Use stat_malus when decreasing stats

This patch applies stat_bonus when increasing and stat_malus when decreasing stats.

Passed STC:
LLR: 2.93 (-2.94,2.94) <0.00,2.00>
Total: 133792 W: 34221 L: 33758 D: 65813
Ptnml(0-2): 477, 15764, 33959, 16211, 485
https://tests.stockfishchess.org/tests/view/654699f3136acbc5735256b2

Passed LTC:
LLR: 2.95 (-2.94,2.94) <0.50,2.50>
Total: 67374 W: 16912 L: 16523 D: 33939
Ptnml(0-2): 42, 7528, 18171, 7891, 55
https://tests.stockfishchess.org/tests/view/65474558136acbc5735263ab

closes #4864

bench: 1114417

src/search.cpp

@@ -94,7 +94,10 @@ constexpr int futility_move_count(bool improving, Depth depth) {
 }
 
 // History and stats update bonus, based on depth
-int stat_bonus(Depth d) { return std::min(357 * d - 483, 1511); }
+int stat_bonus(Depth d) { return std::min(364 * d - 438, 1501); }
+
+// History and stats update malus, based on depth
+int stat_malus(Depth d) { return std::min(452 * d - 452, 1478); }
 
 // Add a small random component to draw evaluations to avoid 3-fold blindness
 Value value_draw(const Thread* thisThread) {
@@ -636,12 +639,12 @@ Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, boo
                 // the previous ply (~0 Elo on STC, ~2 Elo on LTC).
                 if (prevSq != SQ_NONE && (ss - 1)->moveCount <= 2 && !priorCapture)
                     update_continuation_histories(ss - 1, pos.piece_on(prevSq), prevSq,
-                                                  -stat_bonus(depth + 1));
+                                                  -stat_malus(depth + 1));
             }
             // Penalty for a quiet ttMove that fails low (~1 Elo)
             else if (!ttCapture)
             {
-                int penalty = -stat_bonus(depth);
+                int penalty = -stat_malus(depth);
                 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
                 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
             }
@@ -1190,7 +1193,7 @@ Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, boo
                 if (newDepth > d)
                     value = -search<NonPV>(pos, ss + 1, -(alpha + 1), -alpha, newDepth, !cutNode);
 
-                int bonus = value <= alpha ? -stat_bonus(newDepth)
+                int bonus = value <= alpha ? -stat_malus(newDepth)
                           : value >= beta  ? stat_bonus(newDepth)
                                            : 0;
 
@@ -1681,6 +1684,7 @@ void update_all_stats(const Position& pos,
     PieceType              captured;
 
     int quietMoveBonus = stat_bonus(depth + 1);
+    int quietMoveMalus = stat_malus(depth + 1);
 
     if (!pos.capture_stage(bestMove))
     {
@@ -1692,15 +1696,18 @@ void update_all_stats(const Position& pos,
         thisThread->pawnHistory[pawn_structure(pos)][moved_piece][to_sq(bestMove)]
           << quietMoveBonus;
 
+        int moveMalus = bestValue > beta + 168 ? quietMoveMalus      // larger malus
+                                               : stat_malus(depth);  // smaller malus
+
         // Decrease stats for all non-best quiet moves
         for (int i = 0; i < quietCount; ++i)
         {
             thisThread->pawnHistory[pawn_structure(pos)][pos.moved_piece(quietsSearched[i])]
                                    [to_sq(quietsSearched[i])]
-              << -bestMoveBonus;
-            thisThread->mainHistory[us][from_to(quietsSearched[i])] << -bestMoveBonus;
+              << -moveMalus;
+            thisThread->mainHistory[us][from_to(quietsSearched[i])] << -moveMalus;
             update_continuation_histories(ss, pos.moved_piece(quietsSearched[i]),
-                                          to_sq(quietsSearched[i]), -bestMoveBonus);
+                                          to_sq(quietsSearched[i]), -moveMalus);
         }
     }
     else
@@ -1716,14 +1723,14 @@ void update_all_stats(const Position& pos,
         && ((ss - 1)->moveCount == 1 + (ss - 1)->ttHit
             || ((ss - 1)->currentMove == (ss - 1)->killers[0]))
         && !pos.captured_piece())
-        update_continuation_histories(ss - 1, pos.piece_on(prevSq), prevSq, -quietMoveBonus);
+        update_continuation_histories(ss - 1, pos.piece_on(prevSq), prevSq, -quietMoveMalus);
 
     // Decrease stats for all non-best capture moves
     for (int i = 0; i < captureCount; ++i)
     {
         moved_piece = pos.moved_piece(capturesSearched[i]);
         captured    = type_of(pos.piece_on(to_sq(capturesSearched[i])));
-        captureHistory[moved_piece][to_sq(capturesSearched[i])][captured] << -quietMoveBonus;
+        captureHistory[moved_piece][to_sq(capturesSearched[i])][captured] << -quietMoveMalus;
     }
 }