           if (value < singularBeta)
-              {
+          {
               extension = ONE_PLY;
-              singularExtensionLMRmultiplier++;
+              singularLMR++;
+
               if (value < singularBeta - std::min(3 * depth / ONE_PLY, 39))
-                 singularExtensionLMRmultiplier++;
-              }
+                  singularLMR++;
+          }
 
           // Multi-cut pruning
           // Our ttMove is assumed to fail high, and now we failed high also on a reduced
@@ -961,7 +954,7 @@ moves_loop: // When in check, search starts from here
 
           if (   !captureOrPromotion
               && !givesCheck
-              && !pos.advanced_pawn_push(move))
+              && (!pos.advanced_pawn_push(move) || pos.non_pawn_material(~us) > BishopValueMg))
           {
               // Move count based pruning (~30 Elo)
               if (moveCountPruning)
@@ -987,7 +980,8 @@ moves_loop: // When in check, search starts from here
               if (!pos.see_ge(move, Value(-29 * lmrDepth * lmrDepth)))
                   continue;
           }
-          else if (!pos.see_ge(move, -PawnValueEg * (depth / ONE_PLY))) // (~20 Elo)
+          else if ((!givesCheck || !extension)
+                  && !pos.see_ge(move, -PawnValueEg * (depth / ONE_PLY))) // (~20 Elo)
                   continue;
       }
 
@@ -1025,8 +1019,9 @@ moves_loop: // When in check, search starts from here
           // Decrease reduction if opponent's move count is high (~10 Elo)
           if ((ss-1)->moveCount > 15)
               r -= ONE_PLY;
+
           // Decrease reduction if move has been singularly extended
-          r -= singularExtensionLMRmultiplier * ONE_PLY;
+          r -= singularLMR * ONE_PLY;
 
           if (!captureOrPromotion)
           {
@@ -1062,7 +1057,7 @@ moves_loop: // When in check, search starts from here
               r -= ss->statScore / 20000 * ONE_PLY;
           }
 
-          Depth d = std::max(newDepth - std::max(r, DEPTH_ZERO), ONE_PLY);
+          Depth d = clamp(newDepth - r, ONE_PLY, newDepth);
 
           value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
 
@@ -1368,6 +1363,7 @@ moves_loop: // When in check, search starts from here
 
       // Don't search moves with negative SEE values
       if (  (!inCheck || evasionPrunable)
+          && (!givesCheck || !(pos.blockers_for_king(~pos.side_to_move()) & from_sq(move)))
           && !pos.see_ge(move))
           continue;
 
@@ -1478,7 +1474,7 @@ moves_loop: // When in check, search starts from here
   void update_capture_stats(const Position& pos, Move move,
                             Move* captures, int captureCount, int bonus) {
 
-      CapturePieceToHistory& captureHistory =  pos.this_thread()->captureHistory;
+      CapturePieceToHistory& captureHistory = pos.this_thread()->captureHistory;
       Piece moved_piece = pos.moved_piece(move);
       PieceType captured = type_of(pos.piece_on(to_sq(move)));
 
@@ -1717,10 +1713,4 @@ void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
         if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
             Cardinality = 0;
     }
-    else
-    {
-        // Assign the same rank to all moves
-        for (auto& m : rootMoves)
-            m.tbRank = 0;
-    }
 }