🔍 Extend on TT hit at low depths when TT cutoff doesn't happen but TT…

… depth > depth (#1342)
lynx-chess · Jan 9, 2025 · f3b7787 · f3b7787
1 parent d43a726
commit f3b7787
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Showing 3 changed files with 23 additions and 5 deletions.
diff --git a/src/Lynx/Configuration.cs b/src/Lynx/Configuration.cs
@@ -235,6 +235,9 @@ public int Threads
     [SPSA<int>(-8192, 0, 512)]
     public int HistoryPrunning_Margin { get; set; } = -1940;
 
+    [SPSA<int>(0, 10, 0.5)]
+    public int TTHit_NoCutoffExtension_MaxDepth { get; set; } = 6;
+
     #endregion
 }
 

diff --git a/src/Lynx/Search/NegaMax.cs b/src/Lynx/Search/NegaMax.cs
@@ -52,12 +52,19 @@ private int NegaMax(int depth, int ply, int alpha, int beta, bool cutnode, Cance
             // TT cutoffs
             if (!pvNode
                 && ttScore != EvaluationConstants.NoHashEntry
-                && ttDepth >= depth
-                && (ttElementType == NodeType.Exact
-                    || (ttElementType == NodeType.Alpha && ttScore <= alpha)
-                    || (ttElementType == NodeType.Beta && ttScore >= beta)))
+                && ttDepth >= depth)
             {
-                return ttScore;
+                if (ttElementType == NodeType.Exact
+                    || (ttElementType == NodeType.Alpha && ttScore <= alpha)
+                    || (ttElementType == NodeType.Beta && ttScore >= beta))
+                {
+                    return ttScore;
+                }
+                else if (depth <= Configuration.EngineSettings.TTHit_NoCutoffExtension_MaxDepth)
+                {
+                    // Extension idea from Stormphrax
+                    ++depth;
+                }
             }
 
             // Internal iterative reduction (IIR)

diff --git a/src/Lynx/UCIHandler.cs b/src/Lynx/UCIHandler.cs
@@ -558,6 +558,14 @@ private void HandleSetOption(ReadOnlySpan<char> command)
                     }
                     break;
                 }
+            case "tthit_nocutoffextension_maxdepth":
+                {
+                    if (length > 4 && int.TryParse(command[commandItems[4]], out var value))
+                    {
+                        Configuration.EngineSettings.TTHit_NoCutoffExtension_MaxDepth = value;
+                    }
+                    break;
+                }
 
             #endregion