Ad-hoc shelter rule

Ralph Stößer · mcostalba · commit 402a7ae151cf · 2014-01-12T22:48:08.000+01:00
This hacky rule allows to get an about right eval out of this position:
r2qk2r/ppp2p2/2npbn2/2b1p3/2P1P1P1/2NB1PPp/PPNP3K/R1BQ1R2 b kq - 0 13

And, more importantly, passed both short TC:
LLR: 2.95 (-2.94,2.94) [-1.50,4.50]
Total: 6239 W: 1249 L: 1127 D: 3863

And long TC:
LLR: 2.96 (-2.94,2.94) [0.00,6.00]
Total: 38371 W: 6165 L: 5888 D: 26318

bench: 8183238
diff --git a/src/pawns.cpp b/src/pawns.cpp
@@ -240,6 +240,7 @@ template<Color Us>
 Value Entry::shelter_storm(const Position& pos, Square ksq) {
 
   const Color Them = (Us == WHITE ? BLACK : WHITE);
+  static const Bitboard MiddleEdges = (FileABB | FileHBB) & (Rank2BB | Rank3BB);
 
   Value safety = MaxSafetyBonus;
   Bitboard b = pos.pieces(PAWN) & (in_front_bb(Us, rank_of(ksq)) | rank_bb(ksq));
@@ -252,11 +253,17 @@ Value Entry::shelter_storm(const Position& pos, Square ksq) {
   {
       b = ourPawns & file_bb(f);
       rkUs = b ? relative_rank(Us, backmost_sq(Us, b)) : RANK_1;
-      safety -= ShelterWeakness[rkUs];
 
       b  = theirPawns & file_bb(f);
       rkThem = b ? relative_rank(Us, frontmost_sq(Them, b)) : RANK_1;
-      safety -= StormDanger[rkUs == RANK_1 ? 0 : rkThem == rkUs + 1 ? 2 : 1][rkThem];
+
+      if (   (MiddleEdges & (f | rkThem))
+          && file_of(ksq) == f
+          && relative_rank(Us, ksq) == rkThem - 1)
+          safety += Value(200);
+      else
+          safety -= ShelterWeakness[rkUs]
+                  + StormDanger[rkUs == RANK_1 ? 0 : rkThem == rkUs + 1 ? 2 : 1][rkThem];
   }
 
   return safety;
