Rewrite Descendants iteratively

src/Markdig.Tests/Markdig.Tests.csproj

@@ -58,6 +58,7 @@
     </Compile>
     <Compile Include="Specs\TestEmphasisPlus.cs" />
     <Compile Include="TestEmphasisExtraOptions.cs" />
+    <Compile Include="TestDescendantsOrder.cs" />
     <Compile Include="TestConfigureNewLine.cs" />
     <Compile Include="TestHtmlAttributes.cs" />
     <Compile Include="TestHtmlHelper.cs" />

src/Markdig.Tests/TestDescendantsOrder.cs

@@ -0,0 +1,85 @@
+using NUnit.Framework;
+using Markdig.Syntax;
+using Markdig.Syntax.Inlines;
+using System.Linq;
+using System.Collections.Generic;
+
+namespace Markdig.Tests
+{
+    [TestFixture]
+    public class TestDescendantsOrder
+    {
+        [Test]
+        public void TestSchemas()
+        {
+            foreach (var markdown in TestParser.SpecsMarkdown)
+            {
+                AssertSameDescendantsOrder(markdown);
+            }
+        }
+
+        private void AssertSameDescendantsOrder(string markdown)
+        {
+            var syntaxTree = Markdown.Parse(markdown, new MarkdownPipelineBuilder().UseAdvancedExtensions().Build());
+
+            var descendants_legacy = Descendants_Legacy(syntaxTree).ToList();
+            var descendants_new = syntaxTree.Descendants().ToList();
+
+            Assert.AreEqual(descendants_legacy.Count, descendants_new.Count);
+
+            for (int i = 0; i < descendants_legacy.Count; i++)
+            {
+                Assert.AreSame(descendants_legacy[i], descendants_new[i]);
+            }
+        }
+
+        private static IEnumerable<MarkdownObject> Descendants_Legacy(MarkdownObject markdownObject)
+        {
+            // TODO: implement a recursiveless method
+
+            var block = markdownObject as ContainerBlock;
+            if (block != null)
+            {
+                foreach (var subBlock in block)
+                {
+                    yield return subBlock;
+
+                    foreach (var sub in Descendants_Legacy(subBlock))
+                    {
+                        yield return sub;
+                    }
+
+                    // Visit leaf block that have inlines
+                    var leafBlock = subBlock as LeafBlock;
+                    if (leafBlock?.Inline != null)
+                    {
+                        foreach (var subInline in Descendants_Legacy(leafBlock.Inline))
+                        {
+                            yield return subInline;
+                        }
+                    }
+                }
+            }
+            else
+            {
+                var inline = markdownObject as ContainerInline;
+                if (inline != null)
+                {
+                    var child = inline.FirstChild;
+                    while (child != null)
+                    {
+                        var next = child.NextSibling;
+                        yield return child;
+
+                        foreach (var sub in Descendants_Legacy(child))
+                        {
+                            yield return sub;
+                        }
+
+                        child = next;
+                    }
+                }
+            }
+        }
+    }
+}

src/Markdig.Tests/TestParser.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using System.IO;
+using System.Linq;
 using System.Text;
 using System.Text.RegularExpressions;
 using Markdig.Extensions.JiraLinks;
@@ -155,5 +156,20 @@ private static string Compact(string html)
             html = html.Normalize(NormalizationForm.FormKD);
             return html;
         }
+
+        public static readonly string[] SpecsMarkdown;
+        static TestParser()
+        {
+            string assemblyDir = Path.GetDirectoryName(typeof(TestParser).Assembly.Location);
+            string specsDir = Path.GetFullPath(Path.Combine(assemblyDir, "../../Specs"));
+
+            var files = Directory.GetFiles(specsDir).Where(file => file.EndsWith(".md", StringComparison.Ordinal)).ToList();
+            SpecsMarkdown = new string[files.Count];
+
+            for (int i = 0; i < files.Count; i++)
+            {
+                SpecsMarkdown[i] = File.ReadAllText(files[i]);
+            }
+        }
     }
 }

src/Markdig/Syntax/Inlines/ContainerInline.cs

@@ -94,25 +94,36 @@ public bool ContainsChild(Inline childToFind)
         /// <returns>An enumeration of T</returns>
         public IEnumerable<T> FindDescendants<T>() where T : Inline
         {
-            var child = FirstChild;
-            while (child != null)
-            {
-                var next = child.NextSibling;
+            // Fast-path an empty container to avoid allocating a Stack
+            if (LastChild == null) yield break;
 
-                if (child  is T)
-                {
-                    yield return (T)child;
-                }
+            Stack<Inline> stack = new Stack<Inline>();
 
-                if (child is ContainerInline)
-                {
-                    foreach (var subChild in ((ContainerInline) child).FindDescendants<T>())
-                    {
-                        yield return subChild;
-                    }
-                }
-
-                child = next;
+            var child = LastChild;
+            while (child != null)
+            {
+                stack.Push(child);
+                child = child.PreviousSibling;
+            }
+
+            while (stack.Count > 0)
+            {
+                child = stack.Pop();
+
+                if (child is T childT)
+                {
+                    yield return childT;
+                }
+
+                if (child is ContainerInline containerInline)
+                {
+                    child = containerInline.LastChild;
+                    while (child != null)
+                    {
+                        stack.Push(child);
+                        child = child.PreviousSibling;
+                    }
+                }
             }
         }