Introduce dynamic contempt

Make contempt dependent on the current score of the root position.

The idea is that we now use a linear formula like the following to decide
on the contempt to use during a search :

    contempt = x + y * eval

where x is the base contempt set by the user in the "Contempt" UCI option,
and y * eval is the dynamic part which adapts itself to the estimation of
the evaluation of the root position returned by the search. In this patch,
we use x = 18 centipawns by default, and the y * eval correction can go
from -20 centipawns if the root eval is less than -2.0 pawns, up to +20
centipawns when the root eval is more than 2.0 pawns.

To summarize, the new contempt goes from -0.02 to 0.38 pawns, depending if
Stockfish is losing or winning, with an average value of 0.18 pawns by default.

STC:
LLR: 2.95 (-2.94,2.94) [0.00,5.00]
Total: 110052 W: 24614 L: 23938 D: 61500
http://tests.stockfishchess.org/tests/view/5a72e6020ebc590f2c86ea20

LTC:
LLR: 2.97 (-2.94,2.94) [0.00,5.00]
Total: 16470 W: 2896 L: 2705 D: 10869
http://tests.stockfishchess.org/tests/view/5a76c5b90ebc5902971a9830

A second match at LTC was organised against the current master:

ELO: 1.45 +-2.9 (95%) LOS: 84.0%
Total: 19369 W: 3350 L: 3269 D: 12750
http://tests.stockfishchess.org/tests/view/5a7acf980ebc5902971a9a2e

Finally, we checked that there is no apparent problem with multithreading,
despite the fact that some threads might have a slightly different contempt
level that the main thread.

Match of this version against master, both using 5 threads, time control 30+0.3:
ELO: 2.18 +-3.2 (95%) LOS: 90.8%
Total: 14840 W: 2502 L: 2409 D: 9929
http://tests.stockfishchess.org/tests/view/5a7bf3e80ebc5902971a9aa2

Include suggestions from Marco Costalba, Aram Tumanian, Ronald de Man, etc.

Bench: 5207156

src/evaluate.cpp

@@ -905,7 +905,7 @@ namespace {
 
 } // namespace
 
-Score Eval::Contempt = SCORE_ZERO;
+std::atomic<Score> Eval::Contempt;
 
 /// evaluate() is the evaluator for the outer world. It returns a static evaluation
 /// of the position from the point of view of the side to move.

src/evaluate.h

@@ -21,6 +21,7 @@
 #ifndef EVALUATE_H_INCLUDED
 #define EVALUATE_H_INCLUDED
 
+#include <atomic>
 #include <string>
 
 #include "types.h"
@@ -31,7 +32,7 @@ namespace Eval {
 
 const Value Tempo = Value(20); // Must be visible to search
 
-extern Score Contempt;
+extern std::atomic<Score> Contempt;
 
 std::string trace(const Position& pos);
 

src/search.cpp

@@ -191,11 +191,6 @@ void MainThread::search() {
   Time.init(Limits, us, rootPos.game_ply());
   TT.new_search();
 
-  int contempt = Options["Contempt"] * PawnValueEg / 100; // From centipawns
-
-  Eval::Contempt = (us == WHITE ?  make_score(contempt, contempt / 2)
-                                : -make_score(contempt, contempt / 2));
-
   if (rootMoves.empty())
   {
       rootMoves.emplace_back(MOVE_NONE);
@@ -282,6 +277,7 @@ void Thread::search() {
   Depth lastBestMoveDepth = DEPTH_ZERO;
   MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
   double timeReduction = 1.0;
+  Color us = rootPos.side_to_move();
 
   std::memset(ss-4, 0, 7 * sizeof(Stack));
   for (int i = 4; i > 0; i--)
@@ -306,6 +302,10 @@ void Thread::search() {
 
   multiPV = std::min(multiPV, rootMoves.size());
 
+  int contempt = Options["Contempt"] * PawnValueEg / 100;  // From centipawns
+  Eval::Contempt = (us == WHITE ?  make_score(contempt, contempt / 2)
+                                : -make_score(contempt, contempt / 2));
+
   // Iterative deepening loop until requested to stop or the target depth is reached
   while (   (rootDepth += ONE_PLY) < DEPTH_MAX
          && !Threads.stop
@@ -340,6 +340,15 @@ void Thread::search() {
               delta = Value(18);
               alpha = std::max(rootMoves[PVIdx].previousScore - delta,-VALUE_INFINITE);
               beta  = std::min(rootMoves[PVIdx].previousScore + delta, VALUE_INFINITE);
+
+              // Adjust contempt based on current situation
+              contempt  = Options["Contempt"] * PawnValueEg / 100;  // From centipawns
+              contempt += bestValue >  500 ?  50:                   // Dynamic contempt
+                          bestValue < -500 ? -50:
+                          bestValue / 10;
+
+              Eval::Contempt = (us == WHITE ?  make_score(contempt, contempt / 2)
+                                            : -make_score(contempt, contempt / 2));
           }
 
           // Start with a small aspiration window and, in the case of a fail

src/ucioption.cpp

@@ -59,7 +59,7 @@ void init(OptionsMap& o) {
   const int MaxHashMB = Is64Bit ? 131072 : 2048;
 
   o["Debug Log File"]        << Option("", on_logger);
-  o["Contempt"]              << Option(20, -100, 100);
+  o["Contempt"]              << Option(18, -100, 100);
   o["Threads"]               << Option(1, 1, 512, on_threads);
   o["Hash"]                  << Option(16, 1, MaxHashMB, on_hash_size);
   o["Clear Hash"]            << Option(on_clear_hash);