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Revert back KBPsK to latest Gary's version

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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
This commit is contained in:
Marco Costalba 2014-05-04 09:31:25 +02:00
parent c0d3010438
commit f1240483fb
1 changed files with 38 additions and 13 deletions
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51
src/endgame.cpp
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@ -364,27 +364,52 @@ ScaleFactor Endgame<KBPsK>::operator()(const Position& pos) const {
// be detected even when the weaker side has some pawns. // be detected even when the weaker side has some pawns.
Bitboard pawns = pos.pieces(strongSide, PAWN); 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) if ( (pawnFile == FILE_A || pawnFile == FILE_H)
&& !(pawns & ~file_bb(pawnsFile))) && !(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) && square_distance(queeningSq, kingSq) <= 1)
{ return SCALE_FACTOR_DRAW;
// 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)));
if ( square_distance(kingSq, queeningSq) <= 1 // If all the pawns are on the same B or G file, then it's potentially a draw
|| relative_rank(strongSide, kingSq) >= rank) 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];
// 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;
} }
} }