using System.Net;
using System.Text.RegularExpressions;

using decePubClient.Models.Mastodon;

using Microsoft.AspNetCore.Components;

namespace decePubClient.Helpers
{
	/// <summary>
	/// A post's or an account's custom emoji (`:shortcode:` among its <c>emojis</c>) shown as their pictures. The shortcode
	/// stays as the picture's alt text, so it is what a screen reader reads and what is copied. Only shortcodes the
	/// server listed for that post or account are replaced, and only with an http(s) picture.
	/// </summary>
	public static partial class CustomEmojis
	{
		[GeneratedRegex(@":([A-Za-z0-9_+-]{1,64}):")]
		private static partial Regex Shortcode();

		[GeneratedRegex(@"(<[^>]*>)")]
		private static partial Regex Tag();

		/// <summary>Plain text (a name, a content warning): escaped, then its shortcodes as pictures.</summary>
		public static MarkupString Text(string text, IReadOnlyCollection<CustomEmoji> emojis) =>
			new(Replace(WebUtility.HtmlEncode(text ?? string.Empty), emojis));

		/// <summary>HTML the server sanitized: the shortcodes in its text, never inside a tag.</summary>
		public static string Html(string html, IReadOnlyCollection<CustomEmoji> emojis) =>
			emojis.Count == 0 || string.IsNullOrEmpty(html)
				? html
				: string.Concat(Tag().Split(html).Select(part => part.StartsWith('<') ? part : Replace(part, emojis)));

		static string Replace(string text, IReadOnlyCollection<CustomEmoji> emojis)
		{
			if (emojis.Count == 0 || !text.Contains(':'))
				return text;
			var known = new Dictionary<string, string>(StringComparer.Ordinal);
			foreach (var emoji in emojis)
				if (!string.IsNullOrEmpty(emoji.Shortcode) && Uri.TryCreate(emoji.Url, UriKind.Absolute, out var url)
					&& url.Scheme is "https" or "http")
					known.TryAdd(emoji.Shortcode, url.AbsoluteUri);
			return Shortcode().Replace(text, match => known.TryGetValue(match.Groups[1].Value, out var url)
				? $"<img class=\"custom-emoji\" src=\"{WebUtility.HtmlEncode(url)}\" alt=\"{match.Value}\" title=\"{match.Value}\" draggable=\"false\" />"
				: match.Value);
		}
	}
}