Blocked pawn storm

In pawn structures like white pawns f6,h6 against black pawns f7,g6,h7
the attack on the king is blocked by the own pawns. So decrease the
penalty for king safety.

See diagram and discussion in
https://github.com/official-stockfish/Stockfish/pull/1434

A sample position that this patch wants to avoid is the following
1rr2bk1/3q1p1p/2n1bPpP/pp1pP3/2pP4/P1P1B3/1PBQN1P1/1K3R1R w - - 0 1

White pawn storm on the king side was a disaster, it locked the king
side completely. Therefore, all the king tropism bonus that white have
on the king side are useless, and kingadjacent attacks too. Master
gives White a static +4.5 advantage, but White cannot win that game.
The patch is lowering this evaluation artefact.

STC:
LLR: 2.94 (-2.94,2.94) [0.00,5.00]
Total: 16467 W: 3750 L: 3537 D: 9180
http://tests.stockfishchess.org/tests/view/5a92102d0ebc590297cc87d0

LTC:
LLR: 2.96 (-2.94,2.94) [0.00,5.00]
Total: 64242 W: 11130 L: 10745 D: 42367
http://tests.stockfishchess.org/tests/view/5a923dc80ebc590297cc8806

This version includes reformatting and speed optimization by Alain Savard.

Bench: 5643527

diff --git a/src/pawns.cpp b/src/pawns.cpp
index 7e90089ce7921ba7d1d666c4eb965b8e04a00676..42819e142e5e7b12e2f7ad2c4a6159ab71962cd3 100644 (file)
--- a/src/pawns.cpp
+++ b/src/pawns.cpp
@@ -237,14 +237,20 @@ template<Color Us>
 Value Entry::shelter_storm(const Position& pos, Square ksq) {
 
   const Color Them = (Us == WHITE ? BLACK : WHITE);
 Value Entry::shelter_storm(const Position& pos, Square ksq) {
 
   const Color Them = (Us == WHITE ? BLACK : WHITE);

+  const Bitboard ShelterMask = (Us == WHITE ? 1ULL << SQ_A2 | 1ULL << SQ_B3 | 1ULL << SQ_C2 | 1ULL << SQ_F2 | 1ULL << SQ_G3 | 1ULL << SQ_H2
+                                            : 1ULL << SQ_A7 | 1ULL << SQ_B6 | 1ULL << SQ_C7 | 1ULL << SQ_F7 | 1ULL << SQ_G6 | 1ULL << SQ_H7);
+  const Bitboard StormMask   = (Us == WHITE ? 1ULL << SQ_A3 | 1ULL << SQ_C3 | 1ULL << SQ_F3 | 1ULL << SQ_H3
+                                            : 1ULL << SQ_A6 | 1ULL << SQ_C6 | 1ULL << SQ_F6 | 1ULL << SQ_H6);

 
   enum { BlockedByKing, Unopposed, BlockedByPawn, Unblocked };
 
 
   enum { BlockedByKing, Unopposed, BlockedByPawn, Unblocked };
 

-  Bitboard b = pos.pieces(PAWN) & (forward_ranks_bb(Us, ksq) | rank_bb(ksq));
+  File center = std::max(FILE_B, std::min(FILE_G, file_of(ksq)));
+  Bitboard b =   pos.pieces(PAWN)
+               & (forward_ranks_bb(Us, ksq) | rank_bb(ksq))
+               & (adjacent_files_bb(center) | file_bb(center));

   Bitboard ourPawns = b & pos.pieces(Us);
   Bitboard theirPawns = b & pos.pieces(Them);
   Value safety = MaxSafetyBonus;
   Bitboard ourPawns = b & pos.pieces(Us);
   Bitboard theirPawns = b & pos.pieces(Them);
   Value safety = MaxSafetyBonus;

-  File center = std::max(FILE_B, std::min(FILE_G, file_of(ksq)));

 
   for (File f = File(center - 1); f <= File(center + 1); ++f)
   {
 
   for (File f = File(center - 1); f <= File(center + 1); ++f)
   {


@@ -263,6 +269,9 @@ Value Entry::shelter_storm(const Position& pos, Square ksq) {
                  [d][rkThem];
   }
 
                  [d][rkThem];
   }
 

+  if (popcount((ourPawns & ShelterMask) | (theirPawns & StormMask)) == 5)
+      safety += Value(300);
+

   return safety;
 }
 
   return safety;
 }