Singular extension and check extension tweak

If singular extension fails to trigger extension then don't consider check extension.

STC:
LLR: 2.96 (-2.94,2.94) [0.00,4.00]
Total: 69428 W: 12663 L: 12271 D: 44494

LTC:
LLR: 2.96 (-2.94,2.94) [0.00,4.00]
Total: 44023 W: 5875 L: 5612 D: 32536

Bench: 6170444

Closes #1043

diff --git a/src/search.cpp b/src/search.cpp
index f07085eda32f6f956cd0a64f3e48acda38cfa2d7..7029ae032ef8ba2703a7fa5080f4f319628fcf46 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -868,12 +868,7 @@ moves_loop: // When in check search starts from here
       moveCountPruning =   depth < 16 * ONE_PLY
                         && moveCount >= FutilityMoveCounts[improving][depth / ONE_PLY];
 
-      // Step 12. Extensions
-      // Extend checks
-      if (    givesCheck
-          && !moveCountPruning
-          &&  pos.see_ge(move, VALUE_ZERO))
-          extension = ONE_PLY;
+      // Step 12. Singular and Gives Check Extensions
 
       // Singular extension search. If all moves but one fail low on a search of
       // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
@@ -882,7 +877,6 @@ moves_loop: // When in check search starts from here
       // ttValue minus a margin then we extend the ttMove.
       if (    singularExtensionNode
           &&  move == ttMove
-          && !extension
           &&  pos.legal(move))
       {
           Value rBeta = std::max(ttValue - 2 * depth / ONE_PLY, -VALUE_MATE);
@@ -894,6 +888,10 @@ moves_loop: // When in check search starts from here
           if (value < rBeta)
               extension = ONE_PLY;
       }
+      else if (   givesCheck
+               && !moveCountPruning
+               &&  pos.see_ge(move, VALUE_ZERO))
+          extension = ONE_PLY;
 
       // Calculate new depth for this move
       newDepth = depth - ONE_PLY + extension;