Fix heading auto-link hijacking nested link labels (#668) (#949)

* Fix heading auto-link hijacking nested link labels (#668)

* Refactor heading reference resolution to init-only state (#949)

---------

Co-authored-by: Alexandre Mutel <alexandre_mutel@live.com>

src/Markdig.Tests/TestAutoLinks.cs

@@ -21,4 +21,60 @@ public class TestAutoLinks
 
         Assert.That(html, Is.EqualTo(expected).IgnoreWhiteSpace);
     }
+
+    // https://github.com/xoofx/markdig/issues/668
+    // A heading's implicit reference must not resolve inside another still-open
+    // link bracket, which would break the outer link.
+    [Test]
+    public void TestAutoIdentifierHeadingLinkDoesNotHijackNestedLinkLabel()
+    {
+        var markdown = "# Testing Markdown\n\n" +
+                        "### Header\n\n" +
+                        "[Testing [Header]](https://www.bing.com)\n\n" +
+                        "[Testing [test]](https://www.google.com)\n";
+        var expected = "<h1 id=\"testing-markdown\">Testing Markdown</h1>\n" +
+                        "<h3 id=\"header\">Header</h3>\n" +
+                        "<p><a href=\"https://www.bing.com\">Testing [Header]</a></p>\n" +
+                        "<p><a href=\"https://www.google.com\">Testing [test]</a></p>";
+
+        var pipeline = new MarkdownPipelineBuilder()
+            .UseAutoLinks()
+            .UseAutoIdentifiers()
+            .Build();
+        var html = Markdown.ToHtml(markdown, pipeline);
+
+        Assert.That(html, Is.EqualTo(expected).IgnoreWhiteSpace);
+    }
+
+    [TestCase("[Header]", "<a href=\"#header\">Header</a>")]
+    [TestCase("[Header][]", "<a href=\"#header\">Header</a>")]
+    [TestCase("[label][Header]", "<a href=\"#header\">label</a>")]
+    [TestCase("[Testing [Header]](/target)", "<a href=\"/target\">Testing [Header]</a>")]
+    [TestCase("[Testing [Header][]](/target)", "<a href=\"/target\">Testing [Header][]</a>")]
+    [TestCase("[Testing [label][Header]](/target)", "<a href=\"/target\">Testing [label][Header]</a>")]
+    [TestCase("![Testing [Header]](/image.png)", "<img src=\"/image.png\" alt=\"Testing [Header]\" />")]
+    public void TestAutoIdentifierReferenceResolution(string markdown, string expected)
+    {
+        foreach (var trackTrivia in new[] { false, true })
+        {
+            var builder = new MarkdownPipelineBuilder().UseAutoLinks().UseAutoIdentifiers();
+            if (trackTrivia)
+            {
+                builder.EnableTrackTrivia();
+            }
+
+            var html = Markdown.ToHtml("# Header\n\n" + markdown, builder.Build());
+
+            Assert.That(html, Is.EqualTo("<h1 id=\"header\">Header</h1>\n<p>" + expected + "</p>").IgnoreWhiteSpace, $"TrackTrivia: {trackTrivia}");
+        }
+    }
+
+    [Test]
+    public void TestExplicitReferenceStillResolvesInsideOpenLink()
+    {
+        var pipeline = new MarkdownPipelineBuilder().UseAutoLinks().UseAutoIdentifiers().Build();
+        var html = Markdown.ToHtml("[Header]: /explicit\n\n# Header\n\n[Testing [Header]](/target)", pipeline);
+
+        Assert.That(html, Is.EqualTo("<h1 id=\"header\">Header</h1>\n<p>[Testing <a href=\"/explicit\">Header</a>](/target)</p>").IgnoreWhiteSpace);
+    }
 }