FEN parsing: add a second check for correctly setting e.p. square

[stockfish] / src / position.cpp

diff --git a/src/position.cpp b/src/position.cpp
index 9f5816d70b1326919867959e077329cb29215bcc..325ffd1114892aa0e4f1598acc74bc877d430adf 100644 (file)
--- a/src/position.cpp
+++ b/src/position.cpp
@@ -176,8 +176,9 @@ 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 regardless of whether
-      there is a pawn in position to make an en passant capture.
+      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.
 
    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
@@ -254,7 +255,8 @@ Position& Position::set(const string& fenStr, bool isChess960, StateInfo* si, Th
   {
       st->epSquare = make_square(File(col - 'a'), Rank(row - '1'));
 
-      if (!(attackers_to(st->epSquare) & pieces(sideToMove, PAWN)))
+      if (   !(attackers_to(st->epSquare) & pieces(sideToMove, PAWN))
+          || !(pieces(~sideToMove, PAWN) & (st->epSquare + pawn_push(~sideToMove))))
           st->epSquare = SQ_NONE;
   }
   else