Revert "Simplify En Passant"

This reverts commit 9f8058bd26df1c3ca37b85f811026f1eb82e6524.

Fixes the memory leak discussed in pull request #3523
https://github.com/official-stockfish/Stockfish/pull/3523

Passed non-regression STC:
LLR: 2.95 (-2.94,2.94) <-2.50,0.50>
Total: 76184 W: 6330 L: 6282 D: 63572
Ptnml(0-2): 202, 5047, 27564, 5059, 220
https://tests.stockfishchess.org/tests/view/60ba146c457376eb8bcaa2e2

closes https://github.com/official-stockfish/Stockfish/pull/3527

Benched to verify there is no functional change.

Bench: 4364128

diff --git a/src/position.cpp b/src/position.cpp
index f1c36156a3b08551393f829ddcf43aad872af91c..56cb34e8ec3c39d7e4219e957c032c1d997627d1 100644 (file)
--- a/src/position.cpp
+++ b/src/position.cpp
@@ -251,8 +251,6 @@ Position& Position::set(const string& fenStr, bool isChess960, StateInfo* si, Th
       set_castling_right(c, rsq);
   }
 
-  set_state(st);
-
   // 4. En passant square.
   // Ignore if square is invalid or not on side to move relative rank 6.
   bool enpassant = false;
@@ -266,24 +264,12 @@ Position& Position::set(const string& fenStr, bool isChess960, StateInfo* si, Th
       // 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
-      // d) enemy pawn didn't block a check of its own color by moving forward
       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))))
-               && (   file_of(square<KING>(sideToMove)) == file_of(st->epSquare)
-                   || !(blockers_for_king(sideToMove) & (st->epSquare + pawn_push(~sideToMove))));
+               && !(pieces() & (st->epSquare | (st->epSquare + pawn_push(sideToMove))));
   }
 
-  // It's necessary for st->previous to be intialized in this way because legality check relies on its existence
-  if (enpassant) {
-      st->previous = new StateInfo();
-      remove_piece(st->epSquare - pawn_push(sideToMove));
-      st->previous->checkersBB = attackers_to(square<KING>(~sideToMove)) & pieces(sideToMove);
-      st->previous->blockersForKing[WHITE] = slider_blockers(pieces(BLACK), square<KING>(WHITE), st->previous->pinners[BLACK]);
-      st->previous->blockersForKing[BLACK] = slider_blockers(pieces(WHITE), square<KING>(BLACK), st->previous->pinners[WHITE]);
-      put_piece(make_piece(~sideToMove, PAWN), st->epSquare - pawn_push(sideToMove));
-  }
-  else
+  if (!enpassant)
       st->epSquare = SQ_NONE;
 
   // 5-6. Halfmove clock and fullmove number
@@ -295,6 +281,7 @@ Position& Position::set(const string& fenStr, bool isChess960, StateInfo* si, Th
 
   chess960 = isChess960;
   thisThread = th;
+  set_state(st);
   st->accumulator.state[WHITE] = Eval::NNUE::INIT;
   st->accumulator.state[BLACK] = Eval::NNUE::INIT;
 
@@ -517,11 +504,23 @@ bool Position::legal(Move m) const {
   assert(color_of(moved_piece(m)) == us);
   assert(piece_on(square<KING>(us)) == make_piece(us, KING));
 
-  // st->previous->blockersForKing consider capsq as empty.
-  // If pinned, it has to move along the king ray.
+  // En passant captures are a tricky special case. Because they are rather
+  // uncommon, we do it simply by testing whether the king is attacked after
+  // the move is made.
   if (type_of(m) == EN_PASSANT)
-      return   !(st->previous->blockersForKing[sideToMove] & from)
-            || aligned(from, to, square<KING>(us));
+  {
+      Square ksq = square<KING>(us);
+      Square capsq = to - pawn_push(us);
+      Bitboard occupied = (pieces() ^ from ^ capsq) | to;
+
+      assert(to == ep_square());
+      assert(moved_piece(m) == make_piece(us, PAWN));
+      assert(piece_on(capsq) == make_piece(~us, PAWN));
+      assert(piece_on(to) == NO_PIECE);
+
+      return   !(attacks_bb<  ROOK>(ksq, occupied) & pieces(~us, QUEEN, ROOK))
+            && !(attacks_bb<BISHOP>(ksq, occupied) & pieces(~us, QUEEN, BISHOP));
+  }
 
   // Castling moves generation does not check if the castling path is clear of
   // enemy attacks, it is delayed at a later time: now!
@@ -654,15 +653,18 @@ bool Position::gives_check(Move m) const {
   case PROMOTION:
       return attacks_bb(promotion_type(m), to, pieces() ^ from) & square<KING>(~sideToMove);
 
-  // The double-pushed pawn blocked a check? En Passant will remove the blocker.
-  // The only discovery check that wasn't handle is through capsq and fromsq
-  // So the King must be in the same rank as fromsq to consider this possibility.
-  // st->previous->blockersForKing consider capsq as empty.
+  // En passant capture with check? We have already handled the case
+  // of direct checks and ordinary discovered check, so the only case we
+  // need to handle is the unusual case of a discovered check through
+  // the captured pawn.
   case EN_PASSANT:
-      return st->previous->checkersBB
-          || (   rank_of(square<KING>(~sideToMove)) == rank_of(from)
-              && st->previous->blockersForKing[~sideToMove] & from);
+  {
+      Square capsq = make_square(file_of(to), rank_of(from));
+      Bitboard b = (pieces() ^ from ^ capsq) | to;
 
+      return  (attacks_bb<  ROOK>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, ROOK))
+            | (attacks_bb<BISHOP>(square<KING>(~sideToMove), b) & pieces(sideToMove, QUEEN, BISHOP));
+  }
   default: //CASTLING
   {
       // Castling is encoded as 'king captures the rook'