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Only record the ep square from a given FEN if ep is legal

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When given a FEN with an ep square set, only actually set the ep square if there is a legal ep capture that can be played.

Fixes #6563

closes https://github.com/official-stockfish/Stockfish/pull/6564

No functional change
This commit is contained in:
rn5f107s2
2026-02-04 17:56:00 +01:00
committed by Joost VandeVondele
parent 9f968446cb
commit 7f0b5d10ee
1 changed files with 12 additions and 4 deletions
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src/position.cpp
+12 -4
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@@ -264,23 +264,31 @@ Position& Position::set(const string& fenStr, bool isChess960, StateInfo* si) {
// 4. En passant square.
// Ignore if square is invalid or not on side to move relative rank 6.
bool enpassant = false;
bool enpassant = false, legalEP = false;
if (((ss >> col) && (col >= 'a' && col <= 'h'))
&& ((ss >> row) && (row == (sideToMove == WHITE ? '6' : '3'))))
{
st->epSquare = make_square(File(col - 'a'), Rank(row - '1'));
Bitboard pawns = attacks_bb<PAWN>(st->epSquare, ~sideToMove) & pieces(sideToMove, PAWN);
Bitboard target = (pieces(~sideToMove, PAWN) & (st->epSquare + pawn_push(~sideToMove)));
Bitboard occ = pieces() ^ target ^ st->epSquare;
// 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 = attacks_bb<PAWN>(st->epSquare, ~sideToMove) & pieces(sideToMove, PAWN)
&& (pieces(~sideToMove, PAWN) & (st->epSquare + pawn_push(~sideToMove)))
enpassant = pawns
&& target
&& !(pieces() & (st->epSquare | (st->epSquare + pawn_push(sideToMove))));
// If no pawn can execute the en passant capture without leaving the king in check, don't record the epSquare
while (pawns)
legalEP |= !(attackers_to(square<KING>(sideToMove), occ ^ pop_lsb(pawns)) & pieces(~sideToMove) & ~target);
}
if (!enpassant)
if (!enpassant || !legalEP)
st->epSquare = SQ_NONE;
// 5-6. Halfmove clock and fullmove number