feat: Reformat YouTube videos to the embeded URL

Extract the YouTube video or list ID and generates the embed URL.
Simplifies embedding YouTube videos by allowing one to copy paste the direct YouTube URL rather than extracting the embed URL from the <iframe> source.
Also supports short youtu.be links.

Add "rel=0" (unless explicitly specified) to only show related videos from the same channel.

src/Docfx.MarkdigEngine.Extensions/QuoteSectionNote/QuoteSectionNoteRender.cs

@@ -1,13 +1,16 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Collections.ObjectModel;
+using System.Diagnostics;
+using System.Text.RegularExpressions;
 using System.Web;
 using Markdig.Renderers;
 using Markdig.Renderers.Html;
 
 namespace Docfx.MarkdigEngine.Extensions;
 
-public class QuoteSectionNoteRender : HtmlObjectRenderer<QuoteSectionNoteBlock>
+public partial class QuoteSectionNoteRender : HtmlObjectRenderer<QuoteSectionNoteBlock>
 {
     private readonly MarkdownContext _context;
     private readonly Dictionary<string, string> _notes;
@@ -107,6 +110,7 @@ public static string FixUpLink(string link)
         if (Uri.TryCreate(link, UriKind.Absolute, out Uri videoLink))
         {
             var host = videoLink.Host;
+            var path = videoLink.LocalPath;
             var query = videoLink.Query;
             if (query.Length > 1)
             {
@@ -125,16 +129,107 @@ public static string FixUpLink(string link)
                     query += "&nocookie=true";
                 }
             }
-            else if (host.Equals("youtube.com", StringComparison.OrdinalIgnoreCase) || host.Equals("www.youtube.com", StringComparison.OrdinalIgnoreCase))
+            else if (hostsYouTube.Contains(host, StringComparer.OrdinalIgnoreCase))
             {
                 // case 2, YouTube video
-                host = "www.youtube-nocookie.com";
+                var idYouTube = GetYouTubeId(host, path, ref query);
+                if (idYouTube != null)
+                {
+                    host = "www.youtube-nocookie.com";
+                    path = "/embed/" + idYouTube;
+                }
+                else
+                {
+                    //YouTube Playlist
+                    var listYouTube = GetYouTubeList(query);
+                    if (listYouTube != null)
+                    {
+                        host = "www.youtube-nocookie.com";
+                        path = "/embed/videoseries";
+                        query = "list=" + listYouTube;
+                        //No need to modify query parameters, "list=..." is the same
+                    }
+                }
+
+                //Only related videos from the same channel
+                //https://developers.google.com/youtube/player_parameters
+                //Add rel=0 unless specified in the original link
+                if (query.Split('&').Any(q => q.StartsWith("rel=")) == false)
+                {
+                    if (query.Length == 0)
+                        query = "rel=0";
+                    else
+                        query += "&rel=0";
+                }
+
+                //Keep this to preserve previous behavior
+                if (host.Equals("youtube.com", StringComparison.OrdinalIgnoreCase) || host.Equals("www.youtube.com", StringComparison.OrdinalIgnoreCase))
+                {
+                    host = "www.youtube-nocookie.com";
+                }
             }
 
-            var builder = new UriBuilder(videoLink) { Host = host, Query = query };
+            var builder = new UriBuilder(videoLink) { Host = host, Path = path, Query = query };
             link = builder.Uri.ToString();
         }
 
         return link;
     }
+
+    static readonly ReadOnlyCollection<string> hostsYouTube = new string[] {
+        "youtube.com",
+        "www.youtube.com",
+        "youtu.be",
+        "www.youtube-nocookie.com",
+    }.AsReadOnly();
+
+    private static string GetYouTubeId(string host, string path, ref string query)
+    {
+        if (host == "youtu.be")
+        {
+            return path.Substring(1);
+        }
+
+        var match = ReYouTubeQueryVideo().Match(query);
+        if (match.Success)
+        {
+            //Remove from query
+            query = query.Replace(match.Groups[0].Value, "").Trim('&').Replace("&&", "&");
+            return match.Groups[2].Value;
+        }
+
+        match = ReYouTubePathId().Match(path);
+        if (match.Success)
+        {
+            var id = match.Groups[1].Value;
+
+            if (id == "videoseries")
+                return null;
+
+            return id;
+        }
+
+        return null;
+    }
+
+    [GeneratedRegex(@"(^|&)v=([^&]+)")]
+    private static partial Regex ReYouTubeQueryVideo();
+
+    [GeneratedRegex(@"(^|&)list=([^&]+)")]
+    private static partial Regex ReYouTubeQueryList();
+
+    [GeneratedRegex(@"/embed/([^/]+)$")]
+    private static partial Regex ReYouTubePathId();
+
+    private static string GetYouTubeList(string query)
+    {
+        var match = ReYouTubeQueryList().Match(query);
+        if (match.Success)
+        {
+            return match.Groups[2].Value;
+        }
+
+        return null;
+    }
+
 }