Fix check for bishop pair in material imbalance

Prefer pos.bishop_pair() to pos.count<BISHOP>(WHITE) > 1
because the first checks that the two bishops are on
different color squares.

Although the change seems to kick in only in very rare cases,
quite surprisingly it was able to pass SPRT test at short TC.

LLR: 2.95 (-2.94,2.94)
Total: 39818 W: 8174 L: 7956 D: 23688

bench: 3453941

diff --git a/src/material.cpp b/src/material.cpp
index 0f1e19b..8888295 100644 (file)
--- a/src/material.cpp
+++ b/src/material.cpp
@@ -263,14 +263,14 @@ Entry* probe(const Position& pos, Table& entries, Endgames& endgames) {
       e->spaceWeight = make_score(minorPieceCount * minorPieceCount, 0);
   }
 
-  // Evaluate the material imbalance. We use PIECE_TYPE_NONE as a place holder
+  // Evaluate the material imbalance. We use NO_PIECE_TYPE as a place holder
   // for the bishop pair "extended piece", this allow us to be more flexible
   // in defining bishop pair bonuses.
   const int pieceCount[COLOR_NB][PIECE_TYPE_NB] = {
-  { pos.count<BISHOP>(WHITE) > 1, pos.count<PAWN>(WHITE), pos.count<KNIGHT>(WHITE),
-    pos.count<BISHOP>(WHITE)    , pos.count<ROOK>(WHITE), pos.count<QUEEN >(WHITE) },
-  { pos.count<BISHOP>(BLACK) > 1, pos.count<PAWN>(BLACK), pos.count<KNIGHT>(BLACK),
-    pos.count<BISHOP>(BLACK)    , pos.count<ROOK>(BLACK), pos.count<QUEEN >(BLACK) } };
+  { pos.bishop_pair(WHITE)  , pos.count<PAWN>(WHITE), pos.count<KNIGHT>(WHITE),
+    pos.count<BISHOP>(WHITE), pos.count<ROOK>(WHITE), pos.count<QUEEN >(WHITE) },
+  { pos.bishop_pair(BLACK)  , pos.count<PAWN>(BLACK), pos.count<KNIGHT>(BLACK),
+    pos.count<BISHOP>(BLACK), pos.count<ROOK>(BLACK), pos.count<QUEEN >(BLACK) } };
 
   e->value = (int16_t)((imbalance<WHITE>(pieceCount) - imbalance<BLACK>(pieceCount)) / 16);
   return e;