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Clean-up en passant processing

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the goal of this PR is to better document how we process
the ep square (if any) given position fen command, and to
output more meaningful (and consistent) debug fen on the "d"
command. The implementation follows
https://en.wikipedia.org/wiki/X-FEN#Encoding_en-passant
following x-fen, it is "valid" to record ep even if ep would put king en prise.

fixes #2784

closes https://github.com/official-stockfish/Stockfish/pull/2797

No functional change
This commit is contained in:
Alain SAVARD 2020-07-06 22:43:54 -04:00 • committed by Joost VandeVondele
parent 7225d254f9
commit 76a039027d
1 changed files with 17 additions and 9 deletions
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26
src/position.cpp
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@ -178,9 +178,9 @@ Position& Position::set(const string& fenStr, bool isChess960, StateInfo* si, Th
4) En passant target square (in algebraic notation). If there's no en passant
target square, this is "-". If a pawn has just made a 2-square move, this
is the position "behind" the pawn. This is recorded only if there is a pawn
in position to make an en passant capture, and if there really is a pawn
that might have advanced two squares.
is the position "behind" the pawn. Following X-FEN standard, this is recorded only
if there is a pawn in position to make an en passant capture, and if there really
is a pawn that might have advanced two squares.
5) Halfmove clock. This is the number of halfmoves since the last pawn advance
or capture. This is used to determine if a draw can be claimed under the
@ -251,17 +251,25 @@ Position& Position::set(const string& fenStr, bool isChess960, StateInfo* si, Th
set_castling_right(c, rsq);
}
// 4. En passant square. Ignore if no pawn capture is possible
// 4. En passant square.
// Ignore if square is invalid or not on side to move relative rank 6.
bool enpassant = false;
if ( ((ss >> col) && (col >= 'a' && col <= 'h'))
&& ((ss >> row) && (row == '3' || row == '6')))
&& ((ss >> row) && (row == (sideToMove == WHITE ? '6' : '3'))))
{
st->epSquare = make_square(File(col - 'a'), Rank(row - '1'));
if ( !(attackers_to(st->epSquare) & pieces(sideToMove, PAWN))
|| !(pieces(~sideToMove, PAWN) & (st->epSquare + pawn_push(~sideToMove))))
st->epSquare = SQ_NONE;
// En passant square will be considered only if
// a) side to move have a pawn threatening epSquare
// b) there is an enemy pawn in front of epSquare
// c) there is no piece on epSquare or behind epSquare
enpassant = pawn_attacks_bb(~sideToMove, st->epSquare) & pieces(sideToMove, PAWN)
&& (pieces(~sideToMove, PAWN) & (st->epSquare + pawn_push(~sideToMove)))
&& !(pieces() & (st->epSquare | (st->epSquare + pawn_push(sideToMove))));
}
else
if (!enpassant)
st->epSquare = SQ_NONE;
// 5-6. Halfmove clock and fullmove number