🔍 NMP, tweak reduction using eval - beta

src/Lynx/Configuration.cs

@@ -142,6 +142,12 @@ public int TranspositionTableSize
     [SPSA<int>(1, 10, 0.5)]
     public int NMP_DepthDivisor { get; set; } = 3;
 
+    [SPSA<int>(150, 350, 10)]
+    public int NMP_StaticEvalBetaDivisor { get; set; } = 100;
+
+    [SPSA<int>(1, 10, 0.5)]
+    public int NMP_StaticEvalBetaMaxReduction { get; set; } = 3;
+
     [SPSA<int>(5, 30, 1)]
     public int AspirationWindow_Base { get; set; } = 13;
 

src/Lynx/Search/NegaMax.cs

@@ -1,6 +1,7 @@
 using Lynx.Model;
 using System.Diagnostics;
 using System.Runtime.CompilerServices;
+using System.Security.Authentication;
 
 namespace Lynx;
 
@@ -170,14 +171,20 @@ private int NegaMax(int depth, int ply, int alpha, int beta, bool parentWasNullM
                 }
             }
 
+            var staticEvalBetaDiff = staticEval - beta;
+
             // 🔍 Null Move Pruning (NMP) - our position is so good that we can potentially afford giving our opponent a double move and still remain ahead of beta
             if (depth >= Configuration.EngineSettings.NMP_MinDepth
-                && staticEval >= beta
+                && staticEvalBetaDiff >= 0
                 && !parentWasNullMove
                 && phase > 2   // Zugzwang risk reduction: pieces other than pawn presents
                 && (ttElementType != NodeType.Alpha || ttScore >= beta))   // TT suggests NMP will fail: entry must not be a fail-low entry with a score below beta - Stormphrax and Ethereal
             {
-                var nmpReduction = Configuration.EngineSettings.NMP_BaseDepthReduction + ((depth + Configuration.EngineSettings.NMP_DepthIncrement) / Configuration.EngineSettings.NMP_DepthDivisor);   // Clarity
+                var nmpReduction = Configuration.EngineSettings.NMP_BaseDepthReduction
+                    + ((depth + Configuration.EngineSettings.NMP_DepthIncrement) / Configuration.EngineSettings.NMP_DepthDivisor)   // Clarity
+                    + Math.Min(
+                        Configuration.EngineSettings.NMP_StaticEvalBetaMaxReduction,
+                        staticEvalBetaDiff / Configuration.EngineSettings.NMP_StaticEvalBetaDivisor);
 
                 // TODO more advanced adaptative reduction, similar to what Ethereal and Stormphrax are doing
                 //var nmpReduction = Math.Min(

src/Lynx/UCIHandler.cs

@@ -336,6 +336,22 @@ private void HandleSetOption(ReadOnlySpan<char> command)
                     }
                     break;
                 }
+            case "nmp_staticevalbetadivisor":
+                {
+                    if (length > 4 && int.TryParse(command[commandItems[4]], out var value))
+                    {
+                        Configuration.EngineSettings.NMP_StaticEvalBetaDivisor = value;
+                    }
+                    break;
+                }
+            case "nmp_staticevalbetamaxreduction":
+                {
+                    if (length > 4 && int.TryParse(command[commandItems[4]], out var value))
+                    {
+                        Configuration.EngineSettings.NMP_StaticEvalBetaMaxReduction = value;
+                    }
+                    break;
+                }
 
             //case "aspirationwindow_delta":
             //    {