Range reductions

[stockfish] / src / search.cpp

diff --git a/src/search.cpp b/src/search.cpp
index 9b56bd2b08325cf3d871a8a49ec57b352c0c67b6..ffeb511095da93cd4e0d125402c018fdf434ced4 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) {
+  Depth reduction(bool i, Depth d, int mn, bool rangeReduction) {
     int r = Reductions[d] * Reductions[mn];
-    return (r + 534) / 1024 + (!i && r > 904);
+    return (r + 534) / 1024 + (!i && r > 904) + rangeReduction;
   }
 
   constexpr int futility_move_count(bool improving, Depth depth) {
@@ -954,6 +954,7 @@ namespace {
 moves_loop: // When in check, search starts here
 
     ttCapture = ttMove && pos.capture_or_promotion(ttMove);
+    int rangeReduction = 0;
 
     // Step 11. A small Probcut idea, when we are in check
     probCutBeta = beta + 409;
@@ -1041,7 +1042,7 @@ 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), 0);
+          int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount, rangeReduction > 2), 0);
 
           if (   captureOrPromotion
               || givesCheck)
@@ -1176,7 +1177,7 @@ moves_loop: // When in check, search starts here
               || !ss->ttPv)
           && (!PvNode || ss->ply > 1 || thisThread->id() % 4 != 3))
       {
-          Depth r = reduction(improving, depth, moveCount);
+          Depth r = reduction(improving, depth, moveCount, rangeReduction > 2);
 
           if (PvNode)
               r--;
@@ -1236,6 +1237,10 @@ moves_loop: // When in check, search starts here
 
           value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
 
+          // Range reductions (~3 Elo)
+          if (ss->staticEval - value < 30 && depth > 7)
+              rangeReduction++;
+
           // If the son is reduced and fails high it will be re-searched at full depth
           doFullDepthSearch = value > alpha && d < newDepth;
           didLMR = true;