Combine increased LMR horizont and fixed null move reduction

Set null move reduction to R=4, but increase the LMR horizon
to 3 plies. The two tweaks are related and should compensate
the combined effect of null move + LMR reduction at shallow
depths.

Idea from Tord.

After 999 games at 1+0

Mod vs Orig  +251 =522 -225 51.30% + 9 ELO

On Tord iMac Core 2 Duo 2.8 GHz, one thread,
Mac OS X 10.6, at 1+0 time control we have:

Mod vs Orig 994-1006  -1.4 ELO

But Orig version is pgo compiled and Mod is not.
The PGO compiled version is about 8% faster, which
corresponds to about 7 Elo points. This means that
results are reasonably consistent.

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

diff --git a/src/search.cpp b/src/search.cpp
index 4449b1be0e2f4408bcb8b792b776221c4aa81517..c8c8d4bda672a45fba916c8ad48168e9c3bcba30 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -1243,9 +1243,8 @@ namespace {
 
         StateInfo st;
         pos.do_null_move(st);
-        int R = (depth >= 5 * OnePly ? 4 : 3); // Null move dynamic reduction
 
-        Value nullValue = -search(pos, ss, -(beta-1), depth-R*OnePly, ply+1, false, threadID);
+        Value nullValue = -search(pos, ss, -(beta-1), depth-4*OnePly, ply+1, false, threadID);
 
         pos.undo_null_move();
 
@@ -1364,7 +1363,7 @@ namespace {
 
       // Try to reduce non-pv search depth by one ply if move seems not problematic,
       // if the move fails high will be re-searched at full depth.
-      if (    depth >= 2*OnePly
+      if (    depth >= 3*OnePly
           &&  moveCount >= LMRNonPVMoves
           && !dangerous
           && !moveIsCapture