Introduce Quadruple Extensions

Viren6 · vondele · commit 741aaf8a38c7 · 2024-05-05T12:53:58.000+02:00
This patch introduces quadruple extensions, with the new condition of not ttPv. It also generalises all margins, so that extensions can still occur if conditions are only partially fulfilled, but with a stricter margin. Failed STC: LLR: -2.94 (-2.94,2.94) <0.00,2.00> Total: 16096 W: 3984 L: 4228 D: 7884 Ptnml(0-2): 72, 2067, 4002, 1847, 60 https://tests.stockfishchess.org/tests/view/66316422d01fb9ac9bcdbdcd Passed VVLTC 1: LLR: 2.94 (-2.94,2.94) <0.00,2.00> Total: 96660 W: 24550 L: 24210 D: 47900 Ptnml(0-2): 5, 8776, 30426, 9120, 3 https://tests.stockfishchess.org/tests/view/66361f2c74fa3f41ef2ee091 Passed VVLTC 2: LLR: 2.94 (-2.94,2.94) <0.50,2.50> Total: 80546 W: 20495 L: 20120 D: 39931 Ptnml(0-2): 6, 7477, 24929, 7858, 3 https://tests.stockfishchess.org/tests/view/66350cf739ba8e443112b3fa closes #5211 bench 2233743
diff --git a/src/search.cpp b/src/search.cpp
@@ -593,7 +593,6 @@ Value Search::Worker::search(
     bestMove             = Move::none();
     (ss + 2)->killers[0] = (ss + 2)->killers[1] = Move::none();
     (ss + 2)->cutoffCnt                         = 0;
-    ss->multipleExtensions                      = (ss - 1)->multipleExtensions;
     Square prevSq = ((ss - 1)->currentMove).is_ok() ? ((ss - 1)->currentMove).to_sq() : SQ_NONE;
     ss->statScore = 0;
 
@@ -1049,17 +1048,16 @@ Value Search::Worker::search(
 
                 if (value < singularBeta)
                 {
-                    extension = 1;
-
-                    // We make sure to limit the extensions in some way to avoid a search explosion
-                    if (!PvNode && ss->multipleExtensions <= 16)
-                    {
-                        extension = 2 + (value < singularBeta - 11 && !ttCapture);
-                        depth += depth < 14;
-                    }
-                    if (PvNode && !ttCapture && ss->multipleExtensions <= 5
-                        && value < singularBeta - 38)
-                        extension = 2;
+                    int doubleMargin = 251 * PvNode - 241 * !ttCapture;
+                    int tripleMargin =
+                      135 + 234 * PvNode - 248 * !ttCapture + 124 * (ss->ttPv || !ttCapture);
+                    int quadMargin = 447 + 354 * PvNode - 300 * !ttCapture + 206 * ss->ttPv;
+
+                    extension = 1 + (value < singularBeta - doubleMargin)
+                              + (value < singularBeta - tripleMargin)
+                              + (value < singularBeta - quadMargin);
+
+                    depth += ((!PvNode) && (depth < 14));
                 }
 
                 // Multi-cut pruning
@@ -1104,7 +1102,6 @@ Value Search::Worker::search(
 
         // Add extension to new depth
         newDepth += extension;
-        ss->multipleExtensions = (ss - 1)->multipleExtensions + (extension >= 2);
 
         // Speculative prefetch as early as possible
         prefetch(tt.first_entry(pos.key_after(move)));
diff --git a/src/search.h b/src/search.h
@@ -74,7 +74,6 @@ struct Stack {
     bool            inCheck;
     bool            ttPv;
     bool            ttHit;
-    int             multipleExtensions;
     int             cutoffCnt;
 };
 
