Tweak Late Move Reduction at root

locutus2 · snicolet · commit 8fec8834715a · 2019-08-26T08:49:55.000+02:00
Maintain best move counter at the root and allow there only moves which has a counter of zero for Late Move Reduction. For compensation only the first three moves are excluded from Late Move Reduction per default instead the first four moves. What we can further do: - here we use a simple counting scheme but perhaps some aging to fade out early iterations could be helpful - use the best move counter also at inner nodes for LMR and/or pruning STC: LLR: 2.95 (-2.94,2.94) [0.50,4.50] Total: 17414 W: 3984 L: 3733 D: 9697 http://tests.stockfishchess.org/tests/view/5d6234bb0ebc5939d09f2aa2 LTC: LLR: 2.96 (-2.94,2.94) [0.00,3.50] Total: 38058 W: 6448 L: 6166 D: 25444 http://tests.stockfishchess.org/tests/view/5d62681a0ebc5939d09f2f27 Closes #2282 Bench: 3568210
diff --git a/src/search.cpp b/src/search.cpp
@@ -472,7 +472,10 @@ void Thread::search() {
                   ++failedHighCnt;
               }
               else
+              {
+                  ++rootMoves[pvIdx].bestMoveCount;
                   break;
+              }
 
               delta += delta / 4 + 5;
 
@@ -1072,7 +1075,8 @@ namespace {
       // Step 16. Reduced depth search (LMR). If the move fails high it will be
       // re-searched at full depth.
       if (    depth >= 3 * ONE_PLY
-          &&  moveCount > 1 + 3 * rootNode
+          &&  moveCount > 1 + 2 * rootNode
+          && (!rootNode || thisThread->best_move_count(move) == 0)
           && (  !captureOrPromotion
               || moveCountPruning
               || ss->staticEval + PieceValue[EG][pos.captured_piece()] <= alpha
diff --git a/src/search.h b/src/search.h
@@ -70,6 +70,7 @@ struct RootMove {
   Value previousScore = -VALUE_INFINITE;
   int selDepth = 0;
   int tbRank = 0;
+  int bestMoveCount = 0;
   Value tbScore;
   std::vector<Move> pv;
 };
diff --git a/src/thread.cpp b/src/thread.cpp
@@ -52,6 +52,15 @@ Thread::~Thread() {
   stdThread.join();
 }
 
+/// Thread::bestMoveCount(Move move) return best move counter for the given root move
+
+int Thread::best_move_count(Move move) {
+
+  auto rm = std::find(rootMoves.begin() + pvIdx,
+                      rootMoves.begin() + pvLast, move);
+
+  return rm != rootMoves.begin() + pvLast ? rm->bestMoveCount : 0;
+}
 
 /// Thread::clear() reset histories, usually before a new game
 
diff --git a/src/thread.h b/src/thread.h
@@ -56,6 +56,7 @@ class Thread {
   void idle_loop();
   void start_searching();
   void wait_for_search_finished();
+  int best_move_count(Move move);
 
   Pawns::Table pawnsTable;
   Material::Table materialTable;
