using Microsoft.Extensions.Options;

using MongoDB.Driver;
using MongoDB.Entities;

using PrivaPub.Models.Jobs;
using PrivaPub.Models.Statistics;

using System.Collections.Concurrent;
using System.Threading.Channels;

namespace PrivaPub.Infrastructure.Statistics
{
	public interface IInteractionLedger
	{
		void Record(InteractionEvent interaction, string actorUri = default, bool hostClaimed = false);
		void Count(string host, string key, long bytes = 0);
		void CountServer(string section, string key, int? latencyMs = default);
		long Dropped { get; }
	}

	public static class ServerSections
	{
		public const string Client = "client";
		public const string Served = "served";
	}

	public class InteractionLedger : BackgroundService, IInteractionLedger
	{
		const int Capacity = 10_000;
		const int BatchSize = 1000;
		static readonly TimeSpan BatchWait = TimeSpan.FromSeconds(2);
		static readonly TimeSpan CounterInterval = TimeSpan.FromSeconds(10);
		static readonly TimeSpan KnownHostsAge = TimeSpan.FromMinutes(10);

		sealed record Pending(InteractionEvent Event, string ActorUri, bool HostClaimed);

		readonly Channel<Pending> _channel = Channel.CreateBounded<Pending>(new BoundedChannelOptions(Capacity)
		{
			FullMode = BoundedChannelFullMode.Wait,
			SingleWriter = false,
			SingleReader = false
		});
		readonly InteractionSalts _salts;
		readonly IOptionsMonitor<StatisticsOptions> _options;
		readonly ILogger<InteractionLedger> _logger;
		readonly SemaphoreSlim _flushing = new(1, 1);
		readonly ConcurrentDictionary<(DateTime Day, string Host, string Key), long> _reads = new();
		readonly ConcurrentDictionary<(DateTime Day, string Field, string Key), long> _server = new();
		HashSet<string> _knownHosts = new(StringComparer.Ordinal);
		DateTime _knownHostsAt = DateTime.MinValue;
		long _dropped;
		long _droppedReported;
		long _written;
		long _failed;

		public InteractionLedger(InteractionSalts salts, IOptionsMonitor<StatisticsOptions> options, ILogger<InteractionLedger> logger)
		{
			_salts = salts;
			_options = options;
			_logger = logger;
		}

		public long Dropped => Interlocked.Read(ref _dropped);

		public void Record(InteractionEvent interaction, string actorUri = default, bool hostClaimed = false)
		{
			try
			{
				if (interaction == default || !_options.CurrentValue.Enabled)
					return;
				if (!_channel.Writer.TryWrite(new Pending(interaction, actorUri, hostClaimed)))
					Interlocked.Increment(ref _dropped);
			}
			catch (Exception ex)
			{
				_logger.LogDebug(ex, "An interaction could not be recorded");
			}
		}

		public void Count(string host, string key, long bytes = 0)
		{
			if (!_options.CurrentValue.Enabled || string.IsNullOrEmpty(key))
				return;
			var day = DateTime.UtcNow.Date;
			host = Interactions.Host(host);
			key = Key(key);
			_reads.AddOrUpdate((day, host, key), 1, (_, count) => count + 1);
			if (bytes > 0)
				_reads.AddOrUpdate((day, host, key + ":bytes"), bytes, (_, count) => count + bytes);
		}

		public void CountServer(string section, string key, int? latencyMs = default)
		{
			if (!_options.CurrentValue.Enabled || string.IsNullOrEmpty(key))
				return;
			var day = DateTime.UtcNow.Date;
			var field = section == ServerSections.Served ? nameof(ServerDay.Served) : nameof(ServerDay.Client);
			_server.AddOrUpdate((day, field, Key(key)), 1, (_, count) => count + 1);
			if (latencyMs is { } latency && field == nameof(ServerDay.Client))
				_server.AddOrUpdate((day, nameof(ServerDay.ClientLatency), Interactions.Latency(latency)), 1, (_, count) => count + 1);
		}

		protected override async Task ExecuteAsync(CancellationToken stoppingToken)
		{
			var countersAt = DateTime.UtcNow;
			while (!stoppingToken.IsCancellationRequested)
			{
				try
				{
					using var wait = CancellationTokenSource.CreateLinkedTokenSource(stoppingToken);
					wait.CancelAfter(BatchWait);
					try
					{
						await _channel.Reader.WaitToReadAsync(wait.Token);
					}
					catch (OperationCanceledException) when (!stoppingToken.IsCancellationRequested)
					{
					}
					await FlushEvents(stoppingToken);
					if (DateTime.UtcNow - countersAt >= CounterInterval)
					{
						countersAt = DateTime.UtcNow;
						await FlushCounters(stoppingToken);
					}
				}
				catch (OperationCanceledException) when (stoppingToken.IsCancellationRequested)
				{
				}
				catch (Exception ex)
				{
					_logger.LogWarning(ex, "The interaction ledger could not flush");
					await Task.Delay(BatchWait, CancellationToken.None);
				}
			}
		}

		public override async Task StopAsync(CancellationToken cancellationToken)
		{
			await base.StopAsync(cancellationToken);
			using var budget = new CancellationTokenSource(TimeSpan.FromSeconds(5));
			try
			{
				await Flush(budget.Token);
			}
			catch (Exception ex)
			{
				_logger.LogWarning(ex, "The interaction ledger could not flush on shutdown");
			}
		}

		public async Task Flush(CancellationToken token)
		{
			await FlushEvents(token);
			await FlushCounters(token);
		}

		async Task FlushEvents(CancellationToken token)
		{
			await _flushing.WaitAsync(token);
			try
			{
				while (_channel.Reader.TryPeek(out _))
				{
					var batch = new List<InteractionEvent>(BatchSize);
					while (batch.Count < BatchSize && _channel.Reader.TryRead(out var pending))
						batch.Add(await Prepare(pending, token));
					if (batch.Count == 0)
						return;
					try
					{
						await DB.Default.InsertAsync(batch, token);
						Interlocked.Add(ref _written, batch.Count);
					}
					catch (Exception ex) when (ex is not OperationCanceledException)
					{
						Interlocked.Add(ref _failed, batch.Count);
						_logger.LogWarning(ex, "{Count} interactions could not be written", batch.Count);
					}
				}
			}
			finally
			{
				_flushing.Release();
			}
		}

		async Task<InteractionEvent> Prepare(Pending pending, CancellationToken token)
		{
			var interaction = Interactions.Sanitize(pending.Event);
			if (pending.HostClaimed && interaction.Host != Interactions.Unknown && !(await KnownHosts(token)).Contains(interaction.Host))
				interaction.Host = Interactions.Unknown;
			if (pending.ActorUri != default)
				interaction.ActorHash = InteractionSalts.Hash(await _salts.For(interaction.At, token), pending.ActorUri);
			return interaction;
		}

		async Task<HashSet<string>> KnownHosts(CancellationToken token)
		{
			if (DateTime.UtcNow - _knownHostsAt < KnownHostsAge)
				return _knownHosts;
			var hosts = await DB.Default.Find<RemoteInstance, string>().Project(i => i.Host).ExecuteAsync(token);
			_knownHosts = new HashSet<string>(hosts.Where(h => h != default), StringComparer.Ordinal);
			_knownHostsAt = DateTime.UtcNow;
			return _knownHosts;
		}

		async Task FlushCounters(CancellationToken token)
		{
			foreach (var key in _reads.Keys.ToList())
			{
				if (!_reads.TryRemove(key, out var count) || count == 0)
					continue;
				await DB.Default.Update<InstanceDay>()
					.Match(d => d.Day == key.Day && d.Host == key.Host)
					.Modify(b => b.Inc($"{nameof(InstanceDay.Reads)}.{key.Key}", count))
					.Option(o => o.IsUpsert = true)
					.ExecuteAsync(token);
			}

			var day = DateTime.UtcNow.Date;
			var written = Interlocked.Exchange(ref _written, 0);
			var failed = Interlocked.Exchange(ref _failed, 0);
			var dropped = Interlocked.Read(ref _dropped);
			var newlyDropped = dropped - Interlocked.Exchange(ref _droppedReported, dropped);
			var server = _server.Keys.ToList();
			if (server.Count == 0 && written == 0 && failed == 0 && newlyDropped == 0)
				return;
			var update = DB.Default.Update<ServerDay>()
				.Match(d => d.Day == day)
				.Modify(b => b.Inc(d => d.LedgerWritten, written))
				.Modify(b => b.Inc(d => d.LedgerFailed, failed))
				.Modify(b => b.Inc(d => d.LedgerDropped, newlyDropped))
				.Option(o => o.IsUpsert = true);
			await update.ExecuteAsync(token);
			foreach (var key in server)
			{
				if (!_server.TryRemove(key, out var count) || count == 0)
					continue;
				await DB.Default.Update<ServerDay>()
					.Match(d => d.Day == key.Day)
					.Modify(b => b.Inc($"{key.Field}.{key.Key}", count))
					.Option(o => o.IsUpsert = true)
					.ExecuteAsync(token);
			}
		}

		static string Key(string key) => key.Replace('.', '_').Replace('$', '_');
	}
}