Added futility pruning to search for the last 4 plies. If eval(child) + margin < alpha then the child is skipped. Worth about 30 ELO. Version bumped to 0.5.8.

lithander · lithander · commit 5f6d03740ded · 2021-08-12T14:00:21.000+02:00
diff --git a/MinimalChess/IterativeSearch.cs b/MinimalChess/IterativeSearch.cs
@@ -77,8 +77,9 @@ private void StorePVinTT(Move[] pv, int depth)
                 return (0, Array.Empty<Move>());
 
             Color color = position.SideToMove;
+            bool isChecked = position.IsChecked(color);
             //should we try null move pruning?
-            if (depth >= 2 && !position.IsChecked(color))
+            if (depth >= 2 && !isChecked)
             {
                 const int R = 2;
                 //skip making a move
@@ -98,8 +99,13 @@ private void StorePVinTT(Move[] pv, int depth)
             {
                 expandedNodes++;
 
-                //moves after the PV node are unlikely to raise alpha.
-                if (expandedNodes > 1 && depth >= 3 && window.Width > 0)
+                //moves after the PV node are unlikely to raise alpha. skip those that appear clearly futile!
+                int futilityMargin = (int)color * depth * 70;
+                if (expandedNodes > 1 && depth <= 4 && !isChecked && !window.Inside(Evaluation.Evaluate(child) + futilityMargin, color) && !child.IsChecked(child.SideToMove))
+                    continue;
+
+                //moves after the PV node are unlikely to raise alpha. searching with a null-sized window can save a lot of nodes
+                if (expandedNodes > 1 && depth >= 3)
                 {
                     //we can save a lot of nodes by searching with "null window" first, proving cheaply that the score is below alpha...
                     SearchWindow nullWindow = window.GetLowerBound(color);
@@ -114,9 +120,7 @@ private void StorePVinTT(Move[] pv, int depth)
                 {
                     Transpositions.Store(position.ZobristHash, depth, window, eval.Score, move);
                     //store the PV beginning with move, followed by the PV of the childnode
-                    pv = new Move[eval.PV.Length + 1];
-                    pv[0] = move;
-                    Array.Copy(eval.PV, 0, pv, 1, eval.PV.Length);
+                    pv = Merge(move, eval.PV);
                     //...and maybe get a beta cutoff
                     if (window.Cut(eval.Score, color))
                     {
@@ -136,6 +140,14 @@ private void StorePVinTT(Move[] pv, int depth)
             return (window.GetScore(color), pv);
         }
 
+        private static Move[] Merge(Move move, Move[] pv)
+        {
+            Move[] result = new Move[pv.Length + 1];
+            result[0] = move;
+            Array.Copy(pv, 0, result, 1, pv.Length);
+            return result;
+        }
+
         private int QEval(Board position, SearchWindow window)
         {
             NodesVisited++;
diff --git a/MinimalChessBoard/MinimalChessBoard.csproj b/MinimalChessBoard/MinimalChessBoard.csproj
@@ -24,6 +24,9 @@
     <None Update="testsuite.txt">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </None>
+    <None Update="wac-revised.epd">
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </None>
     <None Update="zugzwang.epd">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </None>
diff --git a/MinimalChessEngine/Program.cs b/MinimalChessEngine/Program.cs
@@ -7,7 +7,7 @@ namespace MinimalChessEngine
 {
     public static class Program
     {
-        const string NAME_VERSION = "MinimalChess 0.5.7";
+        const string NAME_VERSION = "MinimalChess 0.5.8";
 
         static Engine _engine = new Engine();
         static async Task Main()
diff --git a/MinimalChessEngine/Properties/PublishProfiles/Windows x64.pubxml b/MinimalChessEngine/Properties/PublishProfiles/Windows x64.pubxml
@@ -8,7 +8,7 @@ https://go.microsoft.com/fwlink/?LinkID=208121.
     <Configuration>Release</Configuration>
     <Platform>Any CPU</Platform>
     <TargetFramework>net5.0</TargetFramework>
-    <PublishDir>D:\Projekte\Chess\Builds\MinimalChess 0.5.7 Windows</PublishDir>
+    <PublishDir>D:\Projekte\Chess\Builds\MinimalChess 0.5.8 Windows</PublishDir>
     <RuntimeIdentifier>win-x64</RuntimeIdentifier>
     <SelfContained>true</SelfContained>
     <PublishSingleFile>True</PublishSingleFile>

Original file line number	Diff line number	Diff line change
`@@ -7,7 +7,7 @@ namespace MinimalChessEngine`
`7`	`7`	`{`
`8`	`8`	`public static class Program`
`9`	`9`	`{`
`10`		`- const string NAME_VERSION = "MinimalChess 0.5.7";`
	`10`	`+ const string NAME_VERSION = "MinimalChess 0.5.8";`
`11`	`11`
`12`	`12`	`static Engine _engine = new Engine();`
`13`	`13`	`static async Task Main()`