Count two times number of attacked pieces in mobility

Now in mobility we count enemy attacked pieces as
empty squares.

With this patch we try to give an higher score to positions
where the number of attacked pieces is higher.

After 999 games at 1+0

Mod vs Orig +262 =517 -219 52.15% 520.5/998 +15 ELO

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

src/evaluate.cpp

@@ -529,31 +529,43 @@ namespace {
   // evaluate_mobility() computes mobility and attacks for every piece
 
   template<PieceType Piece, Color Us, bool HasPopCnt>
-  int evaluate_mobility(const Position& pos, const Bitboard& mob_bb, EvalInfo& ei) {
+  int evaluate_mobility(const Position& pos, Bitboard b, EvalInfo& ei) {
 
     const Color Them = (Us == WHITE ? BLACK : WHITE);
     static const int AttackWeight[] = { 0, 0, KnightAttackWeight, BishopAttackWeight, RookAttackWeight, QueenAttackWeight };
     static const Value* MgBonus[] = { 0, 0, MidgameKnightMobilityBonus, MidgameBishopMobilityBonus, MidgameRookMobilityBonus, MidgameQueenMobilityBonus };
     static const Value* EgBonus[] = { 0, 0, EndgameKnightMobilityBonus, EndgameBishopMobilityBonus, EndgameRookMobilityBonus, EndgameQueenMobilityBonus };
+    static const int lastIndex[] = { 0, 0, 8, 15, 15, 31 };
 
     // Update attack info
-    ei.attackedBy[Us][Piece] |= mob_bb;
+    ei.attackedBy[Us][Piece] |= b;
 
     // King attacks
-    if (mob_bb & ei.kingZone[Us])
+    if (b & ei.kingZone[Us])
     {
         ei.kingAttackersCount[Us]++;
         ei.kingAttackersWeight[Us] += AttackWeight[Piece];
-        Bitboard b = (mob_bb & ei.attackedBy[Them][KING]);
-        if (b)
-            ei.kingAdjacentZoneAttacksCount[Us] += count_1s_max_15<HasPopCnt>(b);
+        Bitboard bb = (b & ei.attackedBy[Them][KING]);
+        if (bb)
+            ei.kingAdjacentZoneAttacksCount[Us] += count_1s_max_15<HasPopCnt>(bb);
     }
 
+    // The squares occupied by enemy pieces will be counted two times instead
+    // of one. The shift (almost) guarantees that intersection with b is zero
+    // so when we 'or' the two bitboards togheter and count we get the correct
+    // sum of '1' in b and attacked bitboards.
+    Bitboard attacked = Us == WHITE ? ((b & pos.pieces_of_color(Them)) >> 1)
+                                    : ((b & pos.pieces_of_color(Them)) << 1);
+
     // Remove squares protected by enemy pawns or occupied by our pieces
-    Bitboard b = mob_bb & ~ei.attackedBy[Them][PAWN] & ~pos.pieces_of_color(Us);
+    b &= ~(ei.attackedBy[Them][PAWN] | pos.pieces_of_color(Us));
 
     // Mobility
-    int mob = (Piece != QUEEN ? count_1s_max_15<HasPopCnt>(b) : count_1s<HasPopCnt>(b));
+    int mob = (Piece != QUEEN ? count_1s_max_15<HasPopCnt>(b | attacked)
+                              : count_1s<HasPopCnt>(b | attacked));
+
+    if (mob > lastIndex[Piece])
+        mob = lastIndex[Piece];
 
     ei.mgMobility += Sign[Us] * MgBonus[Piece][mob];
     ei.egMobility += Sign[Us] * EgBonus[Piece][mob];
@@ -592,7 +604,7 @@ namespace {
   template<PieceType Piece, Color Us, bool HasPopCnt>
   void evaluate_pieces(const Position& pos, EvalInfo& ei) {
 
-    Bitboard mob_bb;
+    Bitboard b;
     Square s, ksq;
     int mob;
     File f;
@@ -603,16 +615,16 @@ namespace {
     while ((s = *ptr++) != SQ_NONE)
     {
         if (Piece == KNIGHT || Piece == QUEEN)
-            mob_bb = pos.attacks_from<Piece>(s);
+            b = pos.attacks_from<Piece>(s);
         else if (Piece == BISHOP)
-            mob_bb = bishop_attacks_bb(s, pos.occupied_squares() & ~pos.pieces(QUEEN, Us));
+            b = bishop_attacks_bb(s, pos.occupied_squares() & ~pos.pieces(QUEEN, Us));
         else if (Piece == ROOK)
-            mob_bb = rook_attacks_bb(s, pos.occupied_squares() & ~pos.pieces(ROOK, QUEEN, Us));
+            b = rook_attacks_bb(s, pos.occupied_squares() & ~pos.pieces(ROOK, QUEEN, Us));
         else
             assert(false);
 
         // Attacks and mobility
-        mob = evaluate_mobility<Piece, Us, HasPopCnt>(pos, mob_bb, ei);
+        mob = evaluate_mobility<Piece, Us, HasPopCnt>(pos, b, ei);
 
         // Bishop and knight outposts squares
         if ((Piece == BISHOP || Piece == KNIGHT) && pos.square_is_weak(s, Them))