More safe checks

Consider a check given by a rook or a minor to be a "safe check"
also in the case where supported by another piece,
and given on a square only defended by a queen

Was yellow STC
http://tests.stockfishchess.org/tests/view/576fcbc80ebc5972faa163e8
LLR: -2.96 (-2.94,2.94) [0.00,5.00]
Total: 55453 W: 10431 L: 10315 D: 34707

Passed LTC
http://tests.stockfishchess.org/tests/view/57733a0b0ebc5972faa164b7
LLR: 2.96 (-2.94,2.94) [0.00,5.00]
Total: 54550 W: 7671 L: 7365 D: 39514

bench: 7398346

src/evaluate.cpp

@@ -77,6 +77,11 @@ namespace {
     // attacked by a given color and piece type (can be also ALL_PIECES).
     Bitboard attackedBy[COLOR_NB][PIECE_TYPE_NB];
 
+    // attackedBy2[color] are the squares attacked by 2 pieces of a given color,
+    // possibly via x-ray or by one pawn and one piece. Diagonal x-ray through
+    // pawn or squares attacked by 2 pawns are not explicitly added.
+    Bitboard attackedBy2[COLOR_NB];
+
     // kingRing[color] is the zone around the king which is considered
     // by the king safety evaluation. This consists of the squares directly
     // adjacent to the king, and the three (or two, for a king on an edge file)
@@ -227,9 +232,10 @@ namespace {
     const Square Down = (Us == WHITE ? DELTA_S : DELTA_N);
 
     ei.pinnedPieces[Us] = pos.pinned_pieces(Us);
-    Bitboard b = ei.attackedBy[Them][KING] = pos.attacks_from<KING>(pos.square<KING>(Them));
+    Bitboard b = ei.attackedBy[Them][KING];
     ei.attackedBy[Them][ALL_PIECES] |= b;
     ei.attackedBy[Us][ALL_PIECES] |= ei.attackedBy[Us][PAWN] = ei.pi->pawn_attacks(Us);
+    ei.attackedBy2[Us] = ei.attackedBy[Us][PAWN] & ei.attackedBy[Us][KING];
 
     // Init king safety tables only if we are going to use them
     if (pos.non_pawn_material(Us) >= QueenValueMg)
@@ -272,6 +278,7 @@ namespace {
         if (ei.pinnedPieces[Us] & s)
             b &= LineBB[pos.square<KING>(Us)][s];
 
+        ei.attackedBy2[Us] |= ei.attackedBy[Us][ALL_PIECES] & b;
         ei.attackedBy[Us][ALL_PIECES] |= ei.attackedBy[Us][Pt] |= b;
 
         if (b & ei.kingRing[Them])
@@ -389,11 +396,9 @@ namespace {
     if (ei.kingAttackersCount[Them])
     {
         // Find the attacked squares which are defended only by the king...
-        undefended =  ei.attackedBy[Them][ALL_PIECES]
+        undefended =   ei.attackedBy[Them][ALL_PIECES]
-                    & ei.attackedBy[Us][KING]
+                    &  ei.attackedBy[Us][KING]
-                    & ~(  ei.attackedBy[Us][PAWN]   | ei.attackedBy[Us][KNIGHT]
+                    & ~ei.attackedBy2[Us];
-                        | ei.attackedBy[Us][BISHOP] | ei.attackedBy[Us][ROOK]
-                        | ei.attackedBy[Us][QUEEN]);
 
         // ... and those which are not defended at all in the larger king ring
         b =  ei.attackedBy[Them][ALL_PIECES] & ~ei.attackedBy[Us][ALL_PIECES]
@@ -414,15 +419,9 @@ namespace {
         // Analyse the enemy's safe queen contact checks. Firstly, find the
         // undefended squares around the king reachable by the enemy queen...
         b = undefended & ei.attackedBy[Them][QUEEN] & ~pos.pieces(Them);
-        if (b)
-        {
-            // ...and then remove squares not supported by another enemy piece
-            b &=  ei.attackedBy[Them][PAWN]   | ei.attackedBy[Them][KNIGHT]
-                | ei.attackedBy[Them][BISHOP] | ei.attackedBy[Them][ROOK]
-                | ei.attackedBy[Them][KING];
 
-            attackUnits += QueenContactCheck * popcount(b);
+        // ...and keep squares supported by another enemy piece
-        }
+        attackUnits += QueenContactCheck * popcount(b & ei.attackedBy2[Them]);
 
         // Analyse the safe enemy's checks which are possible on next move...
         safe  = ~(ei.attackedBy[Us][ALL_PIECES] | pos.pieces(Them));
@@ -439,6 +438,12 @@ namespace {
         if ((b1 | b2) & ei.attackedBy[Them][QUEEN] & safe)
             attackUnits += QueenCheck, score -= SafeCheck;
 
+        // For other pieces, also consider the square safe if attacked twice,
+        // and only defended by a queen.
+        safe |=  ei.attackedBy2[Them]
+               & ~(ei.attackedBy2[Us] | pos.pieces(Them))
+               & ei.attackedBy[Us][QUEEN];
+
         // Enemy rooks safe and other checks
         if (b1 & ei.attackedBy[Them][ROOK] & safe)
             attackUnits += RookCheck, score -= SafeCheck;
@@ -770,6 +775,8 @@ Value Eval::evaluate(const Position& pos) {
 
   // Initialize attack and king safety bitboards
   ei.attackedBy[WHITE][ALL_PIECES] = ei.attackedBy[BLACK][ALL_PIECES] = 0;
+  ei.attackedBy[WHITE][KING] = pos.attacks_from<KING>(pos.square<KING>(WHITE));
+  ei.attackedBy[BLACK][KING] = pos.attacks_from<KING>(pos.square<KING>(BLACK));
   eval_init<WHITE>(pos, ei);
   eval_init<BLACK>(pos, ei);