Simplify the use of classical eval

[stockfish] / src / evaluate.cpp

diff --git a/src/evaluate.cpp b/src/evaluate.cpp
index 718c7bc035da95f92718e73fe599ee376ff793d7..eaad4d556722cb0b2f99d8e0984865f2c549f8c1 100644 (file)
--- a/src/evaluate.cpp
+++ b/src/evaluate.cpp
@@ -192,7 +192,6 @@ using namespace Trace;
 
 namespace {
 
-
   // Threshold for lazy and space evaluation
   constexpr Value LazyThreshold1    =  Value(3631);
   constexpr Value LazyThreshold2    =  Value(2084);
@@ -1043,74 +1042,40 @@ make_v:
     return v;
   }
 
-
-  /// Fisher Random Chess: correction for cornered bishops, to fix chess960 play with NNUE
-
-  Value fix_FRC(const Position& pos) {
-
-    constexpr Bitboard Corners =  1ULL << SQ_A1 | 1ULL << SQ_H1 | 1ULL << SQ_A8 | 1ULL << SQ_H8;
-
-    if (!(pos.pieces(BISHOP) & Corners))
-        return VALUE_ZERO;
-
-    int correction = 0;
-
-    if (   pos.piece_on(SQ_A1) == W_BISHOP
-        && pos.piece_on(SQ_B2) == W_PAWN)
-        correction -= CorneredBishop;
-
-    if (   pos.piece_on(SQ_H1) == W_BISHOP
-        && pos.piece_on(SQ_G2) == W_PAWN)
-        correction -= CorneredBishop;
-
-    if (   pos.piece_on(SQ_A8) == B_BISHOP
-        && pos.piece_on(SQ_B7) == B_PAWN)
-        correction += CorneredBishop;
-
-    if (   pos.piece_on(SQ_H8) == B_BISHOP
-        && pos.piece_on(SQ_G7) == B_PAWN)
-        correction += CorneredBishop;
-
-    return pos.side_to_move() == WHITE ?  Value(3 * correction)
-                                       : -Value(3 * correction);
-  }
-
 } // namespace Eval
 
 
 /// evaluate() is the evaluator for the outer world. It returns a static
 /// evaluation of the position from the point of view of the side to move.
 
-Value Eval::evaluate(const Position& pos) {
+Value Eval::evaluate(const Position& pos, int* complexity) {
 
   Value v;
-  bool useClassical = false;
+  Color stm = pos.side_to_move();
+  Value psq = pos.psq_eg_stm();
 
-  // Deciding between classical and NNUE eval (~10 Elo): for high PSQ imbalance we use classical,
+  // Deciding between classical and NNUE eval: for high PSQ imbalance we use classical,
   // but we switch to NNUE during long shuffling or with high material on the board.
-  if (  !useNNUE
-      || ((pos.this_thread()->depth > 9 || pos.count<ALL_PIECES>() > 7) &&
-          abs(eg_value(pos.psq_score())) * 5 > (856 + pos.non_pawn_material() / 64) * (10 + pos.rule50_count())))
-  {
-      v = Evaluation<NO_TRACE>(pos).value();          // classical
-      useClassical = abs(v) >= 297;
-  }
+  bool useClassical = !useNNUE ||
+                      ((pos.count<ALL_PIECES>() > 7)
+                       && abs(psq) * 5 > (856 + pos.non_pawn_material() / 64) * (10 + pos.rule50_count()));
 
-  // If result of a classical evaluation is much lower than threshold fall back to NNUE
-  if (useNNUE && !useClassical)
+  if (useClassical)
+      v = Evaluation<NO_TRACE>(pos).value();
+  else
   {
-       Value nnue     = NNUE::evaluate(pos, true);     // NNUE
-       int scale      = 1014 + 21 * pos.non_pawn_material() / 1024;
-       Color stm      = pos.side_to_move();
-       Value optimism = pos.this_thread()->optimism[stm];
-       Value psq      = (stm == WHITE ? 1 : -1) * eg_value(pos.psq_score());
-       int complexity = 35 * abs(nnue - psq) / 256;
-
-       optimism = optimism * (32 + complexity) / 32;
-       v = (nnue * scale + optimism * (scale - 846)) / 1024;
-
-       if (pos.is_chess960())
-           v += fix_FRC(pos);
+      int nnueComplexity;
+      int scale = 1064 + 106 * pos.non_pawn_material() / 5120;
+      Value optimism = pos.this_thread()->optimism[stm];
+
+      Value nnue = NNUE::evaluate(pos, true, &nnueComplexity);
+      // Blend nnue complexity with (semi)classical complexity
+      nnueComplexity = (104 * nnueComplexity + 131 * abs(nnue - psq)) / 256;
+      if (complexity) // Return hybrid NNUE complexity to caller
+          *complexity = nnueComplexity;
+
+      optimism = optimism * (269 + nnueComplexity) / 256;
+      v = (nnue * scale + optimism * (scale - 754)) / 1024;
   }
 
   // Damp down the evaluation linearly when shuffling
@@ -1119,6 +1084,10 @@ Value Eval::evaluate(const Position& pos) {
   // Guarantee evaluation does not hit the tablebase range
   v = std::clamp(v, VALUE_TB_LOSS_IN_MAX_PLY + 1, VALUE_TB_WIN_IN_MAX_PLY - 1);
 
+  // When not using NNUE, return classical complexity to caller
+  if (complexity && (!useNNUE || useClassical))
+      *complexity = abs(v - psq);
+
   return v;
 }
 
@@ -1140,7 +1109,6 @@ std::string Eval::trace(Position& pos) {
   std::memset(scores, 0, sizeof(scores));
 
   // Reset any global variable used in eval
-  pos.this_thread()->depth           = 0;
   pos.this_thread()->trend           = SCORE_ZERO;
   pos.this_thread()->bestValue       = VALUE_ZERO;
   pos.this_thread()->optimism[WHITE] = VALUE_ZERO;