Reduce more if multiple moves exceed alpha

Idea of this patch is the following: in case we already have four moves that
exceeded alpha in the current node, the probability of finding fifth should
be reasonably low. Note that four is completely arbitrary - there could and
probably should be some tweaks, both in tweaking best move count threshold
for more reductions and tweaking how they work - for example making more
reductions with best move count linearly.

passed STC:
https://tests.stockfishchess.org/tests/view/615f614783dd501a05b0aee2
LLR: 2.94 (-2.94,2.94) <-0.50,2.50>
Total: 141816 W: 36056 L: 35686 D: 70074
Ptnml(0-2): 499, 15131, 39273, 15511, 494

passed LTC:
https://tests.stockfishchess.org/tests/view/615fdff683dd501a05b0af35
LLR: 2.94 (-2.94,2.94) <0.50,3.50>
Total: 68536 W: 17221 L: 16891 D: 34424
Ptnml(0-2): 38, 6573, 20725, 6885, 47

closes https://github.com/official-stockfish/Stockfish/pull/3736

Bench: 6131513

diff --git a/src/search.cpp b/src/search.cpp
index ac6b8608848afdf17aecd894236fbf79fc5d03d7..73fc8a55fef18d57212ff4bf0deaf7a87ffa846f 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -591,13 +591,13 @@ namespace {
     bool captureOrPromotion, doFullDepthSearch, moveCountPruning,
          ttCapture, singularQuietLMR, noLMRExtension;
     Piece movedPiece;
     bool captureOrPromotion, doFullDepthSearch, moveCountPruning,
          ttCapture, singularQuietLMR, noLMRExtension;
     Piece movedPiece;

-    int moveCount, captureCount, quietCount;
+    int moveCount, captureCount, quietCount, bestMoveCount;

 
     // Step 1. Initialize node
     ss->inCheck        = pos.checkers();
     priorCapture       = pos.captured_piece();
     Color us           = pos.side_to_move();
 
     // Step 1. Initialize node
     ss->inCheck        = pos.checkers();
     priorCapture       = pos.captured_piece();
     Color us           = pos.side_to_move();

-    moveCount          = captureCount = quietCount = ss->moveCount = 0;
+    moveCount          = bestMoveCount = captureCount = quietCount = ss->moveCount = 0;

     bestValue          = -VALUE_INFINITE;
     maxValue           = VALUE_INFINITE;
 
     bestValue          = -VALUE_INFINITE;
     maxValue           = VALUE_INFINITE;
 


@@ -1186,8 +1186,9 @@ moves_loop: // When in check, search starts here
       {
           Depth r = reduction(improving, depth, moveCount, rangeReduction > 2);
 
       {
           Depth r = reduction(improving, depth, moveCount, rangeReduction > 2);
 

-          // Decrease reduction if on the PV (~1 Elo)
-          if (PvNode)
+          // Decrease reduction if on the PV (~2 Elo)
+          if (   PvNode
+              && bestMoveCount <= 3)

               r--;
 
           // Decrease reduction if the ttHit running average is large (~0 Elo)
               r--;
 
           // Decrease reduction if the ttHit running average is large (~0 Elo)


@@ -1340,7 +1341,10 @@ moves_loop: // When in check, search starts here
                   update_pv(ss->pv, move, (ss+1)->pv);
 
               if (PvNode && value < beta) // Update alpha! Always alpha < beta
                   update_pv(ss->pv, move, (ss+1)->pv);
 
               if (PvNode && value < beta) // Update alpha! Always alpha < beta

+              {

                   alpha = value;
                   alpha = value;

+                  bestMoveCount++;
+              }

               else
               {
                   assert(value >= beta); // Fail high
               else
               {
                   assert(value >= beta); // Fail high