Remove the Contempt UCI option

This patch removes the UCI option for setting Contempt in classical evaluation.

It is exactly equivalent to using Contempt=0 for the UCI contempt value and keeping
the dynamic part in the algo (renaming this dynamic part `trend` to better describe
what it does). We have tried quite hard to implement a working Contempt feature for
NNUE but nothing really worked, so it is probably time to give up.

Interested chess fans wishing to keep playing with the UCI option for Contempt and
use it with the classical eval are urged to download the version tagged "SF_Classical"
of Stockfish (dated 31 July 2020), as it was the last version where our search
algorithm was tuned for the classical eval and is probably our strongest classical
player ever: https://github.com/official-stockfish/Stockfish/tags

Passed STC:
LLR: 2.95 (-2.94,2.94) <-2.50,0.50>
Total: 72904 W: 6228 L: 6175 D: 60501
Ptnml(0-2): 221, 5006, 25971, 5007, 247
https://tests.stockfishchess.org/tests/view/60c98bf9457376eb8bcab18d

Passed LTC:
LLR: 2.93 (-2.94,2.94) <-2.50,0.50>
Total: 45168 W: 1601 L: 1547 D: 42020
Ptnml(0-2): 38, 1331, 19786, 1397, 32
https://tests.stockfishchess.org/tests/view/60c9c7fa457376eb8bcab1bb

closes https://github.com/official-stockfish/Stockfish/pull/3575

Bench: 4947716

README.md

@@ -120,14 +120,6 @@ change them via a chess GUI. This is a list of available UCI options in Stockfis
     Limit Syzygy tablebase probing to positions with at most this many pieces left
     (including kings and pawns).
 
-  * #### Contempt
-    A positive value for contempt favors middle game positions and avoids draws,
-    effective for the classical evaluation only.
-
-  * #### Analysis Contempt
-    By default, contempt is set to prefer the side to move. Set this option to "White"
-    or "Black" to analyse with contempt for that side, or "Off" to disable contempt.
-
   * #### Move Overhead
     Assume a time delay of x ms due to network and GUI overheads. This is useful to
     avoid losses on time in those cases.

src/evaluate.cpp

@@ -979,7 +979,7 @@ namespace {
     // Initialize score by reading the incrementally updated scores included in
     // the position object (material + piece square tables) and the material
     // imbalance. Score is computed internally from the white point of view.
-    Score score = pos.psq_score() + me->imbalance() + pos.this_thread()->contempt;
+    Score score = pos.psq_score() + me->imbalance() + pos.this_thread()->trend;
 
     // Probe the pawn hash table
     pe = Pawns::probe(pos);
@@ -1139,7 +1139,7 @@ std::string Eval::trace(Position& pos) {
 
   std::memset(scores, 0, sizeof(scores));
 
-  pos.this_thread()->contempt = SCORE_ZERO; // Reset any dynamic contempt
+  pos.this_thread()->trend = SCORE_ZERO; // Reset any dynamic contempt
 
   v = Evaluation<TRACE>(pos).value();
 

src/search.cpp

@@ -312,19 +312,7 @@ void Thread::search() {
   multiPV = std::min(multiPV, rootMoves.size());
   ttHitAverage = TtHitAverageWindow * TtHitAverageResolution / 2;
 
-  int ct = int(Options["Contempt"]) * PawnValueEg / 100; // From centipawns
-
-  // In analysis mode, adjust contempt in accordance with user preference
-  if (Limits.infinite || Options["UCI_AnalyseMode"])
-      ct =  Options["Analysis Contempt"] == "Off"  ? 0
-          : Options["Analysis Contempt"] == "Both" ? ct
-          : Options["Analysis Contempt"] == "White" && us == BLACK ? -ct
-          : Options["Analysis Contempt"] == "Black" && us == WHITE ? -ct
-          : ct;
-
-  // Evaluation score is from the white point of view
-  contempt = (us == WHITE ?  make_score(ct, ct / 2)
-                          : -make_score(ct, ct / 2));
+  trend = SCORE_ZERO;
 
   int searchAgainCounter = 0;
 
@@ -370,11 +358,11 @@ void Thread::search() {
               alpha = std::max(prev - delta,-VALUE_INFINITE);
               beta  = std::min(prev + delta, VALUE_INFINITE);
 
-              // Adjust contempt based on root move's previousScore (dynamic contempt)
-              int dct = ct + (113 - ct / 2) * prev / (abs(prev) + 147);
+              // Adjust trend based on root move's previousScore (dynamic contempt)
+              int tr = 113 * prev / (abs(prev) + 147);
 
-              contempt = (us == WHITE ?  make_score(dct, dct / 2)
-                                      : -make_score(dct, dct / 2));
+              trend = (us == WHITE ?  make_score(tr, tr / 2)
+                                   : -make_score(tr, tr / 2));
           }
 
           // Start with a small aspiration window and, in the case of a fail

src/thread.h

@@ -74,7 +74,7 @@ public:
   LowPlyHistory lowPlyHistory;
   CapturePieceToHistory captureHistory;
   ContinuationHistory continuationHistory[2][2];
-  Score contempt;
+  Score trend;
 };
 
 

src/ucioption.cpp

@@ -61,8 +61,6 @@ void init(OptionsMap& o) {
   constexpr int MaxHashMB = Is64Bit ? 33554432 : 2048;
 
   o["Debug Log File"]        << Option("", on_logger);
-  o["Contempt"]              << Option(24, -100, 100);
-  o["Analysis Contempt"]     << Option("Both var Off var White var Black var Both", "Both");
   o["Threads"]               << Option(1, 1, 512, on_threads);
   o["Hash"]                  << Option(16, 1, MaxHashMB, on_hash_size);
   o["Clear Hash"]            << Option(on_clear_hash);