Add support for custom heading anchors in Markdown.

docs/source/syntax/links.md

@@ -6,13 +6,13 @@ A link contains link text (the visible text) and a link destination (the URI tha
 
 ## Inline link
 
-```
+```markdown
 [Link title](links.md)
 ```
 
 [Link title](links.md)
 
-```
+```markdown
 [**Hi**, _I'm md_](links.md)
 ```
 
@@ -22,12 +22,39 @@ A link contains link text (the visible text) and a link destination (the URI tha
 
 You can link to a heading on a page with an anchor link. The link destination should be a `#` followed by the header text. Convert spaces to dashes (`-`).
 
-```
+```markdown
 I link to the [Inline link](#inline-link) heading above.
 ```
 
 I link to the [Inline link](#inline-link) heading above.
 
-```
+```markdown
 I link to the [Notes](tables.md#notes) heading on the [Tables](tables.md) page.
-```
+```
+
+## Heading anchors
+
+Headings will automatically create anchor links in the resulting html. 
+
+```markdown
+## This Is A Header
+```
+
+Will have an anchor link injected with the name `this-is-an-header`.
+
+
+If you need more control over the anchor name you may specify it inline
+
+```markdown
+## This Is A Header [#but-this-is-my-anchor]
+```
+
+Will result in an anchor link named `but-this-my-anchor` to be injected instead. 
+
+Do note that these inline anchors will be normalized. 
+
+```markdown
+## This Is A Header [What about this for an anchor!]
+```
+
+Will result in the anchor `what-about-this-for-an-anchor`.

src/Elastic.Markdown/IO/MarkdownFile.cs

@@ -90,9 +90,12 @@ private void ReadDocumentInstructions(MarkdownDocument document)
 		var contents = document
 			.Where(block => block is HeadingBlock { Level: >= 2 })
 			.Cast<HeadingBlock>()
-			.Select(h => h.Inline?.FirstChild?.ToString())
-			.Where(title => !string.IsNullOrWhiteSpace(title))
-			.Select(title => new PageTocItem { Heading = title!, Slug = _slugHelper.GenerateSlug(title) })
+			.Select(h => (h.GetData("header") as string, h.GetData("anchor") as string))
+			.Select(h => new PageTocItem
+			{
+				Heading = h.Item1!.Replace("`", "").Replace("*", ""),
+				Slug = _slugHelper.GenerateSlug(h.Item2 ?? h.Item1)
+			})
 			.ToList();
 		_tableOfContent.Clear();
 		foreach (var t in contents)

src/Elastic.Markdown/Myst/InlineParsers/HeadingBlockWithSlugParser.cs

@@ -0,0 +1,70 @@
+// Licensed to Elasticsearch B.V under one or more agreements.
+// Elasticsearch B.V licenses this file to you under the Apache 2.0 License.
+// See the LICENSE file in the project root for more information
+
+using System.Text.RegularExpressions;
+using Markdig;
+using Markdig.Helpers;
+using Markdig.Parsers;
+using Markdig.Parsers.Inlines;
+using Markdig.Renderers;
+using Markdig.Syntax;
+
+namespace Elastic.Markdown.Myst.InlineParsers;
+
+public static class HeadingBlockWithSlugBuilderExtensions
+{
+	public static MarkdownPipelineBuilder UseHeadingsWithSlugs(this MarkdownPipelineBuilder pipeline)
+	{
+		pipeline.Extensions.AddIfNotAlready<HeadingBlockWithSlugBuilderExtension>();
+		return pipeline;
+	}
+}
+
+public class HeadingBlockWithSlugBuilderExtension : IMarkdownExtension
+{
+	public void Setup(MarkdownPipelineBuilder pipeline) =>
+		pipeline.BlockParsers.Replace<HeadingBlockParser>(new HeadingBlockWithSlugParser());
+
+	public void Setup(MarkdownPipeline pipeline, IMarkdownRenderer renderer) { }
+}
+
+public class HeadingBlockWithSlugParser : HeadingBlockParser
+{
+	public override bool Close(BlockProcessor processor, Block block)
+	{
+		if (block is not HeadingBlock headerBlock)
+			return base.Close(processor, block);
+
+		var text = headerBlock.Lines.Lines[0].Slice.AsSpan();
+		headerBlock.SetData("header", text.ToString());
+
+		if (!HeadingAnchorParser.MatchAnchorLine().IsMatch(text))
+			return base.Close(processor, block);
+
+		var splits = HeadingAnchorParser.MatchAnchor().EnumerateMatches(text);
+
+		foreach (var match in splits)
+		{
+			var header = text.Slice(0, match.Index);
+			var anchor = text.Slice(match.Index, match.Length);
+
+			var newSlice = new StringSlice(header.ToString());
+			headerBlock.Lines.Lines[0] = new StringLine(ref newSlice);
+			headerBlock.SetData("anchor", anchor.ToString());
+			headerBlock.SetData("header", header.ToString());
+			return base.Close(processor, block);
+		}
+
+		return base.Close(processor, block);
+	}
+}
+
+public static partial class HeadingAnchorParser
+{
+	[GeneratedRegex(@"^.*(?:\[[^[]+\])\s*$", RegexOptions.IgnoreCase, "en-US")]
+	public static partial Regex MatchAnchorLine();
+
+	[GeneratedRegex(@"(?:\[[^[]+\])\s*$", RegexOptions.IgnoreCase, "en-US")]
+	public static partial Regex MatchAnchor();
+}

src/Elastic.Markdown/Myst/MarkdownParser.cs

@@ -30,6 +30,7 @@ public class MarkdownParser(
 	public static MarkdownPipeline MinimalPipeline { get; } =
 		new MarkdownPipelineBuilder()
 			.UseYamlFrontMatter()
+			.UseHeadingsWithSlugs()
 			.UseDirectives()
 			.Build();
 
@@ -38,6 +39,7 @@ public class MarkdownParser(
 			.EnableTrackTrivia()
 			.UsePreciseSourceLocation()
 			.UseDiagnosticLinks()
+			.UseHeadingsWithSlugs()
 			.UseEmphasisExtras(EmphasisExtraOptions.Default)
 			.UseSoftlineBreakAsHardlineBreak()
 			.UseSubstitution()

src/Elastic.Markdown/Myst/SectionedHeadingRenderer.cs

@@ -4,6 +4,7 @@
 using Markdig.Renderers;
 using Markdig.Renderers.Html;
 using Markdig.Syntax;
+using Markdig.Syntax.Inlines;
 using Slugify;
 
 namespace Elastic.Markdown.Myst;
@@ -29,15 +30,14 @@ protected override void Write(HtmlRenderer renderer, HeadingBlock obj)
 			? headings[index]
 			: $"h{obj.Level}";
 
-		var slug = string.Empty;
-		if (headingText == "h2")
-		{
-			renderer.Write(@"<section id=""");
-			slug = _slugHelper.GenerateSlug(obj.Inline?.FirstChild?.ToString());
-			renderer.Write(slug);
-			renderer.Write(@""">");
+		var header = obj.GetData("header") as string;
+		var anchor = obj.GetData("anchor") as string;
 
-		}
+		var slug = _slugHelper.GenerateSlug(anchor ?? header);
+
+		renderer.Write(@"<section id=""");
+		renderer.Write(slug);
+		renderer.Write(@""">");
 
 		renderer.Write('<');
 		renderer.Write(headingText);
@@ -47,16 +47,14 @@ protected override void Write(HtmlRenderer renderer, HeadingBlock obj)
 		renderer.WriteLeafInline(obj);
 
 
-		if (headingText == "h2")
-			// language=html
-			renderer.WriteLine($@"<a class=""headerlink"" href=""#{slug}"" title=""Link to this heading"">¶</a>");
+		// language=html
+		renderer.WriteLine($@"<a class=""headerlink"" href=""#{slug}"" title=""Link to this heading"">¶</a>");
 
 		renderer.Write("</");
 		renderer.Write(headingText);
 		renderer.WriteLine('>');
 
-		if (headingText == "h2")
-			renderer.Write("</section>");
+		renderer.Write("</section>");
 
 		renderer.EnsureLine();
 	}

tests/Elastic.Markdown.Tests/Inline/AnchorLinkTests.cs

@@ -33,6 +33,10 @@ protected override void AddToFileSystem(MockFileSystem fileSystem)
 ## Sub Requirements
 
 To follow this tutorial you will need to install the following components:
+
+## New Requirements [#new-reqs]
+
+These are new requirements
 """;
 		fileSystem.AddFile(@"docs/source/testing/req.md", inclusion);
 		fileSystem.AddFile(@"docs/source/_static/img/observability.png", new MockFileData(""));
@@ -74,6 +78,24 @@ public void GeneratesHtml() =>
 	public void HasNoErrors() => Collector.Diagnostics.Should().HaveCount(0);
 }
 
+
+public class ExternalPageCustomAnchorTests(ITestOutputHelper output) : AnchorLinkTestBase(output,
+"""
+[Sub Requirements](testing/req.md#new-reqs)
+"""
+)
+{
+	[Fact]
+	public void GeneratesHtml() =>
+		// language=html
+		Html.Should().Contain(
+			"""<p><a href="testing/req.html#new-reqs">Sub Requirements</a></p>"""
+		);
+
+	[Fact]
+	public void HasNoErrors() => Collector.Diagnostics.Should().HaveCount(0);
+}
+
 public class ExternalPageAnchorAutoTitleTests(ITestOutputHelper output) : AnchorLinkTestBase(output,
 """
 [](testing/req.md#sub-requirements)