Further simplify stat bonuses

Based on recent simplification by linrock Since it completely removed any
special bonuses based on values difference and made it flat stat_bonus(depth +
1) I got an idea that we might as well remove all (depth + 1) bonuses and make
them usual depth bonuses.  Also removes weird negative bonus for depth 1 as a
side effect.

Passed STC:
https://tests.stockfishchess.org/tests/view/6689d817eca84f4d25863746
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 18080 W: 4789 L: 4552 D: 8739
Ptnml(0-2): 46, 1987, 4727, 2244, 36

Passed LTC:
https://tests.stockfishchess.org/tests/view/6689daa4eca84f4d258639d7
LLR: 2.95 (-2.94,2.94) <-1.75,0.25>
Total: 109062 W: 27548 L: 27417 D: 54097
Ptnml(0-2): 58, 11983, 30293, 12164, 33

Passed direct LTC vs linrock patch:
https://tests.stockfishchess.org/tests/view/668a46f8eca84f4d25866fe9
LLR: 2.95 (-2.94,2.94) <-1.75,0.25>
Total: 100002 W: 25351 L: 25209 D: 49442
Ptnml(0-2): 54, 11119, 27529, 11229, 70

closes https://github.com/official-stockfish/Stockfish/pull/5461

Bench 1175744

diff --git a/src/search.cpp b/src/search.cpp
index d22761e3ef04b19760d046295fe55cb4f1f65f2c..ac0e59b5e7a842ae36bcd3b84d7ca46629504017 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -86,7 +86,7 @@ Value to_corrected_static_eval(Value v, const Worker& w, const Position& pos) {
 }
 
 // History and stats update bonus, based on depth
-int stat_bonus(Depth d) { return std::clamp(190 * d - 298, 20, 1596); }
+int stat_bonus(Depth d) { return std::min(190 * d - 108, 1596); }
 
 // History and stats update malus, based on depth
 int stat_malus(Depth d) { return (d < 4 ? 736 * d - 268 : 2044); }
@@ -1782,7 +1782,7 @@ void update_all_stats(const Position& pos,
     Piece                  moved_piece    = pos.moved_piece(bestMove);
     PieceType              captured;
 
-    int quietMoveBonus = stat_bonus(depth + 1);
+    int quietMoveBonus = stat_bonus(depth);
     int quietMoveMalus = stat_malus(depth);
 
     if (!pos.capture_stage(bestMove))