fix: prevent infinite recursion in expression evaluation

Add evaluation stack and recursion detection in InputInterpreter to short-circuit self/cyclic expression references. Return zero when recursion is detected during evaluation and unwind stack state after parse. Add unit tests covering self-recursion and mutual recursion cases.

src/Core/InputInterpreter.cs

@@ -20,6 +20,11 @@ namespace CSMic
         // Function registry
         private readonly Dictionary<string, ICodedFunction> functions;
 
+        // Tracks expression variables currently being evaluated to prevent recursion
+        private readonly List<string> evaluationStack;
+        // Tracks whether recursion was encountered during evaluation
+        private int recursionHitCounter = 0;
+
         #endregion
 
         #region Constructors
@@ -30,6 +35,7 @@ namespace CSMic
             numericArrayVariables = new Dictionary<string, decimal[]>(StringComparer.Ordinal);
             expressionVariables = new Dictionary<string, string>(StringComparer.Ordinal);
             functions = new Dictionary<string, ICodedFunction>(StringComparer.Ordinal);
+            evaluationStack = new List<string>();
         }
 
         // Internal constructor to create a child interpreter that shares stores
@@ -39,6 +45,8 @@ namespace CSMic
             this.numericArrayVariables = parent.numericArrayVariables;
             this.expressionVariables = parent.expressionVariables;
             this.functions = parent.functions;
+            // Share the evaluation stack so recursion is tracked across nested parses
+            this.evaluationStack = parent.evaluationStack;
         }
 
         #endregion
@@ -118,7 +126,27 @@ namespace CSMic
             => numericArrayVariables.TryGetValue(name, out values!);
 
         internal bool TryGetExpression(string name, out string expr)
-            => expressionVariables.TryGetValue(name, out expr!);
+        {
+            // Short-circuit self or cyclic references by evaluating to zero
+            // If name is already in the stack, report a zero expression to caller
+            if (evaluationStack.Contains(name))
+            {
+                expr = "0";
+                // Mark that a recursion was detected so caller can zero the whole evaluation
+                recursionHitCounter++;
+                return true;
+            }
+
+            if (expressionVariables.TryGetValue(name, out expr!))
+            {
+                // Mark as currently evaluating; EvaluateExpression will unwind
+                evaluationStack.Add(name);
+                return true;
+            }
+
+            expr = string.Empty;
+            return false;
+        }
 
         internal void AssignNumeric(string name, decimal value)
         {
@@ -147,14 +175,32 @@ namespace CSMic
         {
             // Create a child interpreter sharing stores, so ProduceOutput doesn't affect parent state
             var child = new InputInterpreter(this);
+            int depth = evaluationStack.Count;
+            int hitStart = recursionHitCounter;
             using var ms = new MemoryStream(Encoding.UTF8.GetBytes(expressionText));
             var scanner = new CSMic.Interpreter.Scanner(ms);
             var parser = new CSMic.Interpreter.Parser(scanner)
             {
                 Interpreter = child
             };
-            parser.Parse();
-            return parser.Result;
+            try
+            {
+                parser.Parse();
+                // If recursion was detected during this evaluation, collapse to zero
+                if (recursionHitCounter > hitStart)
+                {
+                    return new FunctionValue(FunctionValueType.Numeric, 0m);
+                }
+                return parser.Result;
+            }
+            finally
+            {
+                // Unwind any names pushed during this evaluation to avoid leaking state
+                while (evaluationStack.Count > depth)
+                {
+                    evaluationStack.RemoveAt(evaluationStack.Count - 1);
+                }
+            }
         }
 
         // Primary developer-facing API: interpret input and return numeric result

src/Tests/InputInterpreterTests.cs

@@ -116,6 +116,26 @@ public class InputInterpreterTests
         AssertSuccess(_interp.Interpret("arr[i+1]"), 3, _interp);
     }
 
+    [Test]
+    public void Expression_SelfRecursion_EvaluatesToZero()
+    {
+        // Define an expression that references itself
+        AssertSuccess(_interp.Interpret("a := a + 1"), 0, _interp);
+        // Using it should not crash and should yield zero
+        AssertSuccess(_interp.Interpret("a"), 0, _interp);
+        // And in larger expressions it should still be zero
+        AssertSuccess(_interp.Interpret("a + 5"), 5, _interp);
+    }
+
+    [Test]
+    public void Expression_MutualRecursion_EvaluatesToZero()
+    {
+        AssertSuccess(_interp.Interpret("a := b + 1"), 0, _interp);
+        AssertSuccess(_interp.Interpret("b := a + 1"), 0, _interp);
+        AssertSuccess(_interp.Interpret("a"), 0, _interp);
+        AssertSuccess(_interp.Interpret("b"), 0, _interp);
+    }
+
     [Test]
     public void StringLiteral_Alone_IsError()
     {