using System.Text.RegularExpressions;

namespace PrivaPub.Federation.Crawler
{
	//RFC 9309: the most specific group naming the crawler (or "*"), the longest matching rule, Allow winning a tie.
	//A 4xx means no rules at all; a 5xx or no answer means keep out.
	public sealed class RobotsTxt
	{
		sealed record Rule(bool Allow, string Pattern, Regex Match);

		readonly List<Rule> _rules;
		readonly bool _everythingDisallowed;

		RobotsTxt(List<Rule> rules, bool everythingDisallowed)
		{
			_rules = rules;
			_everythingDisallowed = everythingDisallowed;
		}

		public static readonly RobotsTxt AllowAll = new(new List<Rule>(), false);
		public static readonly RobotsTxt DisallowAll = new(new List<Rule>(), true);

		public static RobotsTxt From(int status, string text, params string[] agents) => status switch
		{
			>= 200 and < 300 => Parse(text, agents),
			>= 400 and < 500 => AllowAll,
			_ => DisallowAll
		};

		public static RobotsTxt Parse(string text, params string[] agents)
		{
			var groups = new List<(List<string> Agents, List<Rule> Rules)>();
			(List<string> Agents, List<Rule> Rules) current = default;
			var collectingAgents = false;
			foreach (var raw in (text ?? string.Empty).Split('\n'))
			{
				var line = raw.Split('#')[0].Trim();
				var colon = line.IndexOf(':');
				if (colon <= 0)
					continue;
				var field = line[..colon].Trim().ToLowerInvariant();
				var value = line[(colon + 1)..].Trim();
				if (field == "user-agent")
				{
					if (!collectingAgents)
					{
						current = (new List<string>(), new List<Rule>());
						groups.Add(current);
						collectingAgents = true;
					}
					current.Agents.Add(value.ToLowerInvariant());
					continue;
				}
				if (field is not ("allow" or "disallow") || current.Agents == default)
					continue;
				collectingAgents = false;
				if (value.Length == 0)
					continue;
				current.Rules.Add(new Rule(field == "allow", value, Pattern(value)));
			}

			foreach (var agent in agents.Select(a => a.ToLowerInvariant()))
			{
				var named = groups.Where(g => g.Agents.Contains(agent)).SelectMany(g => g.Rules).ToList();
				if (groups.Any(g => g.Agents.Contains(agent)))
					return new RobotsTxt(named, false);
			}
			return new RobotsTxt(groups.Where(g => g.Agents.Contains("*")).SelectMany(g => g.Rules).ToList(), false);
		}

		public bool Allows(string path)
		{
			if (_everythingDisallowed)
				return false;
			path = string.IsNullOrEmpty(path) ? "/" : path;
			Rule best = default;
			foreach (var rule in _rules.Where(r => r.Match.IsMatch(path)))
				if (best == default || rule.Pattern.Length > best.Pattern.Length || rule.Pattern.Length == best.Pattern.Length && rule.Allow)
					best = rule;
			return best?.Allow != false;
		}

		static Regex Pattern(string value)
		{
			var anchored = value.EndsWith('$');
			var body = Regex.Escape(anchored ? value[..^1] : value).Replace("\\*", ".*");
			return new Regex("^" + body + (anchored ? "$" : string.Empty), RegexOptions.CultureInvariant, TimeSpan.FromMilliseconds(100));
		}
	}
}