Bonus for a prior capture that causes a fail low.

This tweak adds a bonus equal to twice the stat_bonus for the current depth for a prior capture that caused a fail high, similar to the prior countermove bonus we currently have.

Passed STC
https://tests.stockfishchess.org/tests/view/673bc14b86d5ee47d953f1f2
LLR: 2.95 (-2.94,2.94) <0.00,2.00>
Total: 105824 W: 27538 L: 27118 D: 51168
Ptnml(0-2): 358, 12370, 27024, 12814, 346

Passed LTC
https://tests.stockfishchess.org/tests/view/673ccbff86d5ee47d953f2d9
LLR: 2.94 (-2.94,2.94) <0.50,2.50>
Total: 134502 W: 34340 L: 33820 D: 66342
Ptnml(0-2): 102, 14634, 37229, 15214, 72

closes https://github.com/official-stockfish/Stockfish/pull/5695

Bench: 1107054

src/search.cpp

@@ -538,7 +538,7 @@ Value Search::Worker::search(
 
     // Dive into quiescence search when the depth reaches zero
     if (depth <= 0)
-        return qsearch<PvNode ? PV : NonPV>(pos, ss, alpha, beta);
+        return qsearch < PvNode ? PV : NonPV > (pos, ss, alpha, beta);
 
     // Limit the depth if extensions made it too large
     depth = std::min(depth, MAX_PLY - 1);
@@ -890,6 +890,7 @@ Value Search::Worker::search(
             prefetch(tt.first_entry(pos.key_after(move)));
 
             ss->currentMove   = move;
+            ss->capturedPiece = captured;
             ss->continuationHistory =
               &this->continuationHistory[ss->inCheck][true][pos.moved_piece(move)][move.to_sq()];
             ss->continuationCorrectionHistory =
@@ -1139,6 +1140,7 @@ moves_loop:  // When in check, search starts here
 
         // Update the current move (this must be done after singular extension search)
         ss->currentMove   = move;
+        ss->capturedPiece = pos.piece_on(move.to_sq());
         ss->continuationHistory =
           &thisThread->continuationHistory[ss->inCheck][capture][movedPiece][move.to_sq()];
         ss->continuationCorrectionHistory =
@@ -1400,6 +1402,14 @@ moves_loop:  // When in check, search starts here
               << stat_bonus(depth) * bonus / 24;
     }
 
+    else if (priorCapture && prevSq != SQ_NONE)
+    {
+        // bonus for prior countermoves that caused the fail low
+        Piece capturedPiece = (ss - 1)->capturedPiece;
+        thisThread->captureHistory[pos.piece_on(prevSq)][prevSq][type_of(capturedPiece)]
+          << stat_bonus(depth) * 2;
+    }
+
     // Bonus when search fails low and there is a TT move
     else if (ttData.move && !allNode)
         thisThread->mainHistory[us][ttData.move.from_to()] << stat_bonus(depth) * 23 / 100;
@@ -1645,6 +1655,7 @@ Value Search::Worker::qsearch(Position& pos, Stack* ss, Value alpha, Value beta)
 
         // Update the current move
         ss->currentMove   = move;
+        ss->capturedPiece = pos.piece_on(move.to_sq());
         ss->continuationHistory =
           &thisThread
              ->continuationHistory[ss->inCheck][capture][pos.moved_piece(move)][move.to_sq()];

src/search.h

@@ -66,6 +66,7 @@ struct Stack {
     CorrectionHistory<PieceTo>* continuationCorrectionHistory;
     int                         ply;
     Move                        currentMove;
+    Piece                       capturedPiece;
     Move                        excludedMove;
     Value                       staticEval;
     int                         statScore;