Sometimes change the (materialist, positional) balance

Our new nets output two values for the side to move in the last layer.
We can interpret the first value as a material evaluation of the
position, and the second one as the dynamic, positional value of the
location of pieces.

This patch changes the balance for the (materialist, positional) parts
of the score from (128, 128) to (121, 135) when the piece material is
equal between the two players, but keeps the standard (128, 128) balance
when one player is at least an exchange up.

Passed STC:
LLR: 2.93 (-2.94,2.94) <-0.50,2.50>
Total: 15936 W: 1421 L: 1266 D: 13249
Ptnml(0-2): 37, 1037, 5694, 1134, 66
https://tests.stockfishchess.org/tests/view/60a82df9ce8ea25a3ef0408f

Passed LTC:
LLR: 2.94 (-2.94,2.94) <0.50,3.50>
Total: 13904 W: 516 L: 410 D: 12978
Ptnml(0-2): 4, 374, 6088, 484, 2
https://tests.stockfishchess.org/tests/view/60a8bbf9ce8ea25a3ef04101

closes https://github.com/official-stockfish/Stockfish/pull/3492

Bench: 3856635

src/evaluate.cpp

@@ -1119,7 +1119,7 @@ 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);

src/evaluate.h

@@ -43,7 +43,7 @@ namespace Eval {
 
   namespace NNUE {
 
-    Value evaluate(const Position& pos);
+    Value evaluate(const Position& pos, bool adjusted = false);
     bool load_eval(std::string name, std::istream& stream);
     bool save_eval(std::ostream& stream);
     void init();

src/nnue/evaluate_nnue.cpp

@@ -134,7 +134,7 @@ namespace Stockfish::Eval::NNUE {
   }
 
   // Evaluation function. Perform differential calculation.
-  Value evaluate(const Position& pos) {
+  Value evaluate(const Position& pos, bool adjusted) {
 
     // We manually align the arrays on the stack because with gcc < 9.3
     // overaligning stack variables with alignas() doesn't work correctly.
@@ -158,13 +158,26 @@ namespace Stockfish::Eval::NNUE {
     ASSERT_ALIGNED(buffer, alignment);
 
     const std::size_t bucket = (pos.count<ALL_PIECES>() - 1) / 4;
-
     const auto [psqt, lazy] = featureTransformer->transform(pos, transformedFeatures, bucket);
-    if (lazy) {
+
+    if (lazy)
       return static_cast<Value>(psqt / OutputScale);
-    } else {
+    else
+    {
       const auto output = network[bucket]->propagate(transformedFeatures, buffer);
-      return static_cast<Value>((output[0] + psqt) / OutputScale);
+
+      int materialist = psqt;
+      int positional  = output[0];
+
+      int delta_npm = abs(pos.non_pawn_material(WHITE) - pos.non_pawn_material(BLACK));
+      int entertainment = (adjusted && delta_npm <= BishopValueMg - KnightValueMg ? 7 : 0);
+
+      int A = 128 - entertainment;
+      int B = 128 + entertainment;
+
+      int sum = (A * materialist + B * positional) / 128;
+
+      return static_cast<Value>( sum / OutputScale );
     }
   }