Replace distanceFromPv with a better logic.

src/search.cpp

@@ -618,7 +618,6 @@ namespace {
     moveCount          = captureCount = quietCount = ss->moveCount = 0;
     bestValue          = -VALUE_INFINITE;
     maxValue           = VALUE_INFINITE;
-    ss->distanceFromPv = (PvNode ? 0 : ss->distanceFromPv);
 
     // Check for the available remaining time
     if (thisThread == Threads.main())
@@ -1180,8 +1179,6 @@ namespace {
       // Step 15. Make the move
       pos.do_move(move, st, givesCheck);
 
-      (ss+1)->distanceFromPv = ss->distanceFromPv + moveCount - 1;
-
       // Step 16. Late moves reduction / extension (LMR, ~200 Elo)
       // We use various heuristics for the sons of a node after the first son has
       // been searched. In general we would like to reduce them, but there are many
@@ -1280,10 +1277,10 @@ namespace {
                   r -= ss->statScore / 14790;
           }
 
-          // In general we want to cap the LMR depth search at newDepth. But for nodes
-          // close to the principal variation the cap is at (newDepth + 1), which will
-          // allow these nodes to be searched deeper than the pv (up to 4 plies deeper).
-          Depth d = std::clamp(newDepth - r, 1, newDepth + ((ss+1)->distanceFromPv <= 4));
+          // In general we want to cap the LMR depth search at newDepth.
+          // But if reductions are really negative and movecount is low we allow this 
+          // moves to be searched deeper than the first move.
+          Depth d = std::clamp(newDepth - r, 1, newDepth + (r < -1 && moveCount <= 5));
 
           value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
 

src/search.h

@@ -49,7 +49,6 @@ struct Stack {
   Value staticEval;
   int statScore;
   int moveCount;
-  int distanceFromPv;
   bool inCheck;
   bool ttPv;
   bool ttHit;