Do more reductions for late quiet moves in case of consecutive fail h…

…ighs.

Idea of this patch can be described as following - in case we have consecutive fail highs and we reach late enough moves at root node probability of remaining quiet moves being able to produce even bigger value than moves that produced previous cutoff (so ones that should be high in move ordering but now they fail to produce beta cutoff because we actually reached high move count) should be quiet small so we can reduce them more.

passed STC
LLR: 2.94 (-2.94,2.94) {-0.25,1.25}
Total: 53392 W: 5681 L: 5474 D: 42237
Ptnml(0-2): 214, 4104, 17894, 4229, 255
https://tests.stockfishchess.org/tests/view/5f88501adcdad978fe8c527e

passed LTC
LLR: 2.94 (-2.94,2.94) {0.25,1.25}
Total: 59136 W: 2773 L: 2564 D: 53799
Ptnml(0-2): 30, 2117, 25078, 2300, 43
https://tests.stockfishchess.org/tests/view/5f884dbfdcdad978fe8c527a

closes #3184

Bench: 4066972

src/search.cpp

@@ -417,7 +417,7 @@ void Thread::search() {
  // Start with a small aspiration window and, in the case of a fail
  // high/low, re-search with a bigger window until we don't fail
  // high/low anymore.
- int failedHighCnt = 0;
+ failedHighCnt = 0;
  while (true)
  {
  Depth adjustedDepth = std::max(1, rootDepth - failedHighCnt - searchAgainCounter);
@@ -1177,6 +1177,9 @@ namespace {
  if (ttCapture)
  r++;
 
+ // Increase reduction at root if failing high
+ r += rootNode ? thisThread->failedHighCnt * thisThread->failedHighCnt * moveCount / 512 : 0;
+
  // Increase reduction for cut nodes (~10 Elo)
  if (cutNode)
  r += 2;

src/thread.h

@@ -73,6 +73,7 @@ class Thread {
  CapturePieceToHistory captureHistory;
  ContinuationHistory continuationHistory[2][2];
  Score contempt;
+ int failedHighCnt;
 };