Simplify NNUE / classical evaluation selection

[stockfish] / src / evaluate.cpp

diff --git a/src/evaluate.cpp b/src/evaluate.cpp
index 543644ee96b201e49f31ec87f35cc17ff312d199..0e66fea62b2f56c7bd4bb9f25fb7f6f95c8ef4c5 100644 (file)
--- a/src/evaluate.cpp
+++ b/src/evaluate.cpp
@@ -214,11 +214,10 @@ using namespace Trace;
 namespace {
 
   // Threshold for lazy and space evaluation
-  constexpr Value LazyThreshold1 =  Value(1565);
-  constexpr Value LazyThreshold2 =  Value(1102);
-  constexpr Value SpaceThreshold = Value(11551);
-  constexpr Value NNUEThreshold1 =   Value(682);
-  constexpr Value NNUEThreshold2 =   Value(176);
+  constexpr Value LazyThreshold1    =  Value(1565);
+  constexpr Value LazyThreshold2    =  Value(1102);
+  constexpr Value SpaceThreshold    = Value(11551);
+  constexpr Value NNUEThreshold1    =   Value(800);
 
   // KingAttackWeights[PieceType] contains king attack weights by piece type
   constexpr int KingAttackWeights[PIECE_TYPE_NB] = { 0, 0, 81, 52, 44, 10 };
@@ -1119,7 +1118,7 @@ Value Eval::evaluate(const Position& pos) {
 
          int scale = 903 + 28 * pos.count<PAWN>() + 28 * pos.non_pawn_material() / 1024;
 
-         Value nnue = NNUE::evaluate(pos) * scale / 1024;
+         Value nnue = NNUE::evaluate(pos, true) * scale / 1024;
 
          if (pos.is_chess960())
              nnue += fix_FRC(pos);
@@ -1127,30 +1126,14 @@ Value Eval::evaluate(const Position& pos) {
          return nnue;
       };
 
-      // If there is PSQ imbalance we use the classical eval. We also introduce
-      // a small probability of using the classical eval when PSQ imbalance is small.
+      // If there is PSQ imbalance we use the classical eval.
       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;
-
-      // Use classical evaluation for really low piece endgames.
-      // One critical case is the draw for bishop + A/H file pawn vs naked king.
-      bool lowPieceEndgame =   pos.non_pawn_material() == BishopValueMg
-                            || (pos.non_pawn_material() < 2 * RookValueMg && pos.count<PAWN>() < 2);
-
-      v = classical || lowPieceEndgame ? Evaluation<NO_TRACE>(pos).value()
-                                       : adjusted_NNUE();
-
-      // If the classical eval is small and imbalance large, use NNUE nevertheless.
-      // For the case of opposite colored bishops, switch to NNUE eval with small
-      // probability if the classical eval is less than the threshold.
-      if (    largePsq
-          && !lowPieceEndgame
-          && (   abs(v) * 16 < NNUEThreshold2 * r50
-              || (   pos.opposite_bishops()
-                  && abs(v) * 16 < (NNUEThreshold1 + pos.non_pawn_material() / 64) * r50)))
-          v = adjusted_NNUE();
+
+      v = largePsq ? Evaluation<NO_TRACE>(pos).value()  // classical
+                   : adjusted_NNUE();                   // NNUE
+
   }
 
   // Damp down the evaluation linearly when shuffling