diff --git a/src/evaluate.cpp b/src/evaluate.cpp index 789e2859..ba3de70b 100644 --- a/src/evaluate.cpp +++ b/src/evaluate.cpp @@ -1104,23 +1104,26 @@ Value Eval::evaluate(const Position& pos) { return nnue; }; - // If there is PSQ imbalance use classical eval, with small probability if it is small + // 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. 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)); // Use classical evaluation for really low piece endgames. - // The most critical case is a bishop + A/H file pawn vs naked king draw. - bool strongClassical = pos.non_pawn_material() < 2 * RookValueMg && pos.count() < 2; + // 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() < 2); - v = classical || strongClassical ? Evaluation(pos).value() : adjusted_NNUE(); + v = classical || lowPieceEndgame ? Evaluation(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 - && !strongClassical + // 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