Merge static history into main history,

thus simplifying and reducing the memory footprint.
I believe using static diff for better move ordering is more suited for
low depths, so restrict writing to low depths.

Todo: probably the condition for writing can be simplified

LTC:
LLR: 2.95 (-2.94,2.94) {-0.75,0.25}
Total: 18752 W: 768 L: 705 D: 17279
Ptnml(0-2): 7, 635, 8034, 688, 12
https://tests.stockfishchess.org/tests/view/5fd631791ac169120188859e

STC:
LLR: 2.95 (-2.94,2.94) {-1.25,0.25}
Total: 36504 W: 3380 L: 3313 D: 29811
Ptnml(0-2): 116, 2667, 12645, 2682, 142
https://tests.stockfishchess.org/tests/view/5fd5ed861ac1691201888569

closes https://github.com/official-stockfish/Stockfish/pull/3262

bench: 4018036

src/movepick.cpp

@@ -54,9 +54,9 @@ namespace {
 /// ordering is at the current node.
 
 /// MovePicker constructor for the main search
-MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const ButterflyHistory* mh, const ButterflyHistory* sh, const LowPlyHistory* lp,
+MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const ButterflyHistory* mh, const LowPlyHistory* lp,
                        const CapturePieceToHistory* cph, const PieceToHistory** ch, Move cm, const Move* killers, int pl)
-           : pos(p), mainHistory(mh), staticHistory(sh), lowPlyHistory(lp), captureHistory(cph), continuationHistory(ch),
+           : pos(p), mainHistory(mh), lowPlyHistory(lp), captureHistory(cph), continuationHistory(ch),
              ttMove(ttm), refutations{{killers[0], 0}, {killers[1], 0}, {cm, 0}}, depth(d), ply(pl) {
 
   assert(d > 0);
@@ -66,9 +66,9 @@ MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const ButterflyHist
 }
 
 /// MovePicker constructor for quiescence search
-MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const ButterflyHistory* mh, const ButterflyHistory* sh,
+MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const ButterflyHistory* mh,
                        const CapturePieceToHistory* cph, const PieceToHistory** ch, Square rs)
-           : pos(p), mainHistory(mh), staticHistory(sh), captureHistory(cph), continuationHistory(ch), ttMove(ttm), recaptureSquare(rs), depth(d) {
+           : pos(p), mainHistory(mh), captureHistory(cph), continuationHistory(ch), ttMove(ttm), recaptureSquare(rs), depth(d) {
 
   assert(d <= 0);
 
@@ -105,7 +105,6 @@ void MovePicker::score() {
 
       else if (Type == QUIETS)
           m.value =      (*mainHistory)[pos.side_to_move()][from_to(m)]
-                   +     (*staticHistory)[pos.side_to_move()][from_to(m)]
                    + 2 * (*continuationHistory[0])[pos.moved_piece(m)][to_sq(m)]
                    + 2 * (*continuationHistory[1])[pos.moved_piece(m)][to_sq(m)]
                    + 2 * (*continuationHistory[3])[pos.moved_piece(m)][to_sq(m)]

src/movepick.h

@@ -123,12 +123,10 @@ public:
   MovePicker& operator=(const MovePicker&) = delete;
   MovePicker(const Position&, Move, Value, const CapturePieceToHistory*);
   MovePicker(const Position&, Move, Depth, const ButterflyHistory*,
-                                           const ButterflyHistory*,
                                            const CapturePieceToHistory*,
                                            const PieceToHistory**,
                                            Square);
   MovePicker(const Position&, Move, Depth, const ButterflyHistory*,
-                                           const ButterflyHistory*,
                                            const LowPlyHistory*,
                                            const CapturePieceToHistory*,
                                            const PieceToHistory**,
@@ -145,7 +143,6 @@ private:
 
   const Position& pos;
   const ButterflyHistory* mainHistory;
-  const ButterflyHistory* staticHistory;
   const LowPlyHistory* lowPlyHistory;
   const CapturePieceToHistory* captureHistory;
   const PieceToHistory** continuationHistory;

src/search.cpp

@@ -815,13 +815,10 @@ namespace {
         tte->save(posKey, VALUE_NONE, ss->ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, eval);
     }
 
-    // Update static history for previous move
+    if ((ss-1)->moveCount > 1 && is_ok((ss-1)->currentMove) && !(ss-1)->inCheck && !priorCapture && depth < 7)
-    if (is_ok((ss-1)->currentMove) && !(ss-1)->inCheck && !priorCapture)
     {
-        int bonus = ss->staticEval > -(ss-1)->staticEval + 2 * Tempo ? -stat_bonus(depth) :
+        int bonus = std::clamp(- (depth+1) * 2 * int((ss-1)->staticEval + ss->staticEval - 2 * Tempo), -1000, 1000);
-                    ss->staticEval < -(ss-1)->staticEval + 2 * Tempo ? stat_bonus(depth) :
+        thisThread->mainHistory[~us][from_to((ss-1)->currentMove)] << bonus;
-                    0;
-        thisThread->staticHistory[~us][from_to((ss-1)->currentMove)] << bonus;
     }
 
     // Step 7. Razoring (~1 Elo)
@@ -985,7 +982,6 @@ moves_loop: // When in check, search starts from here
     Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
 
     MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
-                                      &thisThread->staticHistory,
                                       &thisThread->lowPlyHistory,
                                       &captureHistory,
                                       contHist,
@@ -1536,7 +1532,6 @@ moves_loop: // When in check, search starts from here
     // queen and checking knight promotions, and other checks(only if depth >= DEPTH_QS_CHECKS)
     // will be generated.
     MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
-                                      &thisThread->staticHistory,
                                       &thisThread->captureHistory,
                                       contHist,
                                       to_sq((ss-1)->currentMove));

src/thread.cpp

@@ -57,7 +57,6 @@ void Thread::clear() {
 
   counterMoves.fill(MOVE_NONE);
   mainHistory.fill(0);
-  staticHistory.fill(0);
   lowPlyHistory.fill(0);
   captureHistory.fill(0);
 

src/thread.h

@@ -69,7 +69,6 @@ public:
   Depth rootDepth, completedDepth;
   CounterMoveHistory counterMoves;
   ButterflyHistory mainHistory;
-  ButterflyHistory staticHistory;
   LowPlyHistory lowPlyHistory;
   CapturePieceToHistory captureHistory;
   ContinuationHistory continuationHistory[2][2];