Smaller null move reduction when depth is high

Lower probability to miss something important.

It seems to increase strenght. Idea form Cyclone.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

diff --git a/src/search.cpp b/src/search.cpp
index 8da12118d21979d3941e984743fb6f5e8794235c..97ed196acede459112754e6892f2b88fba8a75cc 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -1148,7 +1148,8 @@ namespace {
 
         UndoInfo u;
         pos.do_null_move(u);
-        Value nullValue = -search(pos, ss, -(beta-1), depth-4*OnePly, ply+1, false, threadID);
+        int R = (depth > 7 ? 4 : 3);
+        Value nullValue = -search(pos, ss, -(beta-1), depth-R*OnePly, ply+1, false, threadID);
         pos.undo_null_move(u);
 
         if (nullValue >= beta)