Simplify new cmh pruning thresholds by using directly a quadratic formula.

This decouples also the stat bonus updates from the threshold which creates less dependencies for tuning of stat bonus parameters.
Perhaps a further fine tuning of the now separated coefficients for constHist[0] and constHist[1] could give further gains.

STC:
LLR: 2.93 (-2.94,2.94) <-2.50,0.50>
Total: 78384 W: 6134 L: 6090 D: 66160
Ptnml(0-2): 207, 5013, 28705, 5063, 204
https://tests.stockfishchess.org/tests/view/6106d235afad2da4f4ae3d4b

LTC:
LLR: 2.93 (-2.94,2.94) <-2.50,0.50>
Total: 38176 W: 1149 L: 1095 D: 35932
Ptnml(0-2): 6, 1000, 17030, 1038, 14
https://tests.stockfishchess.org/tests/view/6107a080afad2da4f4ae3def

closes https://github.com/official-stockfish/Stockfish/pull/3639

Bench: 5098146

diff --git a/src/search.cpp b/src/search.cpp
index 187f21694ef932d543eb474f8914719d9c2a46f1..af99c90271a689d09517e68db133fb2a2c904535 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -1017,8 +1017,8 @@ moves_loop: // When in check, search starts here
           {
               // Continuation history based pruning (~20 Elo)
               if (   lmrDepth < 5
-                  && (*contHist[0])[movedPiece][to_sq(move)] < (depth == 1 ? 0 : -stat_bonus(depth-1))
-                  && (*contHist[1])[movedPiece][to_sq(move)] < (depth == 1 ? 0 : -stat_bonus(depth-1)))
+                  && (*contHist[0])[movedPiece][to_sq(move)] < 23 - 23 * depth * depth
+                  && (*contHist[1])[movedPiece][to_sq(move)] < 23 - 23 * depth * depth)
                   continue;
 
               // Futility pruning: parent node (~5 Elo)