Keep pawns on both flanks

Positions with pawns on only one flank tend to be more drawish. We add
a term to the initiative bonus to help the attacking player keep pawns
on both flanks.

STC: yellowish run stopped after 257137 games
LLR: -0.92 (-2.94,2.94) [0.00,5.00]
Total: 257137 W: 46560 L: 45511 D: 165066

LTC:
LLR: 2.95 (-2.94,2.94) [0.00,5.00]
Total: 15602 W: 2125 L: 1956 D: 11521

Bench : 6976310

Closes #1009

diff --git a/src/evaluate.cpp b/src/evaluate.cpp
index 62620a77bad7f0ef0b4add0e073c099c6a478249..6180c482ff79e7372c98940c6c5d01c4f1cec6d3 100644 (file)
--- a/src/evaluate.cpp
+++ b/src/evaluate.cpp
@@ -377,11 +377,12 @@ namespace {
 
   // evaluate_king() assigns bonuses and penalties to a king of a given color
 
 
   // evaluate_king() assigns bonuses and penalties to a king of a given color
 

+  const Bitboard QueenSide   = FileABB | FileBBB | FileCBB | FileDBB;

   const Bitboard CenterFiles = FileCBB | FileDBB | FileEBB | FileFBB;
   const Bitboard CenterFiles = FileCBB | FileDBB | FileEBB | FileFBB;

+  const Bitboard KingSide    = FileEBB | FileFBB | FileGBB | FileHBB;

 
   const Bitboard KingFlank[FILE_NB] = {
 
   const Bitboard KingFlank[FILE_NB] = {

-    CenterFiles >> 2, CenterFiles >> 2, CenterFiles >> 2, CenterFiles, CenterFiles,
-    CenterFiles << 2, CenterFiles << 2, CenterFiles << 2
+    QueenSide, QueenSide, QueenSide, CenterFiles, CenterFiles, KingSide, KingSide, KingSide

   };
 
   template<Color Us, bool DoTrace>
   };
 
   template<Color Us, bool DoTrace>


@@ -724,14 +725,15 @@ namespace {
     int kingDistance =  distance<File>(pos.square<KING>(WHITE), pos.square<KING>(BLACK))
                       - distance<Rank>(pos.square<KING>(WHITE), pos.square<KING>(BLACK));
     int pawns = pos.count<PAWN>(WHITE) + pos.count<PAWN>(BLACK);
     int kingDistance =  distance<File>(pos.square<KING>(WHITE), pos.square<KING>(BLACK))
                       - distance<Rank>(pos.square<KING>(WHITE), pos.square<KING>(BLACK));
     int pawns = pos.count<PAWN>(WHITE) + pos.count<PAWN>(BLACK);

+    bool bothFlanks = (pos.pieces(PAWN) & QueenSide) && (pos.pieces(PAWN) & KingSide);

 
     // Compute the initiative bonus for the attacking side
 
     // Compute the initiative bonus for the attacking side

-    int initiative = 8 * (asymmetry + kingDistance - 15) + 12 * pawns;
+    int initiative = 8 * (asymmetry + kingDistance - 17) + 12 * pawns + 16 * bothFlanks;

 
     // Now apply the bonus: note that we find the attacking side by extracting
     // the sign of the endgame value, and that we carefully cap the bonus so
 
     // Now apply the bonus: note that we find the attacking side by extracting
     // the sign of the endgame value, and that we carefully cap the bonus so

-    // that the endgame score will never be divided by more than two.
-    int value = ((eg > 0) - (eg < 0)) * std::max(initiative, -abs(eg / 2));
+    // that the endgame score will never change sign after the bonus.
+    int value = ((eg > 0) - (eg < 0)) * std::max(initiative, -abs(eg));

 
     return make_score(0, value);
   }
 
     return make_score(0, value);
   }