Refactor pawns shelter and storm

Renamed stuff and added comments. The aim is to make more
readable, at least by me ;-) , this newly added part of code.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

diff --git a/src/bitboard.h b/src/bitboard.h
index 85852e5e5b8567557a75ac289025073a2df61ef3..6dd07d5407f85bc431328915c8c459e3a043d7f7 100644 (file)
--- a/src/bitboard.h
+++ b/src/bitboard.h
@@ -263,16 +263,6 @@ extern Square pop_1st_bit(Bitboard* b);
 
 #endif
 
-// relative_rank() returns the relative rank of the closest bit set on the Bitboard.
-// Only to be used with bitboards that contain a single file.
-
-template<Color Us>
-inline Rank relative_rank(Bitboard b) {
-  Square s = Us == WHITE ?  first_1(b)
-                         : ~last_1(b);
-  return rank_of(s);
-}
-
 extern void print_bitboard(Bitboard b);
 extern void bitboards_init();
 

diff --git a/src/evaluate.cpp b/src/evaluate.cpp
index 6e515d88998f612c8dea4669e3d577cb9802636b..0a500c92ebbf4c8ef69c4288ab81638fa6f94e30 100644 (file)
--- a/src/evaluate.cpp
+++ b/src/evaluate.cpp
@@ -751,8 +751,8 @@ Value do_evaluate(const Position& pos, Value& margin) {
     int attackUnits;
     const Square ksq = pos.king_square(Us);
 
-    // King shelter
-    Score score = ei.pi->king_shelter<Us>(pos, ksq);
+    // King shelter and enemy pawns storm
+    Score score = ei.pi->king_safety<Us>(pos, ksq);
 
     // King safety. This is quite complicated, and is almost certainly far
     // from optimally tuned.
@@ -774,7 +774,7 @@ Value do_evaluate(const Position& pos, Value& margin) {
         attackUnits =  std::min(25, (ei.kingAttackersCount[Them] * ei.kingAttackersWeight[Them]) / 2)
                      + 3 * (ei.kingAdjacentZoneAttacksCount[Them] + popcount<Max15>(undefended))
                      + InitKingDanger[relative_square(Us, ksq)]
-                     - mg_value(ei.pi->king_shelter<Us>(pos, ksq)) / 32;
+                     - mg_value(ei.pi->king_safety<Us>(pos, ksq)) / 32;
 
         // Analyse enemy's safe queen contact checks. First find undefended
         // squares around the king attacked by enemy queen...

diff --git a/src/pawns.cpp b/src/pawns.cpp
index eeabcbae84df6e81ed22a5bbff8c5c1b684cb1fd..41d2e6f8148234d46d86a729c08a4468a737b597 100644 (file)
--- a/src/pawns.cpp
+++ b/src/pawns.cpp
@@ -65,18 +65,21 @@ namespace {
 
   #undef S
 
-  const File ShelterFile[8] = { FILE_B, FILE_B, FILE_C, FILE_D, FILE_E, FILE_F, FILE_G, FILE_G };
+  typedef Value V;
 
-  inline Value score_non_center_file(const Value v) {
-               return Value(v * 7 / 16);
-  }
+  // Weakness of our pawn shelter in front of the king indexed by [king pawn][rank]
+  const Value ShelterWeakness[2][8] =
+  { { V(141), V(0), V(38), V(102), V(128), V(141), V(141), V(141) },
+    { V( 61), V(0), V(16), V( 44), V( 56), V( 61), V( 61), V( 61) } };
 
-  typedef Value V;
-  // Arrays are indexed by rank.  Zeroth value is for when no pawn on that file.
-  const Value PawnShelter[8] =  { V(141), V(0), V( 38), V(102), V(128), V(141), V(141), V(141) };
-  const Value PawnStorm[8] =    { V( 26), V(0), V(128), V( 51), V( 26), V(  0), V(  0), V(  0) };
-  // We compute shelter as a penalty for the given color, but shelter is used as a bonus, so we invert it using this as the basis.
-  const Value PawnShelterBasis =  PawnShelter[0] + score_non_center_file(PawnShelter[0]) * 2;
+  // Danger of enemy pawns moving toward our king indexed by [pawn blocked][rank]
+  const Value StormDanger[2][8] =
+  { { V(26), V(0), V(128), V(51), V(26), V(0), V(0), V(0) },
+    { V(13), V(0), V( 64), V(25), V(13), V(0), V(0), V(0) } };
+
+  // Max bonus for king safety. Corresponds to start position with all the pawns
+  // in front of the king and no enemy pawns on the horizont.
+  const Value MaxSafetyBonus = V(263);
 
   inline Score apply_weight(Score v, Score w) {
     return make_score((int(mg_value(v)) * mg_value(w)) / 0x100,
@@ -222,58 +225,67 @@ Score PawnInfoTable::evaluate_pawns(const Position& pos, Bitboard ourPawns,
   return value;
 }
 
+
 template<Color Us>
-int computePawnShelter(const Position &pos, Square ksq) {
-  const Color Them          = (Us == WHITE ? BLACK : WHITE);
-  const File kingFile       = ShelterFile[file_of(ksq)];
-  const Bitboard ourPawns   = pos.pieces(PAWN, Us) & in_front_bb(Us, ksq);
-  const Bitboard theirPawns = pos.pieces(PAWN, Them) & (RankBB[rank_of(ksq)] | in_front_bb(Us, ksq));
-
-  int shelter = 0;
-
-  // Compute king shelter and storm values for the file the king is on, as well as the two adjacent files.
-  for (int fileOffset = -1; fileOffset <= 1; fileOffset++) {
-    // Shelter takes full penalty for center file, otherwise it's half penalty
-    Bitboard shelterFile  = ourPawns & FileBB[kingFile + fileOffset];
-    Rank shelterClosest = shelterFile ? relative_rank<Us>(shelterFile)
-                                      : RANK_1;
-
-    shelter += fileOffset == 0 ? PawnShelter[shelterClosest]
-                               : score_non_center_file(PawnShelter[shelterClosest]);
-
-    // Storm takes full penalty, unless there is an enemy pawn blocking us
-    Bitboard stormFile  = theirPawns & FileBB[kingFile + fileOffset];
-    Rank stormClosest   = stormFile ? relative_rank<Us>(stormFile)
-                                    : RANK_1;
-
-    shelter += shelterClosest + 1 == stormClosest ? PawnStorm[stormClosest] / 2
-                                                  : PawnStorm[stormClosest];
+int PawnInfo::shelter_storm(const Position& pos, Square ksq) {
+
+  const Color Them = (Us == WHITE ? BLACK : WHITE);
+
+  int safety = 0;
+  Bitboard b = pos.pieces(PAWN) & (in_front_bb(Us, ksq) | RankBB[rank_of(ksq)]);
+  Bitboard ourPawns = b & pos.pieces(Us) & ~RankBB[rank_of(ksq)];
+  Bitboard theirPawns = b & pos.pieces(Them);
+  Rank rkUs, rkThem;
+  File kf = file_of(ksq);
+
+  // Compute shelter and storm values for the file the king is on, as well as
+  // the two adjacent files. Computation is done from the white point of view.
+  kf = (kf == FILE_A) ? kf++ : (kf == FILE_H) ? kf-- : kf;
+
+  for (int f = kf - 1; f <= kf + 1; f++)
+  {
+      // Shelter penalty is higher for the pawn in front of the king
+      b = ourPawns & FileBB[f];
+      rkUs = b ? rank_of(Us == WHITE ? first_1(b) : ~last_1(b)) : RANK_1;
+      safety -= ShelterWeakness[f != kf][rkUs];
+
+      // Storm danger is smaller if enemy pawn is blocked
+      b  = theirPawns & FileBB[f];
+      rkThem = b ? rank_of(Us == WHITE ? first_1(b) : ~last_1(b)) : RANK_1;
+      safety -= StormDanger[rkThem == rkUs + 1][rkThem];
   }
-  
-  return shelter;
+
+  return safety;
 }
 
-/// PawnInfo::updateShelter() calculates and caches king shelter. It is called
-/// only when king square changes, about 20% of total king_shelter() calls.
+
+/// PawnInfo::update_safety() calculates and caches a bonus for king safety. It is
+/// called only when king square changes, about 20% of total king_safety() calls.
+
 template<Color Us>
-Score PawnInfo::updateShelter(const Position& pos, Square ksq) {
-  int shelter = 0;
+Score PawnInfo::update_safety(const Position& pos, Square ksq) {
+
+  int bonus = 0;
 
   if (relative_rank(Us, ksq) <= RANK_4)
   {
-    shelter = computePawnShelter<Us>(pos, ksq);
-    if (pos.can_castle(Us == WHITE ? WHITE_OO : BLACK_OO))
-      shelter = std::min(shelter, computePawnShelter<Us>(pos, relative_square(Us, SQ_G1)));
-    if (pos.can_castle(Us == WHITE ? WHITE_OOO : BLACK_OOO))
-      shelter = std::min(shelter, computePawnShelter<Us>(pos, relative_square(Us, SQ_C1)));
-    shelter = PawnShelterBasis - shelter;
+      bonus = shelter_storm<Us>(pos, ksq);
+
+      // If we can castle use the bonus after the castle if is bigger
+      if (pos.can_castle(Us == WHITE ? WHITE_OO : BLACK_OO))
+          bonus = std::max(bonus, shelter_storm<Us>(pos, relative_square(Us, SQ_G1)));
+
+      if (pos.can_castle(Us == WHITE ? WHITE_OOO : BLACK_OOO))
+          bonus = std::max(bonus, shelter_storm<Us>(pos, relative_square(Us, SQ_C1)));
+
+      bonus += MaxSafetyBonus; // Offset to be sure bonus is always positive
   }
 
   kingSquares[Us] = ksq;
-  kingShelters[Us] = make_score(shelter, 0);
+  kingShelters[Us] = make_score(bonus, 0);
   return kingShelters[Us];
 }
 
 // Explicit template instantiation
-template Score PawnInfo::updateShelter<WHITE>(const Position& pos, Square ksq);
-template Score PawnInfo::updateShelter<BLACK>(const Position& pos, Square ksq);
+template Score PawnInfo::update_safety<WHITE>(const Position& pos, Square ksq);
+template Score PawnInfo::update_safety<BLACK>(const Position& pos, Square ksq);

diff --git a/src/pawns.h b/src/pawns.h
index f2b4441a5d46259909acf06fa91ecb20d644b524..1e49a307f64d3748ea742a618869585b0cc46fdb 100644 (file)
--- a/src/pawns.h
+++ b/src/pawns.h
@@ -46,11 +46,14 @@ public:
   int has_open_file_to_right(Color c, File f) const;
 
   template<Color Us>
-  Score king_shelter(const Position& pos, Square ksq);
+  Score king_safety(const Position& pos, Square ksq);
 
 private:
   template<Color Us>
-  Score updateShelter(const Position& pos, Square ksq);
+  Score update_safety(const Position& pos, Square ksq);
+
+  template<Color Us>
+  int shelter_storm(const Position& pos, Square ksq);
 
   Key key;
   Bitboard passedPawns[2];
@@ -100,8 +103,8 @@ inline int PawnInfo::has_open_file_to_right(Color c, File f) const {
 }
 
 template<Color Us>
-inline Score PawnInfo::king_shelter(const Position& pos, Square ksq) {
-  return kingSquares[Us] == ksq ? kingShelters[Us] : updateShelter<Us>(pos, ksq);
+inline Score PawnInfo::king_safety(const Position& pos, Square ksq) {
+  return kingSquares[Us] == ksq ? kingShelters[Us] : update_safety<Us>(pos, ksq);
 }
 
 #endif // !defined(PAWNS_H_INCLUDED)