Increase null reduction at high depths

Linear rule, less aggressive then Dann's one.

It seems it scales well with depth. We will need to
verify against weaker engine if it keeps the score.

After 999 games at 1+0 on my Dual Core
Mod vs Orig +232 =534 -207  +9 ELO

After 1000 games by Martin Thoresen on his QUAD at 1+0
Mod vs Orig 521/479 52.10%

Functionality Signature: 17655312

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

diff --git a/src/search.cpp b/src/search.cpp
index 1668a74..5fc9be3 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -1376,7 +1376,7 @@ namespace {
         pos.do_null_move(st);
 
         // Null move dynamic reduction based on depth
-        int R = (depth >= 5 * OnePly ? 4 : 3);
+        int R = 3 + (depth >= 5 * OnePly ? depth / 8 : 0);
 
         // Null move dynamic reduction based on value
         if (approximateEval - beta > PawnValueMidgame)