Simplify the use of classical eval

no benefit of the fallback term (exercised rarely).
Cleanup the associated code.

passed STC
https://tests.stockfishchess.org/tests/view/62f62c2b6f0a08af9f776367
LLR: 2.96 (-2.94,2.94) <-1.75,0.25>
Total: 67832 W: 18334 L: 18148 D: 31350
Ptnml(0-2): 369, 7171, 18609, 7439, 328

passed LTC
https://tests.stockfishchess.org/tests/view/62f68beb6f0a08af9f77710e
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 104664 W: 28363 L: 28233 D: 48068
Ptnml(0-2): 169, 10162, 31511, 10350, 140

closes https://github.com/official-stockfish/Stockfish/pull/4132

Bench: 6079565

diff --git a/src/evaluate.cpp b/src/evaluate.cpp
index 7d5876758c0836e5ad43e363e4dd9f5f719502aa..eaad4d556722cb0b2f99d8e0984865f2c549f8c1 100644 (file)
--- a/src/evaluate.cpp
+++ b/src/evaluate.cpp
@@ -1053,34 +1053,29 @@ Value Eval::evaluate(const Position& pos, int* complexity) {
   Value v;
   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,
-  // but we switch to NNUE during long shuffling or with high material on the board.
-  bool useClassical =    (pos.count<ALL_PIECES>() > 7)
-                      && abs(psq) * 5 > (856 + pos.non_pawn_material() / 64) * (10 + pos.rule50_count());
 
-  // 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 || useClassical)
-  {
-      v = Evaluation<NO_TRACE>(pos).value();
-      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
   {
-       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;
+      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
@@ -1091,7 +1086,7 @@ Value Eval::evaluate(const Position& pos, int* complexity) {
 
   // When not using NNUE, return classical complexity to caller
   if (complexity && (!useNNUE || useClassical))
-       *complexity = abs(v - psq);
+      *complexity = abs(v - psq);
 
   return v;
 }