Simplify the ShelterStrength[] array

Remove the distinction between the king file and the two neighbours
files in the ShelterStrength[] array. Instead we initialize the safety
variable in the evaluate_shelter() function with a -10 penalty if our
king is on a semi-open file (ie. if our king is on a file without a pawn
protection).

Also rename shelter_storm() to evaluate_shelter() while there.

STC:
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 23153 W: 4795 L: 4677 D: 13681
http://tests.stockfishchess.org/tests/view/5adcb83d0ebc595ec7ff8aa7

LTC:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 25728 W: 3934 L: 3821 D: 17973
http://tests.stockfishchess.org/tests/view/5adcdcb60ebc595ec7ff8adb

See the commit history in PR#1559 for the proof that the committed
version is equivalent to the version in the tests above:
#1559

Full credit to @protonspring for the renormalized values of the
ShelterStrength[] array used for the simplification. Thanks!

Bench: 4703935

src/pawns.cpp

@@ -43,17 +43,13 @@ namespace {
   // Doubled pawn penalty
   constexpr Score Doubled = S(18, 38);
 
-  // Weakness of our pawn shelter in front of the king by [isKingFile][distance from edge][rank].
-  // RANK_1 = 0 is used for files where we have no pawns or our pawn is behind our king.
-  constexpr Value ShelterWeakness[][int(FILE_NB) / 2][RANK_NB] = {
-    { { V( 98), V(20), V(11), V(42), V( 83), V( 84), V(101) }, // Not On King file
-      { V(103), V( 8), V(33), V(86), V( 87), V(105), V(113) },
-      { V(100), V( 2), V(65), V(95), V( 59), V( 89), V(115) },
-      { V( 72), V( 6), V(52), V(74), V( 83), V( 84), V(112) } },
-    { { V(105), V(19), V( 3), V(27), V( 85), V( 93), V( 84) }, // On King file
-      { V(121), V( 7), V(33), V(95), V(112), V( 86), V( 72) },
-      { V(121), V(26), V(65), V(90), V( 65), V( 76), V(117) },
-      { V( 79), V( 0), V(45), V(65), V( 94), V( 92), V(105) } }
+  // Strength of pawn shelter for our king by [distance from edge][rank].
+  // RANK_1 = 0 is used for files where we have no pawn, or pawn is behind our king.
+  Value ShelterStrength[int(FILE_NB) / 2][RANK_NB] = {
+    { V( -9), V(64), V(77), V( 44), V( 4), V( -1), V(-11) },
+    { V(-15), V(83), V(51), V(-10), V( 1), V(-10), V(-28) },
+    { V(-18), V(84), V(27), V(-12), V(21), V( -7), V(-36) },
+    { V( 12), V(79), V(25), V( 19), V( 9), V( -6), V(-33) }
   };
 
   // Danger of enemy pawns moving toward our king by [type][distance from edge][rank].
@@ -78,10 +74,6 @@ namespace {
       { V(21),  V(  23), V( 116), V(41), V(15) } }
   };
 
-  // Max bonus for king safety. Corresponds to start position with all the pawns
-  // in front of the king and no enemy pawn on the horizon.
-  constexpr Value MaxSafetyBonus = V(258);
-
   #undef S
   #undef V
 
@@ -230,21 +222,21 @@ Entry* probe(const Position& pos) {
 }
 
 
-/// Entry::shelter_storm() calculates shelter and storm penalties for the file
-/// the king is on, as well as the two closest files.
+/// Entry::evaluate_shelter() calculates the shelter bonus and the storm
+/// penalty for a king, looking at the king file and the two closest files.
 
 template<Color Us>
-Value Entry::shelter_storm(const Position& pos, Square ksq) {
+Value Entry::evaluate_shelter(const Position& pos, Square ksq) {
 
+  enum { BlockedByKing, Unopposed, BlockedByPawn, Unblocked };
   constexpr Color     Them = (Us == WHITE ? BLACK : WHITE);
   constexpr Direction Down = (Us == WHITE ? SOUTH : NORTH);
 
-  enum { BlockedByKing, Unopposed, BlockedByPawn, Unblocked };
-
   Bitboard b = pos.pieces(PAWN) & (forward_ranks_bb(Us, ksq) | rank_bb(ksq));
   Bitboard ourPawns = b & pos.pieces(Us);
   Bitboard theirPawns = b & pos.pieces(Them);
-  Value safety = MaxSafetyBonus;
+
+  Value safety = (ourPawns & file_bb(ksq)) ? Value(5) : Value(-5);
 
   File center = std::max(FILE_B, std::min(FILE_G, file_of(ksq)));
   for (File f = File(center - 1); f <= File(center + 1); ++f)
@@ -256,8 +248,8 @@ Value Entry::shelter_storm(const Position& pos, Square ksq) {
       Rank rkThem = b ? relative_rank(Us, frontmost_sq(Them, b)) : RANK_1;
 
       int d = std::min(f, ~f);
-      safety -=  ShelterWeakness[f == file_of(ksq)][d][rkUs]
-               + StormDanger
+      safety +=  ShelterStrength[d][rkUs]
+               - StormDanger
                  [(shift<Down>(b) & ksq) ? BlockedByKing :
                   rkUs   == RANK_1       ? Unopposed     :
                   rkThem == (rkUs + 1)   ? BlockedByPawn : Unblocked]
@@ -282,14 +274,14 @@ Score Entry::do_king_safety(const Position& pos, Square ksq) {
   if (pawns)
       while (!(DistanceRingBB[ksq][minKingPawnDistance++] & pawns)) {}
 
-  Value bonus = shelter_storm<Us>(pos, ksq);
+  Value bonus = evaluate_shelter<Us>(pos, ksq);
 
   // If we can castle use the bonus after the castling if it is bigger
   if (pos.can_castle(MakeCastling<Us, KING_SIDE>::right))
-      bonus = std::max(bonus, shelter_storm<Us>(pos, relative_square(Us, SQ_G1)));
+      bonus = std::max(bonus, evaluate_shelter<Us>(pos, relative_square(Us, SQ_G1)));
 
   if (pos.can_castle(MakeCastling<Us, QUEEN_SIDE>::right))
-      bonus = std::max(bonus, shelter_storm<Us>(pos, relative_square(Us, SQ_C1)));
+      bonus = std::max(bonus, evaluate_shelter<Us>(pos, relative_square(Us, SQ_C1)));
 
   return make_score(bonus, -16 * minKingPawnDistance);
 }

src/pawns.h

@@ -59,7 +59,7 @@ struct Entry {
   Score do_king_safety(const Position& pos, Square ksq);
 
   template<Color Us>
-  Value shelter_storm(const Position& pos, Square ksq);
+  Value evaluate_shelter(const Position& pos, Square ksq);
 
   Key key;
   Score scores[COLOR_NB];