Scale factor in opposite-color bishop endings

This change varies the scale factor with the total number of pieces and pawns on the strongSide.

STC :
LLR: 2.94 (-2.94,2.94) {-0.50,1.50}
Total: 150920 W: 28828 L: 28422 D: 93670 +0.65 Elo
Ptnml(0-2): 2507, 17548, 35030, 17782, 2593
https://tests.stockfishchess.org/tests/view/5e983eb2c00499c5410f4951

LTC :
LLR: 2.95 (-2.94,2.94) {0.25,1.75}
Total: 69238 W: 8810 L: 8446 D: 51982 +1.58 Elo
Ptnml(0-2): 451, 6276, 20879, 6484, 529
https://tests.stockfishchess.org/tests/view/5e985b27c00499c5410f4987

closes https://github.com/official-stockfish/Stockfish/pull/2637

Bench 4821332

diff --git a/src/evaluate.cpp b/src/evaluate.cpp
index 0b1956f10824cd6cb66c8cf6d2c20067312723b5..cbcebd045f684d392975bbe7373648d6e152b09c 100644 (file)
--- a/src/evaluate.cpp
+++ b/src/evaluate.cpp
@@ -761,11 +761,16 @@ namespace {
     // If scale is not already specific, scale down the endgame via general heuristics
     if (sf == SCALE_FACTOR_NORMAL)
     {
     // If scale is not already specific, scale down the endgame via general heuristics
     if (sf == SCALE_FACTOR_NORMAL)
     {

-        if (   pos.opposite_bishops()
-            && pos.non_pawn_material() == 2 * BishopValueMg)
-            sf = 22;
+        if (pos.opposite_bishops())
+        {
+            if (   pos.non_pawn_material(WHITE) == BishopValueMg
+                && pos.non_pawn_material(BLACK) == BishopValueMg)
+                sf = 22;
+            else
+                sf = 22 + 3 * pos.count<ALL_PIECES>(strongSide);
+        }

         else
         else

-            sf = std::min(sf, 36 + (pos.opposite_bishops() ? 2 : 7) * pos.count<PAWN>(strongSide));
+            sf = std::min(sf, 36 + 7 * pos.count<PAWN>(strongSide));

 
         sf = std::max(0, sf - (pos.rule50_count() - 12) / 4);
     }
 
         sf = std::max(0, sf - (pos.rule50_count() - 12) / 4);
     }