using Microsoft.Extensions.Caching.Memory;

using MongoDB.Entities;

using PrivaPub.Models.Jobs;

namespace PrivaPub.Infrastructure.Jobs
{
	public interface IHostCircuitBreaker
	{
		Task<DateTime?> UnavailableUntil(string host, CancellationToken token);
		Task Succeeded(string host, CancellationToken token);
		Task Failed(string host, string error, CancellationToken token);
	}

	public class HostCircuitBreaker : IHostCircuitBreaker
	{
		public const int Threshold = 10;
		static readonly TimeSpan CacheLifetime = TimeSpan.FromSeconds(30);

		readonly IMemoryCache _cache;

		public HostCircuitBreaker(IMemoryCache cache)
		{
			_cache = cache;
		}

		public async Task<DateTime?> UnavailableUntil(string host, CancellationToken token)
		{
			var instance = await Instance(host, token);
			return instance?.UnavailableUntil > DateTime.UtcNow ? instance.UnavailableUntil : default;
		}

		public async Task Succeeded(string host, CancellationToken token)
		{
			var instance = await Instance(host, token);
			if (instance is not { ConsecutiveFailures: > 0 } && instance?.LastSuccessAt > DateTime.UtcNow.AddHours(-1))
				return;
			await DB.Default.Update<RemoteInstance>()
				.Match(i => i.Host == host)
				.Modify(i => i.ConsecutiveFailures, 0)
				.Modify(i => i.UnavailableUntil, null)
				.Modify(i => i.LastSuccessAt, DateTime.UtcNow)
				.Option(o => o.IsUpsert = true)
				.ExecuteAsync(token);
			_cache.Remove(Key(host));
		}

		public async Task Failed(string host, string error, CancellationToken token)
		{
			var now = DateTime.UtcNow;
			var instance = await DB.Default.UpdateAndGet<RemoteInstance>()
				.Match(i => i.Host == host)
				.Modify(b => b.Inc(i => i.ConsecutiveFailures, 1))
				.Modify(i => i.LastFailureAt, now)
				.Modify(i => i.LastError, error)
				.Option(o => o.IsUpsert = true)
				.ExecuteAsync(token);
			var quarantine = Backoff.HostQuarantine(instance.ConsecutiveFailures, Threshold);
			if (quarantine > TimeSpan.Zero)
				await DB.Default.Update<RemoteInstance>().MatchID(instance.ID)
					.Modify(i => i.UnavailableUntil, now + quarantine)
					.ExecuteAsync(token);
			_cache.Remove(Key(host));
		}

		async Task<RemoteInstance> Instance(string host, CancellationToken token) =>
			await _cache.GetOrCreateAsync(Key(host), async entry =>
			{
				entry.AbsoluteExpirationRelativeToNow = CacheLifetime;
				return await DB.Default.Find<RemoteInstance>().Match(i => i.Host == host).ExecuteFirstAsync(token);
			});

		static string Key(string host) => "remote-instance:" + host;
	}
}