Keepstealing (#501)

Passed STC:
Elo | 3.50 +- 2.35 (95%)
SPRT | 8.0+0.08s Threads=1 Hash=16MB
LLR | 2.91 (-2.25, 2.89) [0.00, 3.00]
Games | N: 21930 W: 5408 L: 5187 D: 11335
Penta | [66, 2508, 5590, 2741, 60]

Passed LTC:
Elo | 6.82 +- 3.44 (95%)
SPRT | 40.0+0.40s Threads=1 Hash=64MB
LLR | 2.89 (-2.25, 2.89) [0.00, 3.00]
Games | N: 9380 W: 2261 L: 2077 D: 5042
Penta | [4, 1018, 2464, 1198, 6]

Bench: 6442362

src/history.cpp

@@ -22,6 +22,13 @@ void updateHHScore(const Position* pos, SearchData* sd, const Move move, int bon
     sd->searchHistory[pos->side][FromTo(move)] += scaledBonus;
 }
 
+void updateOppHHScore(const Position* pos, SearchData* sd, const Move move, int bonus) {
+    // Scale bonus to fix it in a [-HH_MAX;HH_MAX] range
+    const int scaledBonus = bonus - sd->searchHistory[pos->side ^ 1][FromTo(move)] * std::abs(bonus) / HH_MAX;
+    // Update move score
+    sd->searchHistory[pos->side ^ 1][FromTo(move)] += scaledBonus;
+}
+
 void updateRHScore(const Position *pos, SearchData *sd, const Move move, int bonus) {
     // Scale bonus to fix it in a [-RH_MAX;RH_MAX] range
     const int scaledBonus = bonus - GetRHScore(pos, sd, move) * std::abs(bonus) / RH_MAX;

src/history.h

@@ -31,6 +31,7 @@ int history_bonus(const int depth);
 void CleanHistories(SearchData* sd);
 // Updates history heuristics for a single move
 void updateHHScore(const Position* pos, SearchData* sd, const Move move, int bonus);
+void updateOppHHScore(const Position* pos, SearchData* sd, const Move move, int bonus);
 void updateCHScore(SearchStack* ss, const Move move, const int bonus);
 void updateCapthistScore(const Position* pos, SearchData* sd, const Move move, int bonus);
 void updateSingleCHScore(SearchStack* ss, const Move move, const int bonus, const int offset);

src/search.cpp

@@ -482,6 +482,14 @@ int Negamax(int alpha, int beta, int depth, const bool cutNode, ThreadData* td,
         StoreTTEntry(pos->posKey, NOMOVE, SCORE_NONE, rawEval, HFNONE, 0, pvNode, ttPv);
     }
 
+    // Use static evaluation difference to improve quiet move ordering (~6 Elo)
+    if ((ss - 1)->staticEval != SCORE_NONE && isQuiet((ss - 1)->move))
+    {
+        int bonus = std::clamp(-10 * int((ss - 1)->staticEval + ss->staticEval), -1830, 1427) + 624;
+        Move move = (ss - 1)->move;
+        updateOppHHScore(pos, sd, move, bonus);
+    }
+
     const int complexity = [&] {
         if (eval == 0 || rawEval == 0)
             return 0;

src/types.h

@@ -4,7 +4,7 @@
 // include the tune stuff here to give it global visibility
 #include "tune.h"
 
-#define NAME "Alexandria-7.1.6"
+#define NAME "Alexandria-7.1.7"
 
 inline int reductions[2][64][64];
 inline int lmp_margin[64][2];