Symmetric King Safety: take 2

Another attempt at retiring current asymmetric
king evaluation and use a much simpler symmetric
one. As a good side effect we can avoid recalculating
eval after a null move.

Tested in no-regression mode and passed

STC
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 21580 W: 3752 L: 3632 D: 14196

LTC
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 18253 W: 2593 L: 2469 D: 13191

And a LTC regression test against SF DD to
verify we don't have regression against
weaker engines due to some kind of 'contempt'
effect:

ELO: 54.69 +-2.1 (95%) LOS: 100.0%
Total: 40000 W: 11072 L: 4827 D: 24101

bench: 8205159

Author: lucasart

src/evaluate.cpp

@@ -90,9 +90,9 @@ namespace {
   }
 
   // Evaluation weights, indexed by evaluation term
-  enum { Mobility, PawnStructure, PassedPawns, Space, KingDangerUs, KingDangerThem };
+  enum { Mobility, PawnStructure, PassedPawns, Space, KingSafety };
   const struct Weight { int mg, eg; } Weights[] = {
-    {289, 344}, {233, 201}, {221, 273}, {46, 0}, {271, 0}, {307, 0}
+    {289, 344}, {233, 201}, {221, 273}, {46, 0}, {318, 0}
   };
 
   typedef Value V;
@@ -150,12 +150,11 @@ namespace {
     S(0, 0), S(0, 0), S(56, 70), S(56, 70), S(76, 99), S(86, 118)
   };
 
-  // Hanging[side to move] contains a bonus for each enemy hanging piece
-  const Score Hanging[2] = { S(23, 20) , S(35, 45) };
+  // Hanging contains a bonus for each enemy hanging piece
+  const Score Hanging = S(23, 20);
 
   #undef S
 
-  const Score Tempo            = make_score(24, 11);
   const Score RookOnPawn       = make_score(10, 28);
   const Score RookOpenFile     = make_score(43, 21);
   const Score RookSemiopenFile = make_score(19, 10);
@@ -194,9 +193,9 @@ namespace {
   const int BishopCheck       = 2;
   const int KnightCheck       = 3;
 
-  // KingDanger[Color][attackUnits] contains the actual king danger weighted
-  // scores, indexed by color and by a calculated integer number.
-  Score KingDanger[COLOR_NB][128];
+  // KingDanger[attackUnits] contains the actual king danger weighted
+  // scores, indexed by a calculated integer number.
+  Score KingDanger[128];
 
 
   // apply_weight() weighs score 'v' by weight 'w' trying to prevent overflow
@@ -426,9 +425,7 @@ namespace {
                   | ei.attackedBy[Them][BISHOP] | ei.attackedBy[Them][ROOK]);
 
             if (b)
-                attackUnits +=  QueenContactCheck
-                              * popcount<Max15>(b)
-                              * (Them == pos.side_to_move() ? 2 : 1);
+                attackUnits +=  QueenContactCheck * popcount<Max15>(b);
         }
 
         // Analyse the enemy's safe rook contact checks. Firstly, find the
@@ -446,9 +443,7 @@ namespace {
                   | ei.attackedBy[Them][BISHOP] | ei.attackedBy[Them][QUEEN]);
 
             if (b)
-                attackUnits +=  RookContactCheck
-                              * popcount<Max15>(b)
-                              * (Them == pos.side_to_move() ? 2 : 1);
+                attackUnits +=  RookContactCheck * popcount<Max15>(b);
         }
 
         // Analyse the enemy's safe distance checks for sliders and knights
@@ -482,7 +477,7 @@ namespace {
 
         // Finally, extract the king danger score from the KingDanger[]
         // array and subtract the score from evaluation.
-        score -= KingDanger[Us == Search::RootColor][attackUnits];
+        score -= KingDanger[attackUnits];
     }
 
     if (Trace)
@@ -521,8 +516,7 @@ namespace {
 
         b = weakEnemies & ~ei.attackedBy[Them][ALL_PIECES];
         if (b)
-            score += more_than_one(b) ? Hanging[Us != pos.side_to_move()] * popcount<Max15>(b)
-                                      : Hanging[Us == pos.side_to_move()];
+            score += more_than_one(b) ? Hanging * popcount<Max15>(b) : Hanging;
     }
 
     if (Trace)
@@ -677,9 +671,9 @@ namespace {
     Thread* thisThread = pos.this_thread();
 
     // Initialize score by reading the incrementally updated scores included
-    // in the position object (material + piece square tables) and adding a
-    // Tempo bonus. Score is computed from the point of view of white.
-    score = pos.psq_score() + (pos.side_to_move() == WHITE ? Tempo : -Tempo);
+    // in the position object (material + piece square tables).
+    // Score is computed from the point of view of white.
+    score = pos.psq_score();
 
     // Probe the material hash table
     ei.mi = Material::probe(pos, thisThread->materialTable, thisThread->endgames);
@@ -831,7 +825,7 @@ namespace {
        << "                     |   MG    EG  |   MG    EG  |   MG    EG  \n"
        << "---------------------+-------------+-------------+-------------\n";
 
-    format_row(ss, "Material, PST, Tempo", PST);
+    format_row(ss, "Material, PST", PST);
     format_row(ss, "Material imbalance", IMBALANCE);
     format_row(ss, "Pawns", PAWN);
     format_row(ss, "Knights", KNIGHT);
@@ -885,9 +879,7 @@ namespace Eval {
     for (int t = 0, i = 1; i < 100; ++i)
     {
         t = int(std::min(Peak, std::min(0.4 * i * i, t + MaxSlope)));
-
-        KingDanger[1][i] = apply_weight(make_score(t, 0), Weights[KingDangerUs]);
-        KingDanger[0][i] = apply_weight(make_score(t, 0), Weights[KingDangerThem]);
+        KingDanger[i] = apply_weight(make_score(t, 0), Weights[KingSafety]);
     }
   }
 

src/search.cpp

@@ -42,7 +42,6 @@ namespace Search {
   LimitsType Limits;
   std::vector<RootMove> RootMoves;
   Position RootPos;
-  Color RootColor;
   Time::point SearchTime;
   StateStackPtr SetupStates;
 }
@@ -77,6 +76,9 @@ namespace {
     return (Depth) Reductions[PvNode][i][std::min(int(d) / ONE_PLY, 63)][std::min(mn, 63)];
   }
 
+  // Tempo bonus. Must be handled by search to preserve eval symmetry.
+  const int Tempo = 17;
+
   size_t MultiPV, PVIdx;
   TimeManager TimeMgr;
   double BestMoveChanges;
@@ -180,12 +182,11 @@ uint64_t Search::perft(Position& pos, Depth depth) {
 
 void Search::think() {
 
-  RootColor = RootPos.side_to_move();
-  TimeMgr.init(Limits, RootPos.game_ply(), RootColor);
+  TimeMgr.init(Limits, RootPos.game_ply(), RootPos.side_to_move());
 
   int cf = Options["Contempt Factor"] * PawnValueEg / 100; // From centipawns
-  DrawValue[ RootColor] = VALUE_DRAW - Value(cf);
-  DrawValue[~RootColor] = VALUE_DRAW + Value(cf);
+  DrawValue[ RootPos.side_to_move()] = VALUE_DRAW - Value(cf);
+  DrawValue[~RootPos.side_to_move()] = VALUE_DRAW + Value(cf);
 
   if (RootMoves.empty())
   {
@@ -203,8 +204,8 @@ void Search::think() {
       log << "\nSearching: "  << RootPos.fen()
           << "\ninfinite: "   << Limits.infinite
           << " ponder: "      << Limits.ponder
-          << " time: "        << Limits.time[RootColor]
-          << " increment: "   << Limits.inc[RootColor]
+          << " time: "        << Limits.time[RootPos.side_to_move()]
+          << " increment: "   << Limits.inc[RootPos.side_to_move()]
           << " moves to go: " << Limits.movestogo
           << "\n" << std::endl;
   }
@@ -472,7 +473,7 @@ namespace {
     bestValue = -VALUE_INFINITE;
     ss->currentMove = ss->ttMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
     ss->ply = (ss-1)->ply + 1;
-    (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
+    (ss+1)->skipNullMove = (ss+1)->nullChild = false; (ss+1)->reduction = DEPTH_ZERO;
     (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
 
     // Used to send selDepth info to GUI
@@ -483,7 +484,7 @@ namespace {
     {
         // Step 2. Check for aborted search and immediate draw
         if (Signals.stop || pos.is_draw() || ss->ply > MAX_PLY)
-            return ss->ply > MAX_PLY && !inCheck ? evaluate(pos) : DrawValue[pos.side_to_move()];
+            return ss->ply > MAX_PLY && !inCheck ? evaluate(pos) + Tempo : DrawValue[pos.side_to_move()];
 
         // Step 3. Mate distance pruning. Even if we mate at the next move our score
         // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
@@ -538,7 +539,7 @@ namespace {
     {
         // Never assume anything on values stored in TT
         if ((ss->staticEval = eval = tte->eval_value()) == VALUE_NONE)
-            eval = ss->staticEval = evaluate(pos);
+            eval = ss->staticEval = evaluate(pos) + Tempo;
 
         // Can ttValue be used as a better position evaluation?
         if (ttValue != VALUE_NONE)
@@ -547,7 +548,7 @@ namespace {
     }
     else
     {
-        eval = ss->staticEval = evaluate(pos);
+        eval = ss->staticEval = ss->nullChild ? -(ss-1)->staticEval + 2 * Tempo : evaluate(pos) + Tempo;
         TT.store(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE, ss->staticEval);
     }
 
@@ -607,10 +608,10 @@ namespace {
                  + int(eval - beta) / PawnValueMg * ONE_PLY;
 
         pos.do_null_move(st);
-        (ss+1)->skipNullMove = true;
+        (ss+1)->skipNullMove = (ss+1)->nullChild = true;
         nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1, DEPTH_ZERO)
                                       : - search<NonPV, false>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
-        (ss+1)->skipNullMove = false;
+        (ss+1)->skipNullMove = (ss+1)->nullChild = false;
         pos.undo_null_move();
 
         if (nullValue >= beta)
@@ -1065,7 +1066,7 @@ namespace {
 
     // Check for an instant draw or if the maximum ply has been reached
     if (pos.is_draw() || ss->ply > MAX_PLY)
-        return ss->ply > MAX_PLY && !InCheck ? evaluate(pos) : DrawValue[pos.side_to_move()];
+        return ss->ply > MAX_PLY && !InCheck ? evaluate(pos) + Tempo : DrawValue[pos.side_to_move()];
 
     // Decide whether or not to include checks: this fixes also the type of
     // TT entry depth that we are going to use. Note that in qsearch we use
@@ -1102,15 +1103,15 @@ namespace {
         {
             // Never assume anything on values stored in TT
             if ((ss->staticEval = bestValue = tte->eval_value()) == VALUE_NONE)
-                ss->staticEval = bestValue = evaluate(pos);
+                ss->staticEval = bestValue = evaluate(pos) + Tempo;
 
             // Can ttValue be used as a better position evaluation?
             if (ttValue != VALUE_NONE)
                 if (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER))
                     bestValue = ttValue;
         }
         else
-            ss->staticEval = bestValue = evaluate(pos);
+            ss->staticEval = bestValue = ss->nullChild ? -(ss-1)->staticEval + 2 * Tempo : evaluate(pos) + Tempo;
 
         // Stand pat. Return immediately if static value is at least beta
         if (bestValue >= beta)

src/search.h

@@ -45,7 +45,8 @@ struct Stack {
   Move killers[2];
   Depth reduction;
   Value staticEval;
-  int skipNullMove;
+  bool skipNullMove;
+  bool nullChild;
 };
 
 
@@ -101,7 +102,6 @@ extern volatile SignalsType Signals;
 extern LimitsType Limits;
 extern std::vector<RootMove> RootMoves;
 extern Position RootPos;
-extern Color RootColor;
 extern Time::point SearchTime;
 extern StateStackPtr SetupStates;
 

src/uci.cpp

@@ -197,11 +197,7 @@ void UCI::loop(int argc, char* argv[]) {
                     << "\n"       << Options
                     << "\nuciok"  << sync_endl;
 
-      else if (token == "eval")
-      {
-          Search::RootColor = pos.side_to_move(); // Ensure it is set
-          sync_cout << Eval::trace(pos) << sync_endl;
-      }
+      else if (token == "eval")       sync_cout << Eval::trace(pos) << sync_endl;
       else if (token == "ucinewgame") TT.clear();
       else if (token == "go")         go(pos, is);
       else if (token == "position")   position(pos, is);