#293 - alternative expression for the recursive func to pass the test

Author: dadhi

src/FastExpressionCompiler/FastExpressionCompiler.cs

@@ -610,7 +610,7 @@ public void AddNonPassedParam(ParameterExpression expr)
 
                 if (NonPassedParameters.Length == 0)
                 {
-                    NonPassedParameters = new[] { expr };
+                    NonPassedParameters = new[] { expr }; // todo: @perf optimize for a single non passed parameter
                     return;
                 }
 
@@ -1044,7 +1044,7 @@ private static bool TryCollectBoundConstants(ref ClosureInfo closure, Expression
         private static bool TryCollectBoundConstants(ref ClosureInfo closure, Expression expr, IReadOnlyList<PE> paramExprs, bool isNestedLambda, 
             ref ClosureInfo rootClosure, CompilerFlags flags)
         {
-            var paramCount = paramExprs.Count;
+            var paramCount = paramExprs.Count; // todo: @perf move closer to usage - don't need it in the most cases
 #endif
             while (true)
             {
@@ -1070,7 +1070,7 @@ private static bool TryCollectBoundConstants(ref ClosureInfo closure, Expression
                     {
                         // if parameter is used BUT is not in passed parameters and not in local variables,
                         // it means parameter is provided by outer lambda and should be put in closure for current lambda
-                        var p = paramCount - 1;
+                        var p = paramCount - 1; // todo: @perf optimize for the 1 parameter - we don't need the while loop or call the methods
                         while (p != -1 && !ReferenceEquals(paramExprs.GetParameter(p), expr)) --p;
                         if (p == -1 && !closure.IsLocalVar(expr))
                         {

test/FastExpressionCompiler.IssueTests/Issue293_Recursive_Methods.cs

@@ -22,24 +22,65 @@ public int Run()
         [Test]
         public void Test_Recursive_Expression()
         {
-            var expr = MakeFactorialExpression<int>();
+            var expr = MakeFactorialExpressionWithTheTrick<int>();
 
             expr.PrintCSharp();
             var fs = expr.CompileSys();
             fs.PrintIL();
             var res = fs(4);
 
-            var f = expr.CompileFast(true, CompilerFlags.NoInvocationLambdaInlining);
+            var f = expr.CompileFast(true);
             f.PrintIL();
             var res2 = f(4);
 
-            // f = expr.CompileFast(true);
-            // f.PrintIL();
-            // res2 = f(4);
-
             Assert.AreEqual(res, res2);
         }
 
+        public Expression<Func<T, T>> MakeFactorialExpressionWithTheTrick<T>()
+        {
+            var nParam = Expression.Parameter(typeof(T), "n");
+            var methodVar  = Expression.Variable(typeof(Func<T, T>),   "fac");
+            var methodsVar = Expression.Variable(typeof(Func<T, T>[]), "facs");
+            var one = Expression.Constant(1, typeof(T));
+
+            // This does not work:
+            // Func<int, int> rec = null;
+            // Func<int, int> tmp = n => n <= 1 ? 1 : n * rec(n - 1);
+            // rec = tmp; // setting the closure variable! means that this closure variable is not readonly
+
+            // This should work:
+            // var recs = new Func<int, int>[1];
+            // Func<int, int> tmp = n => n <= 1 ? 1 : n * recs[0](n - 1);
+            // recs[0] = tmp; // setting the item inside the closure variable of array type should work because of reference semantic
+
+
+            return Expression.Lambda<Func<T, T>>(
+                Expression.Block(
+                    new[] { methodsVar, methodVar },
+                    Expression.Assign(methodsVar, Expression.NewArrayBounds(typeof(Func<T, T>), Expression.Constant(1))),
+                    Expression.Assign(
+                        methodVar,
+                        Expression.Lambda<Func<T, T>>(
+                            Expression.Condition(
+                                // ( n <= 1 )
+                                Expression.LessThanOrEqual(nParam, one),
+                                // 1
+                                one,
+                                // n * method( n - 1 )
+                                Expression.Multiply(
+                                    // n
+                                    nParam,
+                                    // method( n - 1 )
+                                    Expression.Invoke(
+                                        Expression.ArrayIndex(methodsVar, Expression.Constant(0)),
+                                        Expression.Subtract(nParam, one)))),
+                            nParam)),
+                    Expression.Assign(Expression.ArrayAccess(methodsVar, Expression.Constant(0)), methodVar),
+                    // return method( n );
+                    Expression.Invoke(methodVar, nParam)),
+                nParam);
+        }
+
         //from https://chriscavanagh.wordpress.com/2012/06/18/recursive-methods-in-expression-trees/
         public Expression<Func<T, T>> MakeFactorialExpression<T>()
         {

test/FastExpressionCompiler.TestsRunner/Program.cs

@@ -196,8 +196,8 @@ void Run(Func<int> run, string name = null)
                 Run(new Issue284_Invalid_Program_after_Coalesce().Run);
                 Run(new FastExpressionCompiler.LightExpression.IssueTests.Issue284_Invalid_Program_after_Coalesce().Run);
 
-                // Run(new Issue293_Recursive_Methods().Run);
-                // Run(new FastExpressionCompiler.LightExpression.IssueTests.Issue293_Recursive_Methods().Run);
+                Run(new Issue293_Recursive_Methods().Run);
+                Run(new FastExpressionCompiler.LightExpression.IssueTests.Issue293_Recursive_Methods().Run);
 
                 Console.WriteLine($"============={Environment.NewLine}IssueTests are passing in {sw.ElapsedMilliseconds} ms.");
             });