Prune all negative see moves at low depths

After 2036 games Mod- Orig:
381 - 278 - 1377 ELO +17 (+- 6.2) LOS 99%

One of the biggest increases ever !

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

diff --git a/src/search.cpp b/src/search.cpp
index f5e1a00..f7718ce 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -1287,6 +1287,17 @@ split_point_start: // At split points actual search starts from here
 
               continue;
           }
+
+          // Prune neg. see moves at low depths
+          if (   predictedDepth < 2 * ONE_PLY
+              && bestValue > value_mated_in(PLY_MAX)
+              && pos.see_sign(move) < 0)
+          {
+              if (SpNode)
+                  lock_grab(&(sp->lock));
+
+              continue;
+          }
       }
 
       // Step 13. Make the move