🔍 Implement razoring (#429)

README.md

@@ -97,6 +97,8 @@ If you're a Linux user and are new to .NET ecosystem, the conversation in [this
 
 - Check extensions [[1](https://www.chessprogramming.org/Check_Extensions)]
 
+- Razoring [[1](https://www.chessprogramming.org/Razoring)]
+
 ## Credits
 
 Lynx development wouldn't have been possible without:

src/Lynx.Cli/appsettings.json

@@ -115,7 +115,10 @@
     "BenchDepth": 5,
     "RFP_MaxDepth": 6,
     "RFP_DepthScalingFactor": 75,
-    "ShowWDL": false
+    "ShowWDL": false,
+    "Razoring_MaxDepth": 2,
+    "Razoring_Depth1Bonus": 125,
+    "Razoring_NotDepth1Bonus": 175
   },
 
   // Logging settings

src/Lynx/Configuration.cs

@@ -304,6 +304,12 @@ public sealed class EngineSettings
     public int RFP_DepthScalingFactor { get; set; } = 75;
 
     public bool ShowWDL { get; set; } = false;
+
+    public int Razoring_MaxDepth { get; set; } = 2;
+
+    public int Razoring_Depth1Bonus { get; set; } = 125;
+
+    public int Razoring_NotDepth1Bonus{ get; set; } = 175;
 }
 
 [JsonSourceGenerationOptions(

src/Lynx/Search/NegaMax.cs

@@ -105,16 +105,46 @@ private int NegaMax(int depth, int ply, int alpha, int beta, bool isVerifyingNul
 
         VerifiedNullMovePruning_SearchAgain:
 
-        // 🔍 Reverse FutilityPrunning (RFP) - https://www.chessprogramming.org/Reverse_Futility_Pruning
-        if (!pvNode && !isInCheck
-            && depth <= Configuration.EngineSettings.RFP_MaxDepth)
+        if (!pvNode && !isInCheck && depth <= Configuration.EngineSettings.RFP_MaxDepth)
         {
-            var staticEval = position.StaticEvaluation(Game.HalfMovesWithoutCaptureOrPawnMove);
+            int staticEval = position.StaticEvaluation(Game.HalfMovesWithoutCaptureOrPawnMove);
 
+            // 🔍 Reverse FutilityPrunning (RFP) - https://www.chessprogramming.org/Reverse_Futility_Pruning
             if (staticEval - (Configuration.EngineSettings.RFP_DepthScalingFactor * depth) >= beta)
             {
                 return staticEval;
             }
+
+            // 🔍 Razoring - Strelka impl (CPW) - https://www.chessprogramming.org/Razoring#Strelka
+            if (depth <= Configuration.EngineSettings.Razoring_MaxDepth)
+            {
+                var score = staticEval + Configuration.EngineSettings.Razoring_Depth1Bonus;
+
+                if (score < beta)               // Static evaluation + bonus indicates fail-low node
+                {
+                    if (depth == 1)
+                    {
+                        var qSearchScore = QuiescenceSearch(ply, alpha, beta);
+
+                        return qSearchScore > score
+                            ? qSearchScore
+                            : score;
+                    }
+
+                    score += Configuration.EngineSettings.Razoring_NotDepth1Bonus;
+
+                    if (score < beta)               // Static evaluation indicates fail-low node
+                    {
+                        var qSearchScore = QuiescenceSearch(ply, alpha, beta);
+                        if (qSearchScore < beta)    // Quiescence score also indicates fail-low node
+                        {
+                            return qSearchScore > score
+                                ? qSearchScore
+                                : score;
+                        }
+                    }
+                }
+            }
         }
 
         var nodeType = NodeType.Alpha;

tests/Lynx.Test/ConfigurationValuesTest.cs

@@ -0,0 +1,29 @@
+using Microsoft.Extensions.Configuration;
+using NUnit.Framework;
+
+namespace Lynx.Test;
+
+/// <summary>
+/// Current logic relies on this
+/// </summary>
+[Explicit]
+[Category(Categories.Configuration)]
+[NonParallelizable]
+public class ConfigurationValuesTest
+{
+    [Test]
+    public void RazoringValues()
+    {
+        Assert.Greater(Configuration.EngineSettings.RFP_MaxDepth, Configuration.EngineSettings.Razoring_MaxDepth);
+
+        var config = new ConfigurationBuilder()
+            .AddJsonFile("appsettings.json", optional: true, reloadOnChange: false)
+            .Build();
+
+        var engineSettingsSection = config.GetRequiredSection(nameof(EngineSettings));
+        Assert.IsNotNull(engineSettingsSection);
+        engineSettingsSection.Bind(Configuration.EngineSettings);
+
+        Assert.Greater(Configuration.EngineSettings.RFP_MaxDepth, Configuration.EngineSettings.Razoring_MaxDepth);
+    }
+}