Add !isInCheck condition

src/Lynx/Search/NegaMax.cs

@@ -42,6 +42,8 @@ private int NegaMax(int depth, int ply, int alpha, int beta, bool parentWasNullM
         NodeType ttElementType = default;
         int ttEvaluation = default;
 
+        bool isInCheck = position.IsInCheck();
+
         if (!isRoot)
         {
             (ttEvaluation, ttBestMove, ttElementType) = _tt.ProbeHash(_ttMask, position, depth, ply, alpha, beta);
@@ -50,12 +52,14 @@ private int NegaMax(int depth, int ply, int alpha, int beta, bool parentWasNullM
                 return ttEvaluation;
             }
 
-            // Internal iterative reduction (IIR)
+            // 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)
+            if (ttElementType == default
+                && depth >= Configuration.EngineSettings.IIR_MinDepth
+                && !isInCheck)
             {
                 --depth;
             }
@@ -64,8 +68,6 @@ private int NegaMax(int depth, int ply, int alpha, int beta, bool parentWasNullM
         // Before any time-consuming operations
         _searchCancellationTokenSource.Token.ThrowIfCancellationRequested();
 
-        bool isInCheck = position.IsInCheck();
-
         if (isInCheck)
         {
             ++depth;