Scale down eval for queen imbalance

We lower the endgame value of the evaluation when we detect that there
is only one queen left on the board (more precisely, we use a scale
factor of 37/64, or about 0.58, for the endgame part of the evaluation).
Hopefully this helps a little bit for the assessment of positions with
queen imbalance, which are one of the well-known Stockfish weaknesses.

STC:
LLR: 2.94 (-2.94,2.94) {-0.50,1.50}
Total: 21600 W: 4176 L: 3955 D: 13469
Ptnml(0-2): 351, 2457, 5003, 2598, 391
https://tests.stockfishchess.org/tests/view/5ef871b6020eec13834a94e8

LTC:
LLR: 2.97 (-2.94,2.94) {0.25,1.75}
Total: 248328 W: 30596 L: 29720 D: 188012
Ptnml(0-2): 1544, 22345, 75665, 22911, 1699
https://tests.stockfishchess.org/tests/view/5ef87aec020eec13834a94fe

Closes https://github.com/official-stockfish/Stockfish/pull/2781

Bench: 4441323

diff --git a/src/evaluate.cpp b/src/evaluate.cpp
index 65f7bddc76e3c4f626ac095db897f3cc9458416d..d19cf34e31ef24ae257251b71e6836ab4d55427d 100644 (file)
--- a/src/evaluate.cpp
+++ b/src/evaluate.cpp
@@ -767,7 +767,6 @@ namespace {
     eg += v;
 
     // Compute the scale factor for the winning side
-
     Color strongSide = eg > VALUE_DRAW ? WHITE : BLACK;
     int sf = me->scale_factor(pos, strongSide);
 
@@ -782,13 +781,15 @@ namespace {
             else
                 sf = 22 + 3 * pos.count<ALL_PIECES>(strongSide);
         }
-        else if(   pos.non_pawn_material(WHITE) == RookValueMg
+        else if (  pos.non_pawn_material(WHITE) == RookValueMg
                 && pos.non_pawn_material(BLACK) == RookValueMg
                 && !pe->passed_pawns(strongSide)
                 && pos.count<PAWN>(strongSide) - pos.count<PAWN>(~strongSide) <= 1
                 && bool(KingSide & pos.pieces(strongSide, PAWN)) != bool(QueenSide & pos.pieces(strongSide, PAWN))
                 && (attacks_bb<KING>(pos.square<KING>(~strongSide)) & pos.pieces(~strongSide, PAWN)))
             sf = 36;
+        else if (pos.count<QUEEN>() == 1)
+            sf = 37;
         else
             sf = std::min(sf, 36 + 7 * pos.count<PAWN>(strongSide));
     }