Introduce movecount pruning for quiet check evasions in qsearch

Vizvezdenec · vondele · commit 95d7369e54f2 · 2022-02-05T07:38:30.000+01:00
Idea of this patch is that we usually don't consider quiet check evasions as "good" ones and prefer capture based ones instead. So it makes sense to think that if in qsearch 2 quiet check evasions failed to produce anything good 3rd and further ones wouldn't be good either. passed STC https://tests.stockfishchess.org/tests/view/61fc1b1ed508ec6a1c9f397c LLR: 2.94 (-2.94,2.94) <0.00,2.50> Total: 58800 W: 15947 L: 15626 D: 27227 Ptnml(0-2): 273, 6568, 15462, 6759, 338 passed LTC https://tests.stockfishchess.org/tests/view/61fcc56dd508ec6a1c9f5619 LLR: 2.95 (-2.94,2.94) <0.50,3.00> Total: 89544 W: 24208 L: 23810 D: 41526 Ptnml(0-2): 81, 9038, 26134, 9440, 79 closes #3920 bench 4830082
diff --git a/src/search.cpp b/src/search.cpp
@@ -1479,6 +1479,8 @@ namespace {
                                       contHist,
                                       prevSq);
 
+    int quietCheckEvasions = 0;
+
     // Loop through the moves until no moves remain or a beta cutoff occurs
     while ((move = mp.next_move()) != MOVE_NONE)
     {
@@ -1540,6 +1542,15 @@ namespace {
           && (*contHist[1])[pos.moved_piece(move)][to_sq(move)] < CounterMovePruneThreshold)
           continue;
 
+      // movecount pruning for quiet check evasions
+      if (  bestValue > VALUE_TB_LOSS_IN_MAX_PLY
+          && quietCheckEvasions > 1
+          && !captureOrPromotion
+          && ss->inCheck)
+          continue;
+
+      quietCheckEvasions += !captureOrPromotion && ss->inCheck;
+
       // Make and search the move
       pos.do_move(move, st, givesCheck);
       value = -qsearch<nodeType>(pos, ss+1, -beta, -alpha, depth - 1);
