Adjust reductions based on current node delta and root delta

This patch is a follow up of previous 2 patches that introduced more reductions for PV nodes with low delta and more pruning for nodes with low delta. Instead of writing separate heuristics now it adjust reductions based on delta / rootDelta - it allows to remove 3 separate adjustements of pruning/LMR in different places and also makes reduction dependence on delta and rootDelta smoother. Also now it works for all pruning heuristics and not just 2.

Passed STC
https://tests.stockfishchess.org/tests/view/61ba9b6c57a0d0f327c2d48b
LLR: 2.94 (-2.94,2.94) <0.00,2.50>
Total: 79192 W: 20513 L: 20163 D: 38516
Ptnml(0-2): 238, 8900, 21024, 9142, 292

passed LTC
https://tests.stockfishchess.org/tests/view/61baf77557a0d0f327c2eb8e
LLR: 2.96 (-2.94,2.94) <0.50,3.00>
Total: 158400 W: 41134 L: 40572 D: 76694
Ptnml(0-2): 101, 16372, 45745, 16828, 154

closes https://github.com/official-stockfish/Stockfish/pull/3862

bench 4651538

diff --git a/src/search.cpp b/src/search.cpp
index 2abfbaf2f3dd35d6f0fb414a03fa13b327bcca90..8a4db35185921776e4288e02e83c8744c2c35db7 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -69,9 +69,9 @@ namespace {
   // Reductions lookup table, initialized at startup
   int Reductions[MAX_MOVES]; // [depth or moveNumber]
 
-  Depth reduction(bool i, Depth d, int mn, bool rangeReduction) {
+  Depth reduction(bool i, Depth d, int mn, bool rangeReduction, Value delta, Value rootDelta) {
     int r = Reductions[d] * Reductions[mn];
-    return (r + 534) / 1024 + (!i && r > 904) + rangeReduction;
+    return (r + 1358 - int(delta) * 1024 / int(rootDelta)) / 1024 + (!i && r > 904) + rangeReduction;
   }
 
   constexpr int futility_move_count(bool improving, Depth depth) {
@@ -1015,6 +1015,8 @@ moves_loop: // When in check, search starts here
       // Calculate new depth for this move
       newDepth = depth - 1;
 
+      Value delta = beta - alpha;
+
       // Step 13. Pruning at shallow depth (~200 Elo). Depth conditions are important for mate finding.
       if (  !rootNode
           && pos.non_pawn_material(us)
@@ -1024,7 +1026,7 @@ moves_loop: // When in check, search starts here
           moveCountPruning = moveCount >= futility_move_count(improving, depth);
 
           // Reduced depth of the next LMR search
-          int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount, rangeReduction > 2), 0);
+          int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount, rangeReduction > 2, delta, thisThread->rootDelta), 0);
 
           if (   captureOrPromotion
               || givesCheck)
@@ -1052,8 +1054,6 @@ moves_loop: // When in check, search starts here
 
               history += thisThread->mainHistory[us][from_to(move)];
 
-              lmrDepth = std::max(0, lmrDepth - (beta - alpha < thisThread->rootDelta / 4));
-
               // Futility pruning: parent node (~5 Elo)
               if (   !ss->inCheck
                   && lmrDepth < 8
@@ -1161,12 +1161,11 @@ moves_loop: // When in check, search starts here
               || !captureOrPromotion
               || (cutNode && (ss-1)->moveCount > 1)))
       {
-          Depth r = reduction(improving, depth, moveCount, rangeReduction > 2);
+          Depth r = reduction(improving, depth, moveCount, rangeReduction > 2, delta, thisThread->rootDelta);
 
           // Decrease reduction at some PvNodes (~2 Elo)
           if (   PvNode
-              && bestMoveCount <= 3
-              && beta - alpha >= thisThread->rootDelta / 4)
+              && bestMoveCount <= 3)
               r--;
 
           // Decrease reduction if position is or has been on the PV
@@ -1175,10 +1174,6 @@ moves_loop: // When in check, search starts here
               && !likelyFailLow)
               r -= 2;
 
-          // Increase reduction at non-PV nodes (~3 Elo)
-          if (!PvNode)
-              r++;
-
           // Decrease reduction if opponent's move count is high (~1 Elo)
           if ((ss-1)->moveCount > 13)
               r--;