Research at intermediate depth if LMR is very high

[stockfish] / src / search.cpp

diff --git a/src/search.cpp b/src/search.cpp
index b8b325e57202f885ee24ee727f35fb11d59d1898..a739b0491c7a9eae9219ba7e404c83483d04c692 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -229,7 +229,7 @@ void Search::think() {
           << std::endl;
   }
 
-  // Reset the threads, still sleeping: will be wake up at split time
+  // Reset the threads, still sleeping: will wake up at split time
   for (size_t i = 0; i < Threads.size(); ++i)
       Threads[i]->maxPly = 0;
 
@@ -348,9 +348,9 @@ namespace {
                 // Bring the best move to the front. It is critical that sorting
                 // is done with a stable algorithm because all the values but the
                 // first and eventually the new best one are set to -VALUE_INFINITE
-                // and we want to keep the same order for all the moves but the new
-                // PV that goes to the front. Note that in case of MultiPV search
-                // the already searched PV lines are preserved.
+                // and we want to keep the same order for all the moves except the
+                // new PV that goes to the front. Note that in case of MultiPV
+                // search the already searched PV lines are preserved.
                 std::stable_sort(RootMoves.begin() + PVIdx, RootMoves.end());
 
                 // Write PV back to transposition table in case the relevant
@@ -776,7 +776,7 @@ moves_loop: // When in check and at SpNode search starts from here
           continue;
 
       // At root obey the "searchmoves" option and skip moves not listed in Root
-      // Move List, as a consequence any illegal move is also skipped. In MultiPV
+      // Move List. As a consequence any illegal move is also skipped. In MultiPV
       // mode we also skip PV moves which have been already searched.
       if (RootNode && !std::count(RootMoves.begin() + PVIdx, RootMoves.end(), move))
           continue;
@@ -907,7 +907,7 @@ moves_loop: // When in check and at SpNode search starts from here
       // Step 14. Make the move
       pos.do_move(move, st, ci, givesCheck);
 
-      // Step 15. Reduced depth search (LMR). If the move fails high will be
+      // Step 15. Reduced depth search (LMR). If the move fails high it will be
       // re-searched at full depth.
       if (    depth >= 3 * ONE_PLY
           && !pvMove
@@ -933,6 +933,13 @@ moves_loop: // When in check and at SpNode search starts from here
 
           value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
 
+          // Research at intermediate depth if reduction is very high
+          if (value > alpha && ss->reduction >= 4 * ONE_PLY)
+          {
+              Depth d2 = std::max(newDepth - 2 * ONE_PLY, ONE_PLY);
+              value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d2, true);
+          }
+
           doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
           ss->reduction = DEPTH_ZERO;
       }
@@ -1401,7 +1408,7 @@ moves_loop: // When in check and at SpNode search starts from here
         Bitboard xray =  (attacks_bb<  ROOK>(m2to, occ) & pos.pieces(color_of(pc), QUEEN, ROOK))
                        | (attacks_bb<BISHOP>(m2to, occ) & pos.pieces(color_of(pc), QUEEN, BISHOP));
 
-        // Verify attackers are triggered by our move and not already exist
+        // Verify attackers are triggered by our move and not already existing
         if (unlikely(xray) && (xray & ~pos.attacks_from<QUEEN>(m2to)))
             return true;
     }