Stat score initialization: grandchildren

[stockfish] / src / search.cpp

diff --git a/src/search.cpp b/src/search.cpp
index 394f228fb4f59956914ab3d014b199934d245716..3874852b1bbcac48a34ea38812b79b047a283d5d 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -514,7 +514,6 @@ namespace {
     Thread* thisThread = pos.this_thread();
     inCheck = pos.checkers();
     moveCount = captureCount = quietCount = ss->moveCount = 0;
-    ss->statScore = 0;
     bestValue = -VALUE_INFINITE;
     maxValue = VALUE_INFINITE;
 
@@ -554,6 +553,13 @@ namespace {
     (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
     Square prevSq = to_sq((ss-1)->currentMove);
 
+    // Initialize statScore to zero for the grandchildren of the current position.
+    // So statScore is shared between all grandchildren and only the first grandchild
+    // starts with statScore = 0. Later grandchildren start with the last calculated
+    // statScore of the previous grandchild. This influences the reduction rules in
+    // LMR which are based on the statScore of parent position.
+    (ss+2)->statScore = 0;
+
     // Step 4. Transposition table lookup. We don't want the score of a partial
     // search to overwrite a previous full search TT value, so we use a different
     // position key in case of an excluded move.