join scale_factor, initiative and mg+eg reduction

Merging this code into one function `winnable()`.
Should allow common concepts used to adjust the eg value,
either by addition or scaling, to be combined more effectively.

Improve trace function.

closes https://github.com/official-stockfish/Stockfish/pull/2710

No functional change.

src/evaluate.cpp

@@ -35,7 +35,7 @@ namespace Trace {
   enum Tracing { NO_TRACE, TRACE };
 
   enum Term { // The first 8 entries are reserved for PieceType
-    MATERIAL = 8, IMBALANCE, MOBILITY, THREAT, PASSED, SPACE, INITIATIVE, TOTAL, TERM_NB
+    MATERIAL = 8, IMBALANCE, MOBILITY, THREAT, PASSED, SPACE, WINNABLE, TOTAL, TERM_NB
   };
 
   Score scores[TERM_NB][COLOR_NB];
@@ -59,7 +59,7 @@ namespace Trace {
 
   std::ostream& operator<<(std::ostream& os, Term t) {
 
-    if (t == MATERIAL || t == IMBALANCE || t == INITIATIVE || t == TOTAL)
+    if (t == MATERIAL || t == IMBALANCE || t == WINNABLE || t == TOTAL)
         os << " ----  ----"    << " | " << " ----  ----";
     else
         os << scores[t][WHITE] << " | " << scores[t][BLACK];
@@ -173,8 +173,7 @@ namespace {
     template<Color Us> Score threats() const;
     template<Color Us> Score passed() const;
     template<Color Us> Score space() const;
-    ScaleFactor scale_factor(Value eg) const;
+    Value winnable(Score score) const;
-    Score initiative(Score score) const;
 
     const Position& pos;
     Material::Entry* me;
@@ -717,12 +716,12 @@ namespace {
   }
 
 
-  // Evaluation::initiative() computes the initiative correction value
+  // Evaluation::winnable() adjusts the mg and eg score components based on the
-  // for the position. It is a second order bonus/malus based on the
   // known attacking/defending status of the players.
+  // A single value is derived from the mg and eg values and returned.
 
   template<Tracing T>
-  Score Evaluation<T>::initiative(Score score) const {
+  Value Evaluation<T>::winnable(Score score) const {
 
     int outflanking =  distance<File>(pos.square<KING>(WHITE), pos.square<KING>(BLACK))
                      - distance<Rank>(pos.square<KING>(WHITE), pos.square<KING>(BLACK));
@@ -756,17 +755,10 @@ namespace {
     int u = ((mg > 0) - (mg < 0)) * Utility::clamp(complexity + 50, -abs(mg), 0);
     int v = ((eg > 0) - (eg < 0)) * std::max(complexity, -abs(eg));
 
-    if (T)
+    mg += u;
-        Trace::add(INITIATIVE, make_score(u, v));
+    eg += v;
 
-    return make_score(u, v);
+    // Compute the scale factor for the winning side
-  }
-
-
-  // Evaluation::scale_factor() computes the scale factor for the winning side
-
-  template<Tracing T>
-  ScaleFactor Evaluation<T>::scale_factor(Value eg) const {
 
     Color strongSide = eg > VALUE_DRAW ? WHITE : BLACK;
     int sf = me->scale_factor(pos, strongSide);
@@ -786,7 +778,18 @@ namespace {
             sf = std::min(sf, 36 + 7 * pos.count<PAWN>(strongSide));
     }
 
-    return ScaleFactor(sf);
+    // Interpolate between the middlegame and (scaled by 'sf') endgame score
+    v =  mg * int(me->game_phase())
+       + eg * int(PHASE_MIDGAME - me->game_phase()) * ScaleFactor(sf) / SCALE_FACTOR_NORMAL;
+    v /= PHASE_MIDGAME;
+
+    if (T)
+    {
+        Trace::add(WINNABLE, make_score(u, eg * ScaleFactor(sf) / SCALE_FACTOR_NORMAL - eg_value(score)));
+        Trace::add(TOTAL, make_score(mg, eg * ScaleFactor(sf) / SCALE_FACTOR_NORMAL));
+    }
+
+    return Value(v);
   }
 
 
@@ -841,14 +844,8 @@ namespace {
             + passed< WHITE>() - passed< BLACK>()
             + space<  WHITE>() - space<  BLACK>();
 
-    score += initiative(score);
+    // Derive single value from mg and eg parts of score
-
+    v = winnable(score);
-    // Interpolate between a middlegame and a (scaled by 'sf') endgame score
-    ScaleFactor sf = scale_factor(eg_value(score));
-    v =  mg_value(score) * int(me->game_phase())
-       + eg_value(score) * int(PHASE_MIDGAME - me->game_phase()) * sf / SCALE_FACTOR_NORMAL;
-
-    v /= PHASE_MIDGAME;
 
     // In case of tracing add all remaining individual evaluation terms
     if (T)
@@ -857,7 +854,6 @@ namespace {
         Trace::add(IMBALANCE, me->imbalance());
         Trace::add(PAWN, pe->pawn_score(WHITE), pe->pawn_score(BLACK));
         Trace::add(MOBILITY, mobility[WHITE], mobility[BLACK]);
-        Trace::add(TOTAL, score);
     }
 
     // Side to move point of view
@@ -909,11 +905,11 @@ std::string Eval::trace(const Position& pos) {
      << "     Threats | " << Term(THREAT)
      << "      Passed | " << Term(PASSED)
      << "       Space | " << Term(SPACE)
-     << "  Initiative | " << Term(INITIATIVE)
+     << "    Winnable | " << Term(WINNABLE)
      << " ------------+-------------+-------------+------------\n"
      << "       Total | " << Term(TOTAL);
 
-  ss << "\nTotal evaluation: " << to_cp(v) << " (white side)\n";
+  ss << "\nFinal evaluation: " << to_cp(v) << " (white side)\n";
 
   return ss.str();
 }