using MaxMind.Db;

using Microsoft.Extensions.Options;

using PrivaPub.Infrastructure.Http;
using PrivaPub.Infrastructure.Statistics;

using System.Net;

namespace PrivaPub.Infrastructure.Geo
{
	public sealed record GeoFix(string Country, string City, double? Latitude, double? Longitude, int? Asn, string AsnOrg);

	public interface IGeoLocator
	{
		GeoFix Locate(IPAddress address);
		string Source { get; }
	}

	public sealed class NoGeoLocator : IGeoLocator
	{
		public GeoFix Locate(IPAddress address) => default;
		public string Source => default;
	}

	public sealed class DbIpLocator : IGeoLocator, IDisposable
	{
		public static readonly string CityFile = GeoUpdater.FileOf("city");
		public static readonly string AsnFile = GeoUpdater.FileOf("asn");
		static readonly TimeSpan CheckInterval = TimeSpan.FromMinutes(10);

		readonly IOptionsMonitor<StatisticsOptions> _options;
		readonly ILogger<DbIpLocator> _logger;
		readonly object _lock = new();
		Reader _city;
		Reader _asn;
		DateTime _cityStamp;
		DateTime _asnStamp;
		DateTime _checkedAt = DateTime.MinValue;
		bool _warned;

		public DbIpLocator(IOptionsMonitor<StatisticsOptions> options, ILogger<DbIpLocator> logger)
		{
			_options = options;
			_logger = logger;
		}

		public string Source
		{
			get
			{
				Refresh();
				var built = _city?.Metadata.BuildDate ?? _asn?.Metadata.BuildDate;
				return built == default ? default : $"DB-IP Lite {built.Value:yyyy-MM}";
			}
		}

		public GeoFix Locate(IPAddress address)
		{
			if (address == default || !IpRangeGuard.IsPublic(address))
				return default;
			Refresh();
			Dictionary<string, object> place, network;
			try
			{
				// under the lock, so a reload cannot dispose a reader in the middle of a lookup; lookups are rare
				lock (_lock)
				{
					if (_city == default && _asn == default)
						return default;
					place = _city?.Find<Dictionary<string, object>>(address);
					network = _asn?.Find<Dictionary<string, object>>(address);
				}
				var location = Section(place, "location");
				return new GeoFix(
					Text(Section(place, "country"), "iso_code"),
					Text(Section(Section(place, "city"), "names"), "en"),
					Round(location?.GetValueOrDefault("latitude")),
					Round(location?.GetValueOrDefault("longitude")),
					Number(network?.GetValueOrDefault("autonomous_system_number")),
					network?.GetValueOrDefault("autonomous_system_organization") as string);
			}
			catch (Exception ex) when (ex is InvalidDatabaseException or ArgumentException)
			{
				_logger.LogWarning(ex, "The geolocation databases could not be read");
				return default;
			}
		}

		// Looks at the files again now instead of within the next ten minutes: GeoUpdater has just replaced one.
		public void Reload()
		{
			lock (_lock)
				_checkedAt = DateTime.MinValue;
			Refresh();
		}

		void Refresh()
		{
			if (DateTime.UtcNow - _checkedAt < CheckInterval)
				return;
			lock (_lock)
			{
				if (DateTime.UtcNow - _checkedAt < CheckInterval)
					return;
				_checkedAt = DateTime.UtcNow;
				var directory = _options.CurrentValue.GeoDirectory;
				(_city, _cityStamp) = Open(Path.Combine(directory ?? string.Empty, CityFile), _city, _cityStamp);
				(_asn, _asnStamp) = Open(Path.Combine(directory ?? string.Empty, AsnFile), _asn, _asnStamp);
				if (_city == default && _asn == default && !_warned)
				{
					_warned = true;
					_logger.LogInformation("No geolocation databases in {Directory}; servers are not located", directory);
				}
			}
		}

		(Reader, DateTime) Open(string path, Reader current, DateTime stamp)
		{
			if (!File.Exists(path))
			{
				current?.Dispose();
				return (default, default);
			}
			var modified = File.GetLastWriteTimeUtc(path);
			if (current != default && modified == stamp)
				return (current, stamp);
			try
			{
				var reader = new Reader(path, FileAccessMode.MemoryMapped);
				current?.Dispose();
				return (reader, modified);
			}
			catch (Exception ex) when (ex is InvalidDatabaseException or IOException)
			{
				_logger.LogWarning(ex, "{Path} is not a readable geolocation database", path);
				return (current, stamp);
			}
		}

		static Dictionary<string, object> Section(Dictionary<string, object> parent, string name) =>
			parent?.GetValueOrDefault(name) as Dictionary<string, object>;

		static string Text(Dictionary<string, object> parent, string name) => parent?.GetValueOrDefault(name) as string;

		static double? Round(object value) => value switch
		{
			double d => Math.Round(d, 1),
			float f => Math.Round(f, 1),
			_ => (double?)null
		};

		static int? Number(object value) => value switch
		{
			long l => (int)l,
			int i => i,
			uint u => (int)u,
			ulong ul => (int)ul,
			_ => (int?)null
		};

		public void Dispose()
		{
			_city?.Dispose();
			_asn?.Dispose();
		}
	}
}