using System.Diagnostics;
using System.Globalization;
using System.Net;
using System.Net.Http.Json;

using collAnon.Client.Services;

using decePubClient.Helpers;
using decePubClient.Models;
using decePubClient.Models.Mastodon;

namespace decePubClient.Services
{
	/// <summary>
	/// Requests to PrivaPub through the "default" HttpClient (AppConfiguration.ApiBaseAddress), at three authentication
	/// levels: Get/Post/Delete need credentials and sign out when PrivaPub refuses them, *Anon add them when there are
	/// some, *TotallyAnon never send them. The route picks the credentials: clientapi/ the root's JWT, api/ the current
	/// persona's Mastodon token (exchanged from the JWT when missing, and once more when PrivaPub refuses it), oauth/
	/// none. It also picks the JSON: snake_case for api/ and oauth/ (SUtility.MastodonSerializer), camelCase for
	/// clientapi/. A payload can be an object (JSON), name/value pairs (a form) or a Func&lt;HttpContent&gt; (e.g. multipart),
	/// called once per attempt.
	/// Nothing throws: a failure comes back as a response carrying an invalid WebResult (SUtility.ClassifiedFailure, read
	/// with ExtensionMethods.ReadWebResult), a cancelled request as HTTP 410. With retryOnError, 408/429/502/503/504 and
	/// transport errors are retried after 1, 2, 4, 8, 16 and 29 seconds, then every 60, plus jitter, for at most 10
	/// minutes. Every request PrivaPub answers is recorded in RequestStats.
	/// </summary>
	public interface IHttpService
	{
		Task<HttpResponseMessage> Get(string uri, string[] queryParams = default, bool retryOnError = false, CancellationToken cancellationToken = default);

		Task<HttpResponseMessage> GetAnon(string uri, string[] queryParams = default, bool retryOnError = false, CancellationToken cancellationToken = default);

		Task<HttpResponseMessage> GetTotallyAnon(string uri, string[] queryParams = default, bool retryOnError = false,
			CancellationToken cancellationToken = default);

		Task<HttpResponseMessage> Post(string uri, object payload = default, bool retryOnError = false, CancellationToken cancellationToken = default);

		Task<HttpResponseMessage> PostAnon(string uri, object payload = default, bool retryOnError = false, CancellationToken cancellationToken = default);

		Task<HttpResponseMessage> PostTotallyAnon(string uri, object payload = default, bool retryOnError = false,
			CancellationToken cancellationToken = default);

		Task<HttpResponseMessage> Delete(string uri, string[] queryParams = default, object payload = default, bool retryOnError = false,
			CancellationToken cancellationToken = default);

		Task<HttpResponseMessage> DeleteTotallyAnon(string uri, string[] queryParams = default, object payload = default, bool retryOnError = false,
			CancellationToken cancellationToken = default);

		/// <summary>
		/// Exchanges the root's JWT for the persona's Mastodon token (PrivaPub's PersonaExchange) and makes the persona
		/// current. A failure comes back like any other.
		/// </summary>
		Task<HttpResponseMessage> ExchangePersonaToken(string personaId, CancellationToken cancellationToken = default);
	}

	public class HttpService(IHttpClientFactory httpClientFactory, TokenAuthStateProvider authStateProvider, AppStatusService statusService,
		AppConfiguration appConfiguration, RequestStats requestStats, ILoggingService logger, CoalescingStringLocalizer localizer) : IHttpService
	{
		static readonly int[] RetryRampUpDelaysMs = [1_000, 2_000, 4_000, 8_000, 16_000, 29_000];
		const int RetryIndefiniteDelayMs = 60_000;
		static readonly TimeSpan RetryDeadline = TimeSpan.FromMinutes(10);
		static readonly int[] RetryableStatusCodes = [408, 429, 502, 503, 504];

		readonly HttpClient httpClient = httpClientFactory.CreateClient("default");

		public Task<HttpResponseMessage> Get(string uri, string[] queryParams = default, bool retryOnError = false, CancellationToken cancellationToken = default) =>
			Send(nameof(Get), () => BuildRequest(HttpMethod.Get, uri, queryParams: queryParams), SendAuthRequest, retryOnError, cancellationToken);

		public Task<HttpResponseMessage> GetAnon(string uri, string[] queryParams = default, bool retryOnError = false, CancellationToken cancellationToken = default) =>
			Send(nameof(GetAnon), () => BuildRequest(HttpMethod.Get, uri, queryParams: queryParams), SendAuthIfAvailableRequest, retryOnError,
				cancellationToken);

		public Task<HttpResponseMessage> GetTotallyAnon(string uri, string[] queryParams = default, bool retryOnError = false,
			CancellationToken cancellationToken = default) =>
			Send(nameof(GetTotallyAnon), () => BuildRequest(HttpMethod.Get, uri, queryParams: queryParams), SendAnonymousRequest, retryOnError,
				cancellationToken);

		public Task<HttpResponseMessage> Post(string uri, object payload = default, bool retryOnError = false, CancellationToken cancellationToken = default) =>
			Send(nameof(Post), () => BuildRequest(HttpMethod.Post, uri, payload, forceContent: true), SendAuthRequest, retryOnError, cancellationToken);

		public Task<HttpResponseMessage> PostAnon(string uri, object payload = default, bool retryOnError = false, CancellationToken cancellationToken = default) =>
			Send(nameof(PostAnon), () => BuildRequest(HttpMethod.Post, uri, payload, forceContent: true), SendAuthIfAvailableRequest, retryOnError,
				cancellationToken);

		public Task<HttpResponseMessage> PostTotallyAnon(string uri, object payload = default, bool retryOnError = false,
			CancellationToken cancellationToken = default) =>
			Send(nameof(PostTotallyAnon), () => BuildRequest(HttpMethod.Post, uri, payload, forceContent: true), SendAnonymousRequest, retryOnError,
				cancellationToken);

		public Task<HttpResponseMessage> Delete(string uri, string[] queryParams = default, object payload = default, bool retryOnError = false,
			CancellationToken cancellationToken = default) =>
			Send(nameof(Delete), () => BuildRequest(HttpMethod.Delete, uri, payload, queryParams), SendAuthRequest, retryOnError, cancellationToken);

		public Task<HttpResponseMessage> DeleteTotallyAnon(string uri, string[] queryParams = default, object payload = default, bool retryOnError = false,
			CancellationToken cancellationToken = default) =>
			Send(nameof(DeleteTotallyAnon), () => BuildRequest(HttpMethod.Delete, uri, payload, queryParams), SendAnonymousRequest, retryOnError,
				cancellationToken);

		public async Task<HttpResponseMessage> ExchangePersonaToken(string personaId, CancellationToken cancellationToken = default)
		{
			var jwt = await authStateProvider.GetToken(cancellationToken);
			if (jwt is null)
				return SUtility.DefaultUnauthorized(localizer["Your session has expired. Sign in again."]);

			var response = await PostTotallyAnon(APIs.OAuth.POST_Token, new Dictionary<string, string>
			{
				["grant_type"] = APIs.OAuth.TokenExchangeGrant,
				["client_id"] = appConfiguration.ClientId,
				["subject_token"] = jwt,
				["subject_token_type"] = APIs.OAuth.JwtTokenType,
				["avatar_id"] = personaId,
				["scope"] = APIs.OAuth.Scopes
			}, cancellationToken: cancellationToken);
			if (!response.IsSuccessStatusCode)
				return response;

			var token = await response.Content.ReadFromJsonAsync<TokenResponse>(SUtility.MastodonSerializer, cancellationToken);
			if (string.IsNullOrEmpty(token?.AccessToken))
				return SUtility.ClassifiedFailure(FailureCodes.ServerError, localizer["The server sent an invalid response."], HttpStatusCode.BadGateway);

			await authStateProvider.SetPersona(personaId, token.AccessToken, cancellationToken);
			return response;
		}

		async Task<HttpResponseMessage> Send(string caller, Func<HttpRequestMessage> requestFactory,
			Func<Func<HttpRequestMessage>, CancellationToken, Task<HttpResponseMessage>> send, bool retryOnError, CancellationToken cancellationToken)
		{
			var deadline = DateTime.UtcNow + RetryDeadline;
			var attempt = 0;
			while (true)
			{
				var where = $"{nameof(HttpService)}.{caller}";
				var failure = default(string);
				var transportException = default(Exception);
				try
				{
					// A request message can be sent once, so every attempt (and every re-authentication) builds its own.
					var response = await send(requestFactory, cancellationToken);
					var status = (int)response.StatusCode;
					if (!retryOnError || !RetryableStatusCodes.Contains(status))
						return status > 499 && !response.Content.Headers.Contains(SUtility.FailureHeader) ? ServerError(response) : response;

					failure = $"HTTP {status}";
					response.Dispose();
				}
				catch (Exception) when (cancellationToken.IsCancellationRequested)
				{
					return SUtility.DefaultTaskCancelled();
				}
				catch (Exception ex) when (ex is HttpRequestException or TaskCanceledException)
				{
					if (!retryOnError)
					{
						await logger.ProcessError(ex, where, cancellationToken);
						return await DescribeUnreachable(ex, 1, cancellationToken);
					}

					failure = ex.Message;
					transportException = ex;
				}
				catch (Exception ex)
				{
					await logger.ProcessError(ex, where, cancellationToken);
					return SUtility.ClassifiedFailure(FailureCodes.ClientError,
						localizer["The request could not be prepared. Reload the page and try again."], HttpStatusCode.ServiceUnavailable);
				}

				var delay = attempt < RetryRampUpDelaysMs.Length ? RetryRampUpDelaysMs[attempt] : RetryIndefiniteDelayMs;
				delay += Random.Shared.Next(0, delay / 4);
				if (deadline - DateTime.UtcNow <= TimeSpan.FromMilliseconds(delay))
				{
					await logger.ProcessError($"Gave up after {attempt + 1} attempts. {failure}", where, cancellationToken);
					if (transportException is not null)
						return await DescribeUnreachable(transportException, attempt + 1, cancellationToken);
					return SUtility.ClassifiedFailure(FailureCodes.RetriesExhausted,
						localizer["The server is still not answering after {0} attempts. Try again later.", attempt + 1], HttpStatusCode.ServiceUnavailable);
				}

				await logger.ProcessWarning($"Attempt {attempt + 1} failed, retrying in {delay}ms. {failure}", where, cancellationToken);
				try
				{
					await Task.Delay(delay, cancellationToken);
				}
				catch (OperationCanceledException)
				{
					return SUtility.DefaultTaskCancelled();
				}

				attempt++;
			}
		}

		HttpResponseMessage ServerError(HttpResponseMessage response)
		{
			var status = response.StatusCode;
			response.Dispose();
			return SUtility.ClassifiedFailure(FailureCodes.ServerError, localizer["The server reported an internal error (HTTP {0}).", (int)status], status);
		}

		/// <summary>Why a request got no answer: the device is offline, it timed out, retries ran out, or the server is unreachable.</summary>
		async Task<HttpResponseMessage> DescribeUnreachable(Exception exception, int attempts, CancellationToken cancellationToken)
		{
			if (!await statusService.IsOnline(cancellationToken))
				return SUtility.ClassifiedFailure(FailureCodes.DeviceOffline,
					localizer["This device is offline. Check the connection and try again."], HttpStatusCode.ServiceUnavailable);

			if (exception is TaskCanceledException { InnerException: TimeoutException })
				return SUtility.ClassifiedFailure(FailureCodes.Timeout,
					localizer["The server took too long to answer. Try again later."], HttpStatusCode.ServiceUnavailable);

			if (attempts > 1)
				return SUtility.ClassifiedFailure(FailureCodes.RetriesExhausted,
					localizer["The server is still not answering after {0} attempts. Try again later.", attempts], HttpStatusCode.ServiceUnavailable);

			return SUtility.ClassifiedFailure(FailureCodes.ApiUnreachable,
				localizer["The server can't be reached. Try again later."], HttpStatusCode.ServiceUnavailable);
		}

		Task<HttpResponseMessage> SendAnonymousRequest(Func<HttpRequestMessage> requestFactory, CancellationToken cancellationToken) =>
			Measure(requestFactory(), cancellationToken);

		async Task<HttpResponseMessage> SendAuthRequest(Func<HttpRequestMessage> requestFactory, CancellationToken cancellationToken)
		{
			var request = requestFactory();
			if (IsMastodon(request))
				return await SendAsPersona(request, requestFactory, cancellationToken);

			var token = await authStateProvider.GetToken(cancellationToken);
			if (token is null)
			{
				logger.Process($"NO_TOKEN:{request.RequestUri?.OriginalString}", nameof(SendAuthRequest), LogLevel.Warning);
				return SUtility.DefaultUnauthorized(localizer["Your session has expired. Sign in again."]);
			}

			request.Headers.Authorization = new("Bearer", token);
			var response = await Measure(request, cancellationToken);
			if (response.StatusCode is not HttpStatusCode.Unauthorized)
				return response;

			response.Dispose();
			await authStateProvider.LogoutAsync(cancellationToken: cancellationToken);
			return SUtility.DefaultUnauthorized(localizer["Your session has expired. Sign in again."]);
		}

		/// <summary>
		/// A Mastodon API request as the current persona. A missing token is exchanged first; a refused one (revoked,
		/// or PrivaPub's database was reset) is exchanged once more and the request sent again. Without a usable JWT the
		/// session ends.
		/// </summary>
		async Task<HttpResponseMessage> SendAsPersona(HttpRequestMessage request, Func<HttpRequestMessage> requestFactory, CancellationToken cancellationToken)
		{
			var session = await authStateProvider.GetAuthData(cancellationToken);
			var token = await authStateProvider.GetPersonaToken(cancellationToken) ?? await Exchange(session.PersonaId, cancellationToken);
			if (token is null)
				return await EndSession(cancellationToken);

			request.Headers.Authorization = new("Bearer", token);
			var response = await Measure(request, cancellationToken);
			if (response.StatusCode is not HttpStatusCode.Unauthorized)
				return response;

			response.Dispose();
			await authStateProvider.ForgetPersonaToken(session.PersonaId, cancellationToken);
			token = await Exchange(session.PersonaId, cancellationToken);
			if (token is null)
				return await EndSession(cancellationToken);

			var retry = requestFactory();
			retry.Headers.Authorization = new("Bearer", token);
			response = await Measure(retry, cancellationToken);
			if (response.StatusCode is not HttpStatusCode.Unauthorized)
				return response;

			response.Dispose();
			return await EndSession(cancellationToken);
		}

		async Task<string> Exchange(string personaId, CancellationToken cancellationToken)
		{
			if (personaId is null)
				return null;
			using var response = await ExchangePersonaToken(personaId, cancellationToken);
			return response.IsSuccessStatusCode ? await authStateProvider.GetPersonaToken(cancellationToken) : null;
		}

		async Task<HttpResponseMessage> EndSession(CancellationToken cancellationToken)
		{
			if (await authStateProvider.GetToken(cancellationToken) is null)
				await authStateProvider.LogoutAsync(cancellationToken: cancellationToken);
			return SUtility.DefaultUnauthorized(localizer["Your session has expired. Sign in again."]);
		}

		async Task<HttpResponseMessage> SendAuthIfAvailableRequest(Func<HttpRequestMessage> requestFactory, CancellationToken cancellationToken)
		{
			var request = requestFactory();
			var token = IsMastodon(request) ? await authStateProvider.GetPersonaToken(cancellationToken) : await authStateProvider.GetToken(cancellationToken);
			if (token is not null)
				request.Headers.Authorization = new("Bearer", token);
			return await Measure(request, cancellationToken);
		}

		/// <summary>Sends the request and records it in RequestStats.</summary>
		async Task<HttpResponseMessage> Measure(HttpRequestMessage request, CancellationToken cancellationToken)
		{
			var started = Stopwatch.GetTimestamp();
			var response = await httpClient.SendAsync(request, cancellationToken);
			var elapsed = Stopwatch.GetElapsedTime(started).TotalMilliseconds;
			requestStats.Record(new(DateTime.UtcNow, request.Method.Method, RequestStats.Template(request.RequestUri?.OriginalString), (int)response.StatusCode,
				elapsed, response.Content.Headers.ContentLength, HeaderInt(response, "X-RateLimit-Remaining"), HeaderTime(response, "X-RateLimit-Reset")));
			return response;
		}

		static int? HeaderInt(HttpResponseMessage response, string name) =>
			response.Headers.TryGetValues(name, out var values) && int.TryParse(values.FirstOrDefault(), out var number) ? number : null;

		static DateTime? HeaderTime(HttpResponseMessage response, string name)
		{
			if (!response.Headers.TryGetValues(name, out var values))
				return null;
			var value = values.FirstOrDefault();
			if (DateTime.TryParse(value, CultureInfo.InvariantCulture, DateTimeStyles.AdjustToUniversal, out var time))
				return time;
			return long.TryParse(value, out var seconds) ? DateTimeOffset.FromUnixTimeSeconds(seconds).UtcDateTime : null;
		}

		static bool IsMastodon(HttpRequestMessage request) =>
			request.RequestUri?.OriginalString.StartsWith(APIs.MastodonPrefix, StringComparison.Ordinal) == true;

		static HttpRequestMessage BuildRequest(HttpMethod method, string uri, object payload = default, string[] queryParams = default,
			bool forceContent = false)
		{
			var target = queryParams is { Length: > 0 } ? $"{uri}?{string.Join('&', queryParams)}" : uri;
			var request = new HttpRequestMessage(method, target);
			request.Content = payload switch
			{
				Func<HttpContent> content => content(),
				IEnumerable<KeyValuePair<string, string>> fields => new FormUrlEncodedContent(fields),
				null when !forceContent => null,
				_ => JsonContent.Create(payload, payload?.GetType() ?? typeof(object), options: SerializerFor(uri))
			};
			return request;
		}

		/// <summary>snake_case for the Mastodon API and /oauth, camelCase for /clientapi.</summary>
		static System.Text.Json.JsonSerializerOptions SerializerFor(string uri) =>
			uri.StartsWith(APIs.ClientApiPrefix, StringComparison.Ordinal) ? SUtility.DefaultSerializer : SUtility.MastodonSerializer;
	}
}