Use more efficient stack for FlattenSpansInReverse

Author: DustinCampbell

src/Compiler/Microsoft.AspNetCore.Razor.Language/src/Legacy/LegacySyntaxNodeExtensions.ChildSyntaxListReversedEnumeratorStack.cs

@@ -0,0 +1,107 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Diagnostics.CodeAnalysis;
+using Microsoft.AspNetCore.Razor.Language.Syntax;
+
+namespace Microsoft.AspNetCore.Razor.Language.Legacy;
+
+internal static partial class LegacySyntaxNodeExtensions
+{
+    /// <summary>
+    ///  This is similar to <see cref="SyntaxNode.ChildSyntaxListEnumeratorStack"/>.
+    ///  However, instead of enumerating descendant nodes in a top-down, left-to-right
+    ///  fashion, the process is reversed; it operates right-to-left and bottom-up.
+    /// </summary>
+    private struct ChildSyntaxListReversedEnumeratorStack : IDisposable
+    {
+        private const int MaxArraySize = 256;
+
+        private static readonly ObjectPool<ChildSyntaxList.Reversed.Enumerator[]> s_stackPool = new(() => new ChildSyntaxList.Reversed.Enumerator[16]);
+
+        private ChildSyntaxList.Reversed.Enumerator[] _stack;
+        private int _stackPtr;
+
+        public ChildSyntaxListReversedEnumeratorStack(SyntaxNode node)
+        {
+            _stack = s_stackPool.Allocate();
+            _stackPtr = -1;
+
+            PushRightmostChildren(node);
+        }
+
+        private void PushRightmostChildren(SyntaxNode node)
+        {
+            var current = node;
+            do
+            {
+                var children = current.ChildNodes();
+                if (children.Count == 0)
+                {
+                    break;
+                }
+
+                if (++_stackPtr == _stack.Length)
+                {
+                    Array.Resize(ref _stack, _stack.Length * 2);
+                }
+
+                _stack[_stackPtr] = children.Reverse().GetEnumerator();
+
+                current = children.Last();
+            }
+            while (current is not null);
+        }
+
+        private bool TryMoveNextAndGetCurrent([NotNullWhen(true)] out SyntaxNode? node)
+        {
+            if (_stackPtr < 0)
+            {
+                node = null;
+                return false;
+            }
+
+            ref var enumerator = ref _stack[_stackPtr];
+
+            if (!enumerator.MoveNext())
+            {
+                node = null;
+                return false;
+            }
+
+            node = enumerator.Current;
+            return true;
+        }
+
+        public bool TryGetNextNode([NotNullWhen(true)] out SyntaxNode? node)
+        {
+            while (!TryMoveNextAndGetCurrent(out node))
+            {
+                _stackPtr--;
+
+                if (_stackPtr < 0)
+                {
+                    node = null;
+                    return false;
+                }
+            }
+
+            PushRightmostChildren(node);
+            return true;
+        }
+
+        public bool IsEmpty
+            => _stackPtr < 0;
+
+        public void Dispose()
+        {
+            // Return only reasonably-sized stacks to the pool.
+            if (_stack.Length < MaxArraySize)
+            {
+                Array.Clear(_stack, 0, _stack.Length);
+                s_stackPool.Free(_stack);
+            }
+        }
+    }
+}

src/Compiler/Microsoft.AspNetCore.Razor.Language/src/Legacy/LegacySyntaxNodeExtensions.NodeStack.cs

@@ -242,14 +242,11 @@ public static bool IsSpanKind(this SyntaxNode node)
 
     private static IEnumerable<SyntaxNode> FlattenSpansInReverse(this SyntaxNode node)
     {
-        using var stack = new NodeStack(node.DescendantNodes());
+        using var stack = new ChildSyntaxListReversedEnumeratorStack(node);
 
-        // Iterate through stack.
-        while (!stack.IsEmpty)
+        while (stack.TryGetNextNode(out var nextNode))
         {
-            var child = stack.Pop();
-
-            if (child is MarkupStartTagSyntax startTag)
+            if (nextNode is MarkupStartTagSyntax startTag)
             {
                 var children = startTag.Children;
 
@@ -262,7 +259,7 @@ private static IEnumerable<SyntaxNode> FlattenSpansInReverse(this SyntaxNode nod
                     }
                 }
             }
-            else if (child is MarkupEndTagSyntax endTag)
+            else if (nextNode is MarkupEndTagSyntax endTag)
             {
                 var children = endTag.Children;
 
@@ -275,9 +272,9 @@ private static IEnumerable<SyntaxNode> FlattenSpansInReverse(this SyntaxNode nod
                     }
                 }
             }
-            else if (child.IsSpanKind())
+            else if (nextNode.IsSpanKind())
             {
-                yield return child;
+                yield return nextNode;
             }
         }
     }