Clean-up en passant processing

[stockfish] / src / position.cpp

diff --git a/src/position.cpp b/src/position.cpp
index 6ef7aedc23f5c3aaa51402e127f07a4dac0b2a60..396bff5f545c0f92bde33d12f23810e9d6d98745 100644 (file)
--- a/src/position.cpp
+++ b/src/position.cpp
@@ -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