Implement QS FP

src/Lynx.Cli/appsettings.json

@@ -57,7 +57,7 @@
     "FP_DepthScalingFactor": 60,
     "FP_Margin": 250,
 
-    "DeltaPruning_Margin": 200,
+    "FP_QS_Margin": 75,
 
     // Evaluation
     "DoubledPawnPenalty": {

src/Lynx/Configuration.cs

@@ -202,7 +202,7 @@ public sealed class EngineSettings
     public int FP_Margin { get; set; } = 250;
 
     [SPSAAttribute<int>(0, 500, 25)]
-    public int DeltaPruning_Margin { get; set; } = 200;
+    public int FP_QS_Margin { get; set; } = 75;
 
     #region Evaluation
 

src/Lynx/SEE.cs

@@ -201,7 +201,7 @@ public static bool HasPositiveScore(Position position, Move move, int threshold
     }
 
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
-    public static int Gain(Move move)
+    private static int Gain(Move move)
     {
         if (move.IsCastle())
         {

src/Lynx/Search/NegaMax.cs

@@ -559,12 +559,11 @@ public int QuiescenceSearch(int ply, int alpha, int beta)
                 continue;
             }
 
-            // Delta pruning
-            // If the captured piece SEE value + stand pat + some the margin aren't enough to raise alpha, this move and the following ones
-            // (which should have lower SEE value) are unlikely to improve
-            if (standPat + SEE.Gain(move) + Configuration.EngineSettings.DeltaPruning_Margin < alpha)
+            // QS Futility Pruning (FP)
+            // Moves without potential to raise alpha are discarded
+            if (!position.IsInCheck() && standPat + Configuration.EngineSettings.FP_QS_Margin < alpha && !SEE.HasPositiveScore(position, move))
             {
-                return alpha;
+                continue;
             }
 
             var gameState = position.MakeMove(move);