Increase reduction if tt-move is a capture

The idea is that chances are the tt-move is best and will be difficult to raise alpha when playing a quiet move.

STC:
LLR: 2.95 (-2.94,2.94) [0.00,5.00]
Total: 7582 W: 1415 L: 1259 D: 4908

LTC:
LLR: 2.97 (-2.94,2.94) [0.00,5.00]
Total: 59553 W: 7885 L: 7573 D: 44095

Bench: 5725676

Closes #1147

src/search.cpp

@@ -554,7 +554,7 @@ namespace {
     Depth extension, newDepth;
     Value bestValue, value, ttValue, eval;
     bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
-    bool captureOrPromotion, doFullDepthSearch, moveCountPruning, skipQuiets;
+    bool captureOrPromotion, doFullDepthSearch, moveCountPruning, skipQuiets, ttCapture;
     Piece moved_piece;
     int moveCount, quietCount;
 
@@ -842,6 +842,7 @@ namespace {
                            && (tte->bound() & BOUND_LOWER)
                            &&  tte->depth() >= depth - 3 * ONE_PLY;
     skipQuiets = false;
+    ttCapture = false;
 
     // Step 11. Loop through moves
     // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
@@ -959,6 +960,9 @@ namespace {
           ss->moveCount = --moveCount;
           continue;
       }
+
+      if (move == ttMove && captureOrPromotion)
+          ttCapture = true;
 
       // Update the current move (this must be done after singular extension search)
       ss->currentMove = move;
@@ -979,6 +983,11 @@ namespace {
               r -= r ? ONE_PLY : DEPTH_ZERO;
           else
           {
+
+              // Increase reduction if ttMove is a capture
+              if (ttCapture)
+                  r += ONE_PLY;
+
               // Increase reduction for cut nodes
               if (cutNode)
                   r += 2 * ONE_PLY;