using AngleSharp.Css.Dom;
using AngleSharp.Dom;

using Ganss.Xss;

using System.Text.RegularExpressions;

namespace PrivaPub.Federation.Objects
{
	public static partial class ContentSanitizer
	{
		static readonly HtmlSanitizer Sanitizer = Build();

		public static string Html(string html) =>
			string.IsNullOrWhiteSpace(html) ? string.Empty : Sanitizer.Sanitize(html).Trim();

		static HtmlSanitizer Build()
		{
			var sanitizer = new HtmlSanitizer(new HtmlSanitizerOptions
			{
				AllowedTags = new HashSet<string>(StringComparer.OrdinalIgnoreCase)
				{
					"p", "br", "span", "a", "abbr", "del", "s", "pre", "blockquote", "code", "b", "strong", "u", "i", "em",
					"sub", "sup", "ul", "ol", "li", "ruby", "rt", "rp", "h1", "h2", "h3", "h4", "h5", "h6"
				},
				AllowedAttributes = new HashSet<string>(StringComparer.OrdinalIgnoreCase)
				{
					"href", "rel", "class", "translate", "start", "reversed", "value", "title"
				},
				AllowedCssProperties = new HashSet<string>(),
				AllowedAtRules = new HashSet<CssRuleType>(),
				AllowedSchemes = new HashSet<string>(StringComparer.OrdinalIgnoreCase)
				{
					"http", "https", "dat", "dweb", "ipfs", "ipns", "ssb", "gopher", "xmpp", "magnet", "gemini"
				},
				UriAttributes = new HashSet<string>(StringComparer.OrdinalIgnoreCase) { "href" }
			})
			{
				KeepChildNodes = true
			};
			foreach (var allowed in new[] { "mention", "hashtag", "ellipsis", "invisible" })
				sanitizer.AllowedClasses.Add(allowed);
			sanitizer.RemovingCssClass += (_, e) => e.Cancel = MicroformatClass().IsMatch(e.CssClass);
			sanitizer.RemovingTag += (_, e) =>
			{
				if (e.Tag.LocalName is "script" or "style" or "template" or "iframe" or "object" or "embed" or "noscript" or "svg" or "math")
					e.Tag.InnerHtml = string.Empty;
			};
			sanitizer.PostProcessNode += (_, e) =>
			{
				if (e.Node is not IElement element)
					return;
				switch (element.LocalName)
				{
					case "a":
						if (!SchemePrefix().IsMatch(element.GetAttribute("href") ?? string.Empty))
							element.RemoveAttribute("href");
						element.SetAttribute("rel", "nofollow noopener noreferrer");
						element.SetAttribute("target", "_blank");
						break;
					case "h1" or "h2" or "h3" or "h4" or "h5" or "h6":
						var paragraph = e.Document.CreateElement("p");
						var strong = e.Document.CreateElement("strong");
						while (element.FirstChild != default)
							strong.AppendChild(element.FirstChild);
						paragraph.AppendChild(strong);
						e.ReplacementNodes.Add(paragraph);
						break;
				}
			};
			return sanitizer;
		}

		[GeneratedRegex("^[a-z][a-z0-9+.-]*:", RegexOptions.IgnoreCase)]
		private static partial Regex SchemePrefix();

		[GeneratedRegex("^(h|p|u|dt|e)-[a-z0-9-]+$")]
		private static partial Regex MicroformatClass();
	}
}