refactor: use short indexes for flat lambda closure usage entries

Agent-Logs-Url: https://github.com/dadhi/FastExpressionCompiler/sessions/775f1746-e6c9-4aef-8b08-84f5734431e5

Co-authored-by: dadhi <39516+dadhi@users.noreply.github.com>

Author: Copilot

src/FastExpressionCompiler.LightExpression/FlatExpression.cs

@@ -46,15 +46,16 @@ public enum ExprNodeKind : byte
 }
 
 /// <summary>Maps a lambda node to a parameter identity used from an outer scope and therefore captured in closure.</summary>
+[StructLayout(LayoutKind.Sequential, Pack = 2)]
 public readonly struct LambdaClosureParameterUsage
 {
     /// <summary>The lambda node index containing the parameter usage.</summary>
-    public readonly int LambdaNodeIndex;
+    public readonly short LambdaNodeIndex;
 
     /// <summary>The parameter identity id (<see cref="ExprNode.ChildIdx"/>) referenced from outer scope.</summary>
-    public readonly int ParameterId;
+    public readonly short ParameterId;
 
-    public LambdaClosureParameterUsage(int lambdaNodeIndex, int parameterId)
+    public LambdaClosureParameterUsage(short lambdaNodeIndex, short parameterId)
     {
         LambdaNodeIndex = lambdaNodeIndex;
         ParameterId = parameterId;
@@ -1170,8 +1171,9 @@ private void CollectLambdaClosureParameterUsages(System.Linq.Expressions.LambdaE
 
             var captured = collector.CapturedParameters;
             for (var i = 0; i < captured.Count; ++i)
-                _tree.LambdaClosureParameterUsages.Add(new LambdaClosureParameterUsage(lambdaNodeIndex,
-                    GetId(ref _parameterIds, captured[i])));
+                _tree.LambdaClosureParameterUsages.Add(new LambdaClosureParameterUsage(
+                    checked((short)lambdaNodeIndex),
+                    checked((short)GetId(ref _parameterIds, captured[i]))));
         }
 
         private sealed class LambdaClosureUsageCollector : System.Linq.Expressions.ExpressionVisitor

test/FastExpressionCompiler.LightExpression.UnitTests/LightExpressionTests.cs

@@ -972,7 +972,7 @@ public void Flat_lambda_closure_parameter_usages_excludes_nested_lambda_locals()
             Asserts.AreEqual(pId, usage.ParameterId);
         }
 
-        private static int GetSingleNestedLambdaIndex(ref ExprTree fe)
+        private static short GetSingleNestedLambdaIndex(ref ExprTree fe)
         {
             var nestedLambdaIndex = -1;
             for (var i = 0; i < fe.LambdaNodes.Count; ++i)
@@ -984,16 +984,16 @@ private static int GetSingleNestedLambdaIndex(ref ExprTree fe)
                     throw new InvalidOperationException("Expected a single nested lambda.");
                 nestedLambdaIndex = lambdaIndex;
             }
-            return nestedLambdaIndex;
+            return checked((short)nestedLambdaIndex);
         }
 
-        private static int GetParameterIdByName(ref ExprTree fe, string name)
+        private static short GetParameterIdByName(ref ExprTree fe, string name)
         {
             for (var i = 0; i < fe.Nodes.Count; ++i)
             {
                 ref var node = ref fe.Nodes[i];
                 if (node.NodeType == ExpressionType.Parameter && string.Equals((string)node.Obj, name, StringComparison.Ordinal))
-                    return node.ChildIdx;
+                    return checked((short)node.ChildIdx);
             }
             throw new InvalidOperationException($"Parameter node '{name}' was not found.");
         }