From 8fec8834715a440ac18e24e130888c2c60bab352 Mon Sep 17 00:00:00 2001
From: Stefan Geschwentner <Stefan-Geschwentner@web.de>
Date: Sun, 25 Aug 2019 21:45:58 +0200
Subject: [PATCH] Tweak Late Move Reduction at root

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 https://github.com/official-stockfish/Stockfish/pull/2282

Bench: 3568210
---
 src/search.cpp | 6 +++++-
 src/search.h   | 1 +
 src/thread.cpp | 9 +++++++++
 src/thread.h   | 1 +
 4 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/src/search.cpp b/src/search.cpp
index f0fa6f05..28cbffd6 100644
--- 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 @@ moves_loop: // When in check, search starts from here
       // 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
index 24c58d30..c77ca3ad 100644
--- 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
index e5043b6e..5165fd90 100644
--- 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
index c11d1787..ed427b10 100644
--- a/src/thread.h
+++ b/src/thread.h
@@ -56,6 +56,7 @@ public:
   void idle_loop();
   void start_searching();
   void wait_for_search_finished();
+  int best_move_count(Move move);
 
   Pawns::Table pawnsTable;
   Material::Table materialTable;
-- 
2.39.2