Improve play for closed positions

This patch give a small bonus to incite the attacking side to keep more
pawns on the board.

A consequence of this bonus is that Stockfish will tend to play positions
slightly more closed on average than master, especially when it believes
that it has an advantage.

To lower the risk of blockades where Stockfish start shuffling without
progress, we also implement a progressive decrease of the evaluation
value with the 50 moves counter (along with the necessary aging of the
transposition table to reduce the impact of the Graph History Interaction
problem): since the evaluation decreases during shuffling phases, the
engine will tend to examine the consequences of pawn breaks faster during
the search.

Passed STC:
LLR: 2.96 (-2.94,2.94) {-0.25,1.25}
Total: 26184 W: 2406 L: 2252 D: 21526
Ptnml(0-2): 85, 1784, 9223, 1892, 108
https://tests.stockfishchess.org/tests/view/600cc08b735dd7f0f0352c06

Passed LCT:
LLR: 2.95 (-2.94,2.94) {0.25,1.25}
Total: 199768 W: 7695 L: 7191 D: 184882
Ptnml(0-2): 85, 6478, 86269, 6952, 100
https://tests.stockfishchess.org/tests/view/600ccd28735dd7f0f0352c10

Closes https://github.com/official-stockfish/Stockfish/pull/3321

Bench: 3988915

diff --git a/src/evaluate.cpp b/src/evaluate.cpp
index 76266937d56717fe047789b49857cd7f199ae350..6bd3c08b5133a672bab8616c57108a0a6704bd70 100644 (file)
--- a/src/evaluate.cpp
+++ b/src/evaluate.cpp
@@ -1052,8 +1052,8 @@ Value Eval::evaluate(const Position& pos) {
   {
       // Scale and shift NNUE for compatibility with search and classical evaluation
       auto  adjusted_NNUE = [&](){
-         int mat = pos.non_pawn_material() + PawnValueMg * pos.count<PAWN>();
-         return NNUE::evaluate(pos) * (679 + mat / 32) / 1024 + Tempo;
+         int mat = pos.non_pawn_material() + 2 * PawnValueMg * pos.count<PAWN>();
+         return NNUE::evaluate(pos) * (641 + mat / 32 - 4 * pos.rule50_count()) / 1024 + Tempo;
       };
 
       // If there is PSQ imbalance use classical eval, with small probability if it is small

diff --git a/src/position.cpp b/src/position.cpp
index 12b1bd9a8c5332996794e1ea9549fa2b23ba10b3..8c97160b8e93545952461fd91135d2c9751a4cfe 100644 (file)
--- a/src/position.cpp
+++ b/src/position.cpp
@@ -1017,7 +1017,7 @@ void Position::do_null_move(StateInfo& newSt) {
   }
 
   st->key ^= Zobrist::side;
-  prefetch(TT.first_entry(st->key));
+  prefetch(TT.first_entry(key()));
 
   ++st->rule50;
   st->pliesFromNull = 0;

diff --git a/src/position.h b/src/position.h
index 93e0f91d675b4d3dae7d9c7102c5478c59382c1f..928366bcf4307036d579ccd1cfd814a28f79e242 100644 (file)
--- a/src/position.h
+++ b/src/position.h
@@ -322,7 +322,8 @@ inline int Position::pawns_on_same_color_squares(Color c, Square s) const {
 }
 
 inline Key Position::key() const {
-  return st->key;
+  return st->rule50 < 14 ? st->key
+                         : st->key ^ make_key((st->rule50 - 14) / 8);
 }
 
 inline Key Position::pawn_key() const {