Tweak cutnode reduction

Less reduction for second move at non-check CUT node with depth <= 10.

STC:
LLR: 2.94 (-2.94,2.94) {-0.50,1.50}
Total: 38680 W: 7490 L: 7245 D: 23945
Ptnml(0-2): 643, 4441, 8967, 4606, 683
https://tests.stockfishchess.org/tests/view/5f21e1782f7e63962b99f451

LTC:
LLR: 2.95 (-2.94,2.94) {0.25,1.75}
Total: 71976 W: 9003 L: 8636 D: 54337
Ptnml(0-2): 440, 6414, 21972, 6663, 499
https://tests.stockfishchess.org/tests/view/5f2245762f7e63962b99f4bd

closes https://github.com/official-stockfish/Stockfish/pull/2868

Bench: 4746616

diff --git a/src/search.cpp b/src/search.cpp
index 6ec4d80322372332e10f909a5c574dbf6be5f3d9..91ac60ad4b2615f47864631e139b1108a4266012 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -1167,6 +1167,13 @@ moves_loop: // When in check, search starts from here
       {
           Depth r = reduction(improving, depth, moveCount);
 
       {
           Depth r = reduction(improving, depth, moveCount);
 

+          // Decrease reduction at non-check cut nodes for second move at low depths
+          if (   cutNode
+              && depth <= 10
+              && moveCount <= 2
+              && !ss->inCheck)
+              r--;
+

           // Decrease reduction if the ttHit running average is large
           if (thisThread->ttHitAverage > 473 * TtHitAverageResolution * TtHitAverageWindow / 1024)
               r--;
           // Decrease reduction if the ttHit running average is large
           if (thisThread->ttHitAverage > 473 * TtHitAverageResolution * TtHitAverageWindow / 1024)
               r--;