Optimize PipeTable parsing: O(n²) → O(n) for 3.7x–85x speedup, enables 10K+ row tables (xoofx#922)

* Optimize PipeTable parsing: O(n²) → O(n) for large tables

Pipe tables were creating deeply nested tree structures where each pipe
delimiter contained all subsequent content as children, causing O(n²)
traversal complexity for n cells. This change restructures the parser to
use a flat sibling-based structure, treating tables as matrices rather
than nested trees.

Key changes:
- Set IsClosed=true on PipeTableDelimiterInline to prevent nesting
- Add PromoteNestedPipesToRootLevel() to flatten pipes nested in emphasis
- Update cell boundary detection to use sibling traversal
- Move EmphasisInlineParser before PipeTableParser in processing order
- Fix EmphasisInlineParser to continue past IsClosed delimiters
- Add ContainsParentOrSiblingOfType<T>() helper for flat structure detection

Performance improvements (measured on typical markdown content):

| Rows | Before    | After   | Speedup |
|------|-----------|---------|---------|
| 100  | 542 μs    | 150 μs  | 3.6x    |
| 500  | 23,018 μs | 763 μs  | 30x     |
| 1000 | 89,418 μs | 1,596 μs| 56x     |
| 1500 | 201,593 μs| 2,740 μs| 74x     |
| 5000 | CRASH     | 10,588 μs| ∞      |
| 10000| CRASH     | 18,551 μs| ∞      |

Tables with 5000+ rows previously crashed due to stack overflow from
recursive depth. They now parse successfully with linear time complexity.

* remove baseline results file

* Do not use System.Index and fix nullabillity checks for older platforms

Author: Mpdreamz

.gitignore

@@ -8,6 +8,8 @@
 *.sln.docstates
 *.nuget.props
 *.nuget.targets
+src/.idea
+BenchmarkDotNet.Artifacts
 
 # User-specific files (MonoDevelop/Xamarin Studio)
 *.userprefs

src/Markdig.Benchmarks/PipeTable/PipeTableBenchmark.cs

@@ -0,0 +1,81 @@
+// Copyright (c) Alexandre Mutel. All rights reserved.
+// This file is licensed under the BSD-Clause 2 license.
+// See the license.txt file in the project root for more information.
+
+using BenchmarkDotNet.Attributes;
+using BenchmarkDotNet.Diagnosers;
+using Markdig;
+
+namespace Testamina.Markdig.Benchmarks.PipeTable;
+
+/// <summary>
+/// Benchmark for pipe table parsing performance, especially for large tables.
+/// Tests the performance of PipeTableParser with varying table sizes.
+/// </summary>
+[MemoryDiagnoser]
+[GcServer(true)] // Use server GC to get more comprehensive GC stats
+public class PipeTableBenchmark
+{
+    private string _100Rows = null!;
+    private string _500Rows = null!;
+    private string _1000Rows = null!;
+    private string _1500Rows = null!;
+    private string _5000Rows = null!;
+    private string _10000Rows = null!;
+    private MarkdownPipeline _pipeline = null!;
+
+    [GlobalSetup]
+    public void Setup()
+    {
+        // Pipeline with pipe tables enabled (part of advanced extensions)
+        _pipeline = new MarkdownPipelineBuilder()
+            .UseAdvancedExtensions()
+            .Build();
+
+        // Generate tables of various sizes
+        // Note: Before optimization, 5000+ rows hit depth limit due to nested tree structure.
+        // After optimization, these should work.
+        _100Rows = PipeTableGenerator.Generate(rows: 100, columns: 5);
+        _500Rows = PipeTableGenerator.Generate(rows: 500, columns: 5);
+        _1000Rows = PipeTableGenerator.Generate(rows: 1000, columns: 5);
+        _1500Rows = PipeTableGenerator.Generate(rows: 1500, columns: 5);
+        _5000Rows = PipeTableGenerator.Generate(rows: 5000, columns: 5);
+        _10000Rows = PipeTableGenerator.Generate(rows: 10000, columns: 5);
+    }
+
+    [Benchmark(Description = "PipeTable 100 rows x 5 cols")]
+    public string Parse100Rows()
+    {
+        return Markdown.ToHtml(_100Rows, _pipeline);
+    }
+
+    [Benchmark(Description = "PipeTable 500 rows x 5 cols")]
+    public string Parse500Rows()
+    {
+        return Markdown.ToHtml(_500Rows, _pipeline);
+    }
+
+    [Benchmark(Description = "PipeTable 1000 rows x 5 cols")]
+    public string Parse1000Rows()
+    {
+        return Markdown.ToHtml(_1000Rows, _pipeline);
+    }
+
+    [Benchmark(Description = "PipeTable 1500 rows x 5 cols")]
+    public string Parse1500Rows()
+    {
+        return Markdown.ToHtml(_1500Rows, _pipeline);
+    }
+
+    [Benchmark(Description = "PipeTable 5000 rows x 5 cols")]
+    public string Parse5000Rows()
+    {
+        return Markdown.ToHtml(_5000Rows, _pipeline);
+    }
+
+    [Benchmark(Description = "PipeTable 10000 rows x 5 cols")]
+    public string Parse10000Rows()
+    {
+        return Markdown.ToHtml(_10000Rows, _pipeline);
+    }
+}

src/Markdig.Benchmarks/PipeTable/PipeTableGenerator.cs

@@ -0,0 +1,61 @@
+// Copyright (c) Alexandre Mutel. All rights reserved.
+// This file is licensed under the BSD-Clause 2 license.
+// See the license.txt file in the project root for more information.
+
+using System.Text;
+
+namespace Testamina.Markdig.Benchmarks.PipeTable;
+
+/// <summary>
+/// Generates pipe table markdown content for benchmarking purposes.
+/// </summary>
+public static class PipeTableGenerator
+{
+    private const int DefaultCellWidth = 10;
+
+    /// <summary>
+    /// Generates a pipe table in markdown format.
+    /// </summary>
+    /// <param name="rows">Number of data rows (excluding header)</param>
+    /// <param name="columns">Number of columns</param>
+    /// <param name="cellWidth">Width of each cell content (default: 10)</param>
+    /// <returns>Pipe table markdown string</returns>
+    public static string Generate(int rows, int columns, int cellWidth = DefaultCellWidth)
+    {
+        var sb = new StringBuilder();
+
+        // Header row
+        sb.Append('|');
+        for (int col = 0; col < columns; col++)
+        {
+            sb.Append(' ');
+            sb.Append($"Header {col + 1}".PadRight(cellWidth));
+            sb.Append(" |");
+        }
+        sb.AppendLine();
+
+        // Separator row (with dashes)
+        sb.Append('|');
+        for (int col = 0; col < columns; col++)
+        {
+            sb.Append(new string('-', cellWidth + 2));
+            sb.Append('|');
+        }
+        sb.AppendLine();
+
+        // Data rows
+        for (int row = 0; row < rows; row++)
+        {
+            sb.Append('|');
+            for (int col = 0; col < columns; col++)
+            {
+                sb.Append(' ');
+                sb.Append($"R{row + 1}C{col + 1}".PadRight(cellWidth));
+                sb.Append(" |");
+            }
+            sb.AppendLine();
+        }
+
+        return sb.ToString();
+    }
+}

src/Markdig.Benchmarks/Program.cs

@@ -7,6 +7,7 @@
 using BenchmarkDotNet.Running;
 
 using Markdig;
+using Testamina.Markdig.Benchmarks.PipeTable;
 
 
 namespace Testamina.Markdig.Benchmarks;
@@ -68,7 +69,16 @@ static void Main(string[] args)
         //config.Add(gcDiagnoser);
 
         //var  config = DefaultConfig.Instance;
-        BenchmarkRunner.Run<Program>(config);
+
+        // Run specific benchmarks based on command line arguments
+        if (args.Length > 0 && args[0] == "--pipetable")
+        {
+            BenchmarkRunner.Run<PipeTableBenchmark>(config);
+        }
+        else
+        {
+            BenchmarkRunner.Run<Program>(config);
+        }
         //BenchmarkRunner.Run<TestDictionary>(config);
         //BenchmarkRunner.Run<TestMatchPerf>();
         //BenchmarkRunner.Run<TestStringPerf>();

src/Markdig/Extensions/Tables/PipeTableExtension.cs

@@ -38,7 +38,7 @@ public void Setup(MarkdownPipelineBuilder pipeline)
         var lineBreakParser = pipeline.InlineParsers.FindExact<LineBreakInlineParser>();
         if (!pipeline.InlineParsers.Contains<PipeTableParser>())
         {
-            pipeline.InlineParsers.InsertBefore<EmphasisInlineParser>(new PipeTableParser(lineBreakParser!, Options));
+            pipeline.InlineParsers.InsertAfter<EmphasisInlineParser>(new PipeTableParser(lineBreakParser!, Options));
         }
     }