Relax TT condition for zugzwang verified null values

In this case use a normal VALUE_TYPE_LOWER TT type instead of
VALUE_TYPE_NS_LO. This allow us to TT cut-off in a bit more nodes.

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

diff --git a/src/search.cpp b/src/search.cpp
index 3f8436415358e04840d4f54736446ae82efa7874..e6ffcb19f42cf73abcbb082f7a4789442ba36a06 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -1388,7 +1388,9 @@ namespace {
             {
                 assert(value_to_tt(nullValue, ply) == nullValue);
 
-                TT.store(posKey, nullValue, VALUE_TYPE_NS_LO, depth, MOVE_NONE);
+                // Special flag null values that are not zugzwang checked
+                ValueType vt = (depth < 6 * OnePly ? VALUE_TYPE_NS_LO : VALUE_TYPE_LOWER);
+                TT.store(posKey, nullValue, vt, depth, MOVE_NONE);
                 return nullValue;
             }
         } else {