Replace distanceFromPv with a better logic

[stockfish] / src / search.cpp

diff --git a/src/search.cpp b/src/search.cpp
index b3bd2f0371f393767cc52a948c26cf79beddfa83..f23db4c1919cf8b7db978b2c6cfee0fb2d494422 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -35,6 +35,8 @@
 #include "uci.h"
 #include "syzygy/tbprobe.h"
 
+namespace Stockfish {
+
 namespace Search {
 
   LimitsType Limits;
@@ -422,7 +424,7 @@ void Thread::search() {
           while (true)
           {
               Depth adjustedDepth = std::max(1, rootDepth - failedHighCnt - searchAgainCounter);
-              bestValue = ::search<PV>(rootPos, ss, alpha, beta, adjustedDepth, false);
+              bestValue = Stockfish::search<PV>(rootPos, ss, alpha, beta, adjustedDepth, false);
 
               // Bring the best move to the front. It is critical that sorting
               // is done with a stable algorithm because all the values but the
@@ -610,13 +612,12 @@ namespace {
 
     // Step 1. Initialize node
     Thread* thisThread = pos.this_thread();
-    ss->inCheck = pos.checkers();
-    priorCapture = pos.captured_piece();
-    Color us = pos.side_to_move();
-    moveCount = captureCount = quietCount = ss->moveCount = 0;
-    bestValue = -VALUE_INFINITE;
-    maxValue = VALUE_INFINITE;
-    ss->distanceFromPv = (PvNode ? 0 : ss->distanceFromPv);
+    ss->inCheck        = pos.checkers();
+    priorCapture       = pos.captured_piece();
+    Color us           = pos.side_to_move();
+    moveCount          = captureCount = quietCount = ss->moveCount = 0;
+    bestValue          = -VALUE_INFINITE;
+    maxValue           = VALUE_INFINITE;
 
     // Check for the available remaining time
     if (thisThread == Threads.main())
@@ -915,6 +916,7 @@ namespace {
             return probCutBeta;
 
         assert(probCutBeta < VALUE_INFINITE);
+
         MovePicker mp(pos, ttMove, probCutBeta - ss->staticEval, &captureHistory);
         int probCutCount = 0;
         bool ttPv = ss->ttPv;
@@ -1119,6 +1121,7 @@ moves_loop: // When in check, search starts from here
       {
           Value singularBeta = ttValue - ((formerPv + 4) * depth) / 2;
           Depth singularDepth = (depth - 1 + 3 * formerPv) / 2;
+
           ss->excludedMove = move;
           value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, singularDepth, cutNode);
           ss->excludedMove = MOVE_NONE;
@@ -1176,8 +1179,6 @@ moves_loop: // When in check, search starts from here
       // 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
@@ -1203,15 +1204,19 @@ moves_loop: // When in check, search starts from here
 
           // Decrease reduction if position is or has been on the PV
           // and node is not likely to fail low. (~10 Elo)
-          if (ss->ttPv && !likelyFailLow)
+          if (   ss->ttPv
+              && !likelyFailLow)
               r -= 2;
 
           // Increase reduction at root and non-PV nodes when the best move does not change frequently
-          if ((rootNode || !PvNode) && thisThread->rootDepth > 10 && thisThread->bestMoveChanges <= 2)
+          if (   (rootNode || !PvNode)
+              && thisThread->rootDepth > 10
+              && thisThread->bestMoveChanges <= 2)
               r++;
 
           // More reductions for late moves if position was not in previous PV
-          if (moveCountPruning && !formerPv)
+          if (   moveCountPruning
+              && !formerPv)
               r++;
 
           // Decrease reduction if opponent's move count is high (~5 Elo)
@@ -1224,7 +1229,7 @@ moves_loop: // When in check, search starts from here
 
           if (captureOrPromotion)
           {
-              // Unless giving check, this capture is likely bad
+              // Increase reduction for non-checking captures likely to be bad
               if (   !givesCheck
                   && ss->staticEval + PieceValue[EG][pos.captured_piece()] + 210 * depth <= alpha)
                   r++;
@@ -1244,7 +1249,7 @@ moves_loop: // When in check, search starts from here
 
               // Decrease reduction for moves that escape a capture. Filter out
               // castling moves, because they are coded as "king captures rook" and
-              // hence break make_move(). (~2 Elo)
+              // hence break reverse_move() (~2 Elo)
               else if (    type_of(move) == NORMAL
                        && !pos.see_ge(reverse_move(move)))
                   r -= 2 + ss->ttPv - (type_of(movedPiece) == PAWN);
@@ -1272,10 +1277,10 @@ moves_loop: // When in check, search starts from here
                   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 move
+          // 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);
 
@@ -1406,8 +1411,9 @@ moves_loop: // When in check, search starts from here
     assert(moveCount || !ss->inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
 
     if (!moveCount)
-        bestValue = excludedMove ? alpha
-                   :     ss->inCheck ? mated_in(ss->ply) : VALUE_DRAW;
+        bestValue = excludedMove ? alpha :
+                    ss->inCheck  ? mated_in(ss->ply)
+                                 : VALUE_DRAW;
 
     // If there is a move which produces search value greater than alpha we update stats of searched moves
     else if (bestMove)
@@ -1573,15 +1579,13 @@ moves_loop: // When in check, search starts from here
 
       moveCount++;
 
-      // Futility pruning
+      // Futility pruning and moveCount pruning
       if (    bestValue > VALUE_TB_LOSS_IN_MAX_PLY
           && !givesCheck
           &&  futilityBase > -VALUE_KNOWN_WIN
-          && !pos.advanced_pawn_push(move))
+          &&  type_of(move) != PROMOTION)
       {
-          assert(type_of(move) != EN_PASSANT); // Due to !pos.advanced_pawn_push
 
-          // moveCount pruning
           if (moveCount > 2)
               continue;
 
@@ -2034,3 +2038,5 @@ void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
             m.tbRank = 0;
     }
 }
+
+} // namespace Stockfish