🔍 Add Internal Iterative Reduction (IIR)

README.md

@@ -110,6 +110,8 @@ If you're a Linux user and are new to .NET ecosystem, the conversation in [this
 
 - PEXT Bitboards [[1](https://www.chessprogramming.org/BMI2#PEXTBitboards)]  [[2](https://analog-hors.github.io/site/magic-bitboards/)]
 
+- Internal Iterative Deepening [[1](https://www.chessprogramming.org/Internal_Iterative_Deepening)]
+
 ## Credits
 
 Lynx development would simply not have been possible without:

src/Lynx.Cli/appsettings.json

@@ -36,6 +36,8 @@
     "Razoring_Depth1Bonus": 125,
     "Razoring_NotDepth1Bonus": 175,
 
+    "IIR_MinDepth": 4,
+
     "History_MaxMoveValue": 8192,
 
     // Evaluation

src/Lynx/Configuration.cs

@@ -147,6 +147,8 @@ public sealed class EngineSettings
 
     public int Razoring_NotDepth1Bonus { get; set; } = 175;
 
+    public int IIR_MinDepth { get; set; } = 4;
+
     public int History_MaxMoveValue { get; set; } = 8_192;
 
     #region Evaluation

src/Lynx/Search/NegaMax.cs

@@ -49,6 +49,16 @@ private int NegaMax(int depth, int ply, int alpha, int beta, bool parentWasNullM
             {
                 return ttEvaluation;
             }
+
+            // Internal iterative reduction (IIR)
+            // If this position isn't found in TT, it has never been searched before,
+            // so the search will be potentially expensive.
+            // Therefore, we search with reduced depth for now, expecting to record a TT move
+            // which we'll be able to use later for the full depth search
+            if (ttElementType == default && depth >= Configuration.EngineSettings.IIR_MinDepth)
+            {
+                --depth;
+            }
         }
 
         // Before any time-consuming operations