Give a reduced bonus for threats by hanging pawns

Passed STC:
LLR: 2.96 (-2.94,2.94) [-1.50,4.50]
Total: 105539 W: 20389 L: 20001 D: 65149

and LTC:
LLR: 2.95 (-2.94,2.94) [0.00,6.00]
Total: 9629 W: 1577 L: 1432 D: 6620

Bench: 7658627

Resolves #317

src/evaluate.cpp

@@ -158,6 +158,8 @@ namespace {
     S(0, 0), S(0, 0), S(107, 138), S(84, 122), S(114, 203), S(121, 217)
   };
 
+  const Score ThreatenedByHangingPawn = S(40, 60);
+
   // Assorted bonuses and penalties used by evaluation
   const Score KingOnOne          = S( 2, 58);
   const Score KingOnMany         = S( 6,125);
@@ -305,11 +307,6 @@ namespace {
 
         mobility[Us] += MobilityBonus[Pt][mob];
 
-        // Decrease score if we are attacked by an enemy pawn. The remaining part
-        // of threat evaluation must be done later when we have full attack info.
-        if (ei.attackedBy[Them][PAWN] & s)
-            score -= ThreatenedByPawn[Pt];
-
         if (Pt == BISHOP || Pt == KNIGHT)
         {
             // Bonus for outpost square
@@ -501,9 +498,26 @@ namespace {
     enum { Defended, Weak };
     enum { Minor, Major };
 
-    Bitboard b, weak, defended;
+    Bitboard b, weak, defended, safe_pawns, safe_pawn_threats, unsafe_pawn_threats;
     Score score = SCORE_ZERO;
 
+    // Pawn Threats
+    b = ei.attackedBy[Us][PAWN] & (pos.pieces(Them) ^ pos.pieces(Them, PAWN));
+    if(b)
+    {
+        safe_pawns = pos.pieces(Us, PAWN) & (~ei.attackedBy[Them][ALL_PIECES] | ei.attackedBy[Us][ALL_PIECES]);
+        safe_pawn_threats = (shift_bb<Right>(safe_pawns) | shift_bb<Left>(safe_pawns)) & (pos.pieces(Them) ^ pos.pieces(Them, PAWN));
+        unsafe_pawn_threats = b ^ safe_pawn_threats;
+	// Unsafe pawn threats
+        if(unsafe_pawn_threats)
+	  score += ThreatenedByHangingPawn;
+
+	// Evaluate safe pawn threats
+        while(safe_pawn_threats)
+	  score += ThreatenedByPawn[type_of(pos.piece_on(pop_lsb(&safe_pawn_threats)))];
+
+    }
+
     // Non-pawn enemies defended by a pawn
     defended = (pos.pieces(Them) ^ pos.pieces(Them, PAWN)) & ei.attackedBy[Them][PAWN];