Revert back KBPsK to latest Gary's version

The bug was found to be elsewhere. This version
is correct and also is able to detect as draw
positions like:

8/8/5b2/8/8/4k1p1/6P1/5K2 b - - 6 133

bench: 8678654

diff --git a/src/endgame.cpp b/src/endgame.cpp
index 4ee515ade71cab5c856f3f6c547d1fd959a8820f..311443ec939f82ac7b2af69c836524ba8e8768a4 100644 (file)
--- a/src/endgame.cpp
+++ b/src/endgame.cpp
@@ -364,27 +364,52 @@ ScaleFactor Endgame<KBPsK>::operator()(const Position& pos) const {
   // be detected even when the weaker side has some pawns.
 
   Bitboard pawns = pos.pieces(strongSide, PAWN);
   // be detected even when the weaker side has some pawns.
 
   Bitboard pawns = pos.pieces(strongSide, PAWN);

-  File pawnsFile = file_of(lsb(pawns));
+  File pawnFile = file_of(pos.list<PAWN>(strongSide)[0]);

 
   // All pawns are on a single rook file ?
 
   // All pawns are on a single rook file ?

-  if (    (pawnsFile == FILE_A || pawnsFile == FILE_H)
-      && !(pawns & ~file_bb(pawnsFile)))
+  if (    (pawnFile == FILE_A || pawnFile == FILE_H)
+      && !(pawns & ~file_bb(pawnFile)))

   {
       Square bishopSq = pos.list<BISHOP>(strongSide)[0];
   {
       Square bishopSq = pos.list<BISHOP>(strongSide)[0];

-      Square queeningSq = relative_square(strongSide, make_square(pawnsFile, RANK_8));
+      Square queeningSq = relative_square(strongSide, make_square(pawnFile, RANK_8));

       Square kingSq = pos.king_square(weakSide);
 
       Square kingSq = pos.king_square(weakSide);
 

-      // If the bishop has the wrong color, and the defending king is on the file
-      // of the pawn(s) or the neighboring file, then it's potentially a draw.

       if (   opposite_colors(queeningSq, bishopSq)
       if (   opposite_colors(queeningSq, bishopSq)

-          && file_distance(kingSq, lsb(pawns)) <= 1)
-      {
-          // If the defending king has distance <= 1 to the promotion square or
-          // is placed somewhere in front of the frontmost pawn, it's a draw.
-          Rank rank = relative_rank(strongSide, (frontmost_sq(strongSide, pawns)));
+          && square_distance(queeningSq, kingSq) <= 1)
+          return SCALE_FACTOR_DRAW;
+  }
+
+  // If all the pawns are on the same B or G file, then it's potentially a draw
+  if (    (pawnFile == FILE_B || pawnFile == FILE_G)
+      && !(pos.pieces(PAWN) & ~file_bb(pawnFile))
+      && pos.non_pawn_material(weakSide) == 0
+      && pos.count<PAWN>(weakSide) >= 1)
+  {
+      // Get weakSide pawn that is closest to the home rank
+      Square weakPawnSq = backmost_sq(weakSide, pos.pieces(weakSide, PAWN));
+
+      Square strongKingSq = pos.king_square(strongSide);
+      Square weakKingSq = pos.king_square(weakSide);
+      Square bishopSq = pos.list<BISHOP>(strongSide)[0];

 
 

-          if (   square_distance(kingSq, queeningSq) <= 1
-              || relative_rank(strongSide, kingSq) >= rank)
+      // There's potential for a draw if our pawn is blocked on the 7th rank,
+      // the bishop cannot attack it or they only have one pawn left
+      if (   relative_rank(strongSide, weakPawnSq) == RANK_7
+          && (pos.pieces(strongSide, PAWN) & (weakPawnSq + pawn_push(weakSide)))
+          && (opposite_colors(bishopSq, weakPawnSq) || pos.count<PAWN>(strongSide) == 1))
+      {
+          int strongKingDist = square_distance(weakPawnSq, strongKingSq);
+          int weakKingDist = square_distance(weakPawnSq, weakKingSq);
+
+          // It's a draw if the weak king is on its back two ranks, within 2
+          // squares of the blocking pawn and the strong king is not
+          // closer. (I think this rule only fails in practically
+          // unreachable positions such as 5k1K/6p1/6P1/8/8/3B4/8/8 w
+          // and positions where qsearch will immediately correct the
+          // problem such as 8/4k1p1/6P1/1K6/3B4/8/8/8 w)
+          if (   relative_rank(strongSide, weakKingSq) >= RANK_7
+              && weakKingDist <= 2
+              && weakKingDist <= strongKingDist)

               return SCALE_FACTOR_DRAW;
       }
   }
               return SCALE_FACTOR_DRAW;
       }
   }