Only set ep square if ep capture is possible

rn5f107s2 · vondele · vondele · commit 94175524b1c0 · 2025-08-04T15:16:21.000+02:00
for positions such as: position fen rr6/2q2p1k/2P1b1pp/bB2P1n1/R2B2PN/p4P1P/P1Q4K/1R6 b - - 2 38 moves f7f5 position fen 8/p2r1pK1/6p1/1kp1P1P1/2p5/2P5/8/4R3 b - - 0 43 moves f7f5 position fen 4k3/4p3/2b3b1/3P1P2/4K3/8/8/8 b - - moves e7e5 ep is not possible for various reasons. Do not set the ep square and the do not change the zobrist key, as if ep were possible. This fixes 3-fold detection, which could in rare cases be observed as a warning generated by fastchess for PVs that continued beyond an actual 3-fold. Also fixes wrong evals for certain moves e.g. a2a1 for position fen 4k3/1b2p2b/8/3P1P2/4K3/8/8/r7 b - - 0 1 moves e7e5 e4e3 a1a2 e3e4 a2a1 e4f3 a1a2 f3e4 fixes #6138 originally written and tested by rn5f107s2, with small buglet as https://tests.stockfishchess.org/tests/view/685af90843ce022d15794400 failed STC LLR: -2.93 (-2.94,2.94) <-1.75,0.25> Total: 130688 W: 33986 L: 34362 D: 62340 Ptnml(0-2): 414, 13292, 38302, 12928, 408 https://tests.stockfishchess.org/tests/view/688bcb35502b34dd5e7111c9 passed LTC LLR: 2.94 (-2.94,2.94) <-1.75,0.25> Total: 99018 W: 25402 L: 25273 D: 48343 Ptnml(0-2): 54, 9097, 31089, 9204, 65 https://tests.stockfishchess.org/tests/view/688c613c502b34dd5e7112d3 #6211 Bench: 2946135 Co-authored-by: Joost VandeVondele <Joost.VandeVondele@gmail.com>
diff --git a/src/Makefile b/src/Makefile
@@ -903,7 +903,7 @@ help:
 	echo "Supported archs:" && \
 	echo "" && \
 	echo "native                  > select the best architecture for the host processor (default)" && \
-	echo "x86-64-avx512icl        > x86 64-bit with minimum avx512 support of Intel Ice Lane or AMD Zen 4" && \
+	echo "x86-64-avx512icl        > x86 64-bit with minimum avx512 support of Intel Ice Lake or AMD Zen 4" && \
 	echo "x86-64-vnni512          > x86 64-bit with vnni 512bit support" && \
 	echo "x86-64-vnni256          > x86 64-bit with vnni 512bit support, limit operands to 256bit wide" && \
 	echo "x86-64-avx512           > x86 64-bit with avx512 support" && \
diff --git a/src/position.cpp b/src/position.cpp
@@ -717,6 +717,8 @@ DirtyPiece Position::do_move(Move                      m,
     Piece  pc       = piece_on(from);
     Piece  captured = m.type_of() == EN_PASSANT ? make_piece(them, PAWN) : piece_on(to);
 
+    bool checkEP = false;
+
     DirtyPiece dp;
     dp.pc     = pc;
     dp.from   = from;
@@ -804,21 +806,14 @@ DirtyPiece Position::do_move(Move                      m,
 
     // Move the piece. The tricky Chess960 castling is handled earlier
     if (m.type_of() != CASTLING)
-    {
-
         move_piece(from, to);
-    }
 
     // If the moving piece is a pawn do some special extra work
     if (type_of(pc) == PAWN)
     {
-        // Set en passant square if the moved pawn can be captured
-        if ((int(to) ^ int(from)) == 16
-            && (attacks_bb<PAWN>(to - pawn_push(us), us) & pieces(them, PAWN)))
-        {
-            st->epSquare = to - pawn_push(us);
-            k ^= Zobrist::enpassant[file_of(st->epSquare)];
-        }
+        // Check later if the en passant square needs to be set
+        if ((int(to) ^ int(from)) == 16)
+            checkEP = true;
 
         else if (m.type_of() == PROMOTION)
         {
@@ -863,11 +858,6 @@ DirtyPiece Position::do_move(Move                      m,
             st->minorPieceKey ^= Zobrist::psq[pc][from] ^ Zobrist::psq[pc][to];
     }
 
-    // Update the key with the final value
-    st->key = k;
-    if (tt)
-        prefetch(tt->first_entry(key()));
-
     // Set capture piece
     st->capturedPiece = captured;
 
@@ -879,6 +869,63 @@ DirtyPiece Position::do_move(Move                      m,
     // Update king attacks used for fast check detection
     set_check_info();
 
+    // Accurate e.p. info is needed for correct zobrist key generation and 3-fold checking
+    while (checkEP)
+    {
+        auto updateEpSquare = [&] {
+            st->epSquare = to - pawn_push(us);
+            k ^= Zobrist::enpassant[file_of(st->epSquare)];
+        };
+
+        Bitboard pawns = attacks_bb<PAWN>(to - pawn_push(us), us) & pieces(them, PAWN);
+
+        // If there are no pawns attacking the ep square, ep is not possible
+        if (!pawns)
+            break;
+
+        // If there are checkers other than the to be captured pawn, ep is never legal
+        if (checkers() & ~square_bb(to))
+            break;
+
+        if (more_than_one(pawns))
+        {
+            // If there are two pawns potentially being abled to capture and at least one
+            // is not pinned, ep is legal as there are no horizontal exposed checks
+            if (!more_than_one(blockers_for_king(them) & pawns))
+            {
+                updateEpSquare();
+                break;
+            }
+
+            // If there is no pawn on our king's file, and thus both pawns are pinned
+            // by bishops, ep is not legal as the king square must be in front of the to square.
+            // And because the ep square and the king are not on a common diagonal, either ep capture
+            // would expose the king to a check from one of the bishops
+            if (!(file_bb(square<KING>(them)) & pawns))
+                break;
+
+            // Otherwise remove the pawn on the king file, as an ep capture by it can never be legal and the
+            // check below relies on there only being one pawn
+            pawns &= ~file_bb(square<KING>(them));
+        }
+
+        Square   ksq      = square<KING>(them);
+        Square   capsq    = to;
+        Bitboard occupied = (pieces() ^ lsb(pawns) ^ capsq) | (to - pawn_push(us));
+
+        // If our king is not attacked after making the move, ep is legal.
+        if (!(attacks_bb<ROOK>(ksq, occupied) & pieces(us, QUEEN, ROOK))
+            && !(attacks_bb<BISHOP>(ksq, occupied) & pieces(us, QUEEN, BISHOP)))
+            updateEpSquare();
+
+        break;
+    }
+
+    // Update the key with the final value
+    st->key = k;
+    if (tt)
+        prefetch(tt->first_entry(key()));
+
     // Calculate the repetition info. It is the ply distance from the previous
     // occurrence of the same position, negative in the 3-fold case, or zero
     // if the position was not repeated.
