Tweak hybrid treshold.

Increase the first hybrid threshold with more material.
Rewrite the hybrid rules for clarity.

STC:
LLR: 2.94 (-2.94,2.94) {-0.25,1.25}
Total: 24416 W: 3039 L: 2848 D: 18529
Ptnml(0-2): 135, 2136, 7503, 2271, 163
https://tests.stockfishchess.org/tests/view/5f6451efbb0cae038ca8f4dc

LTC;
LLR: 2.95 (-2.94,2.94) {0.25,1.25}
Total: 65016 W: 3702 L: 3455 D: 57859
Ptnml(0-2): 66, 2991, 26157, 3218, 76
https://tests.stockfishchess.org/tests/view/5f64b143bb0cae038ca8f51f

closes https://github.com/official-stockfish/Stockfish/pull/3140

Bench: 3973739

diff --git a/src/evaluate.cpp b/src/evaluate.cpp
index faf71d2701dd83ad152e2d82f260750a904e5809..a9159477a077fd626f9acc584292307b985b4cbf 100644 (file)
--- a/src/evaluate.cpp
+++ b/src/evaluate.cpp
@@ -1015,20 +1015,28 @@ make_v:
 
 Value Eval::evaluate(const Position& pos) {
 
-  // Use classical eval if there is a large imbalance
-  // If there is a moderate imbalance, use classical eval with probability (1/8),
-  // as derived from the node counter.
-  bool useClassical = abs(eg_value(pos.psq_score())) * 16 > NNUEThreshold1 * (16 + pos.rule50_count());
-  bool classical = !Eval::useNNUE
-                ||  useClassical
-                || (abs(eg_value(pos.psq_score())) > PawnValueMg / 4 && !(pos.this_thread()->nodes & 0xB));
-  Value v = classical ? Evaluation<NO_TRACE>(pos).value()
-                      : NNUE::evaluate(pos) * 5 / 4 + Tempo;
-
-  if (   useClassical 
-      && Eval::useNNUE 
-      && abs(v) * 16 < NNUEThreshold2 * (16 + pos.rule50_count()))
-      v = NNUE::evaluate(pos) * 5 / 4 + Tempo;
+  Value v;
+
+  if (!Eval::useNNUE)
+      v = Evaluation<NO_TRACE>(pos).value();
+  else
+  {
+      // scale and shift NNUE for compatibility with search and classical evaluation
+      auto  adjusted_NNUE = [&](){ return NNUE::evaluate(pos) * 5 / 4 + Tempo; };
+
+      // if there is PSQ imbalance use classical eval, with small probability if it is small
+      Value psq = Value(abs(eg_value(pos.psq_score())));
+      int   r50 = 16 + pos.rule50_count();
+      bool  largePsq = psq * 16 > (NNUEThreshold1 + pos.non_pawn_material() / 64) * r50;
+      bool  classical = largePsq || (psq > PawnValueMg / 4 && !(pos.this_thread()->nodes & 0xB));
+
+      v = classical ? Evaluation<NO_TRACE>(pos).value() : adjusted_NNUE();
+
+      // if the classical eval is small and imbalance large, use NNUE nevertheless.
+      if (   largePsq
+          && abs(v) * 16 < NNUEThreshold2 * r50)
+          v = adjusted_NNUE();
+  }
 
   // Damp down the evaluation linearly when shuffling
   v = v * (100 - pos.rule50_count()) / 100;