using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using CommunityToolkit.Mvvm.Messaging;
using MailKit;
using MailKit.Net.Imap;
using MailKit.Search;
using MimeKit;
using MoreLinq;
using Serilog;
using Wino.Core.Domain.Entities.Mail;
using Wino.Core.Domain.Entities.Shared;
using Wino.Core.Domain.Enums;
using Wino.Core.Domain.Exceptions;
using Wino.Core.Domain.Interfaces;
using Wino.Core.Domain.Models.Connectivity;
using Wino.Core.Domain.Models.MailItem;
using Wino.Core.Domain.Models.Synchronization;
using Wino.Core.Extensions;
using Wino.Core.Integration;
using Wino.Core.Integration.Processors;
using Wino.Core.Mime;
using Wino.Core.Requests.Bundles;
using Wino.Core.Requests.Folder;
using Wino.Core.Requests.Mail;
using Wino.Messaging.UI;
using Wino.Services.Extensions;

namespace Wino.Core.Synchronizers.Mail
{
    public class ImapSynchronizer : WinoSynchronizer<ImapRequest, ImapMessageCreationPackage, object>
    {
        private CancellationTokenSource idleDoneToken;
        private CancellationTokenSource cancelInboxListeningToken = new CancellationTokenSource();

        private IMailFolder inboxFolder;

        private readonly ILogger _logger = Log.ForContext<ImapSynchronizer>();
        private readonly ImapClientPool _clientPool;
        private readonly IImapChangeProcessor _imapChangeProcessor;
        private readonly IApplicationConfiguration _applicationConfiguration;

        // Minimum summary items to Fetch for mail synchronization from IMAP.
        private readonly MessageSummaryItems mailSynchronizationFlags =
            MessageSummaryItems.Flags |
            MessageSummaryItems.UniqueId |
            MessageSummaryItems.ThreadId |
            MessageSummaryItems.EmailId |
            MessageSummaryItems.Headers |
            MessageSummaryItems.PreviewText |
            MessageSummaryItems.GMailThreadId |
            MessageSummaryItems.References |
            MessageSummaryItems.ModSeq;

        /// <summary>
        /// Timer that keeps the <see cref="InboxClient"/> alive for the lifetime of the pool.
        /// Sends NOOP command to the server periodically.
        /// </summary>
        private Timer _noOpTimer;

        /// <summary>
        /// ImapClient that keeps the Inbox folder opened all the time for listening notifications.
        /// </summary>
        private ImapClient _inboxIdleClient;

        public override uint BatchModificationSize => 1000;
        public override uint InitialMessageDownloadCountPerFolder => 250;

        public ImapSynchronizer(MailAccount account,
                                IImapChangeProcessor imapChangeProcessor,
                                IApplicationConfiguration applicationConfiguration) : base(account)
        {
            // Create client pool with account protocol log.
            _imapChangeProcessor = imapChangeProcessor;
            _applicationConfiguration = applicationConfiguration;

            var poolOptions = ImapClientPoolOptions.CreateDefault(Account.ServerInformation, CreateAccountProtocolLogFileStream());

            _clientPool = new ImapClientPool(poolOptions);
            idleDoneToken = new CancellationTokenSource();
        }

        private Stream CreateAccountProtocolLogFileStream()
        {
            if (Account == null) throw new ArgumentNullException(nameof(Account));

            var logFile = Path.Combine(_applicationConfiguration.ApplicationDataFolderPath, $"Protocol_{Account.Address}.log");

            // Each session should start a new log.
            if (File.Exists(logFile)) File.Delete(logFile);

            return new FileStream(logFile, FileMode.CreateNew);
        }

        // TODO
        // private async void NoOpTimerTriggered(object state) => await AwaitInboxIdleAsync();

        private async Task AwaitInboxIdleAsync()
        {
            if (_inboxIdleClient == null)
            {
                _logger.Warning("InboxClient is null. Cannot send NOOP command.");
                return;
            }

            await _clientPool.EnsureConnectedAsync(_inboxIdleClient);
            await _clientPool.EnsureAuthenticatedAsync(_inboxIdleClient);

            try
            {
                if (inboxFolder == null)
                {
                    inboxFolder = _inboxIdleClient.Inbox;
                    await inboxFolder.OpenAsync(FolderAccess.ReadOnly, cancelInboxListeningToken.Token);
                }

                idleDoneToken = new CancellationTokenSource();

                await _inboxIdleClient.IdleAsync(idleDoneToken.Token, cancelInboxListeningToken.Token);
            }
            finally
            {
                idleDoneToken.Dispose();
                idleDoneToken = null;
            }
        }

        private async Task StopInboxListeningAsync()
        {
            if (inboxFolder != null)
            {
                inboxFolder.CountChanged -= InboxFolderCountChanged;
                inboxFolder.MessageExpunged -= InboxFolderMessageExpunged;
                inboxFolder.MessageFlagsChanged -= InboxFolderMessageFlagsChanged;
            }

            if (_noOpTimer != null)
            {
                _noOpTimer.Dispose();
                _noOpTimer = null;
            }

            if (idleDoneToken != null)
            {
                idleDoneToken.Cancel();
                idleDoneToken.Dispose();
                idleDoneToken = null;
            }

            if (_inboxIdleClient != null)
            {
                await _inboxIdleClient.DisconnectAsync(true);
                _inboxIdleClient.Dispose();
                _inboxIdleClient = null;
            }
        }

        /// <summary>
        /// Tries to connect & authenticate with the given credentials.
        /// Prepares synchronizer for active listening of Inbox folder.
        /// </summary>
        public async Task StartInboxListeningAsync()
        {
            _inboxIdleClient = await _clientPool.GetClientAsync();

            // Run it every 8 minutes after 1 minute delay.
            // _noOpTimer = new Timer(NoOpTimerTriggered, null, 60000, 8 * 60 * 1000);

            await _clientPool.EnsureConnectedAsync(_inboxIdleClient);
            await _clientPool.EnsureAuthenticatedAsync(_inboxIdleClient);

            if (!_inboxIdleClient.Capabilities.HasFlag(ImapCapabilities.Idle))
            {
                _logger.Information("Imap server does not support IDLE command. Listening live changes is not supported for {Name}", Account.Name);
                return;
            }

            inboxFolder = _inboxIdleClient.Inbox;

            if (inboxFolder == null)
            {
                _logger.Information("Inbox folder is null. Cannot listen for changes.");
                return;
            }

            inboxFolder.CountChanged += InboxFolderCountChanged;
            inboxFolder.MessageExpunged += InboxFolderMessageExpunged;
            inboxFolder.MessageFlagsChanged += InboxFolderMessageFlagsChanged;

            while (!cancelInboxListeningToken.IsCancellationRequested)
            {
                await AwaitInboxIdleAsync();
            }

            await StopInboxListeningAsync();
        }

        private void InboxFolderMessageFlagsChanged(object sender, MessageFlagsChangedEventArgs e)
        {
            Console.WriteLine("Flags have changed for message #{0} ({1}).", e.Index, e.Flags);
        }

        private void InboxFolderMessageExpunged(object sender, MessageEventArgs e)
        {
            _logger.Information("Inbox folder message expunged");
        }

        private void InboxFolderCountChanged(object sender, EventArgs e)
        {
            _logger.Information("Inbox folder count changed.");
        }

        /// <summary>
        /// Parses List of string of mail copy ids and return valid uIds.
        /// Follow the rules for creating arbitrary unique id for mail copies.
        /// </summary>
        private UniqueIdSet GetUniqueIds(IEnumerable<string> mailCopyIds)
            => new(mailCopyIds.Select(a => new UniqueId(MailkitClientExtensions.ResolveUid(a))));

        /// <summary>
        /// Returns UniqueId for the given mail copy id.
        /// </summary>
        private UniqueId GetUniqueId(string mailCopyId)
            => new(MailkitClientExtensions.ResolveUid(mailCopyId));

        #region Mail Integrations

        // Items are grouped before being passed to this method.
        // Meaning that all items will come from and to the same folder.
        // It's fine to assume that here.

        public override List<IRequestBundle<ImapRequest>> Move(BatchMoveRequest requests)
        {
            return CreateTaskBundle(async (client, item) =>
            {
                var sourceFolder = await client.GetFolderAsync(item.FromFolder.RemoteFolderId);
                var destinationFolder = await client.GetFolderAsync(item.ToFolder.RemoteFolderId);

                // Only opening source folder is enough.
                await sourceFolder.OpenAsync(FolderAccess.ReadWrite).ConfigureAwait(false);
                await sourceFolder.MoveToAsync(GetUniqueId(item.Item.Id), destinationFolder).ConfigureAwait(false);
                await sourceFolder.CloseAsync().ConfigureAwait(false);
            }, requests);
        }

        public override List<IRequestBundle<ImapRequest>> ChangeFlag(BatchChangeFlagRequest requests)
        {
            return CreateTaskBundle(async (client, item) =>
            {
                var folder = item.Item.AssignedFolder;
                var remoteFolder = await client.GetFolderAsync(folder.RemoteFolderId);

                await remoteFolder.OpenAsync(FolderAccess.ReadWrite).ConfigureAwait(false);
                await remoteFolder.StoreAsync(GetUniqueId(item.Item.Id), new StoreFlagsRequest(item.Item.IsFlagged ? StoreAction.Add : StoreAction.Remove, MessageFlags.Flagged) { Silent = true }).ConfigureAwait(false);
                await remoteFolder.CloseAsync().ConfigureAwait(false);
            }, requests);
        }

        public override List<IRequestBundle<ImapRequest>> Delete(BatchDeleteRequest requests)
        {
            return CreateTaskBundle(async (client, request) =>
            {
                var folder = request.Item.AssignedFolder;
                var remoteFolder = await client.GetFolderAsync(folder.RemoteFolderId).ConfigureAwait(false);

                await remoteFolder.OpenAsync(FolderAccess.ReadWrite).ConfigureAwait(false);
                await remoteFolder.StoreAsync(GetUniqueId(request.Item.Id), new StoreFlagsRequest(StoreAction.Add, MessageFlags.Deleted) { Silent = true }).ConfigureAwait(false);
                await remoteFolder.ExpungeAsync().ConfigureAwait(false);
                await remoteFolder.CloseAsync().ConfigureAwait(false);
            }, requests);
        }

        public override List<IRequestBundle<ImapRequest>> MarkRead(BatchMarkReadRequest requests)
        {
            return CreateTaskBundle(async (client, request) =>
            {
                var folder = request.Item.AssignedFolder;
                var remoteFolder = await client.GetFolderAsync(folder.RemoteFolderId);

                await remoteFolder.OpenAsync(FolderAccess.ReadWrite).ConfigureAwait(false);
                await remoteFolder.StoreAsync(GetUniqueId(request.Item.Id), new StoreFlagsRequest(request.IsRead ? StoreAction.Add : StoreAction.Remove, MessageFlags.Seen) { Silent = true }).ConfigureAwait(false);
                await remoteFolder.CloseAsync().ConfigureAwait(false);
            }, requests);
        }

        public override List<IRequestBundle<ImapRequest>> CreateDraft(CreateDraftRequest request)
        {
            return CreateSingleTaskBundle(async (client, item) =>
            {
                var remoteDraftFolder = await client.GetFolderAsync(request.DraftPreperationRequest.CreatedLocalDraftCopy.AssignedFolder.RemoteFolderId).ConfigureAwait(false);

                await remoteDraftFolder.OpenAsync(FolderAccess.ReadWrite).ConfigureAwait(false);
                await remoteDraftFolder.AppendAsync(request.DraftPreperationRequest.CreatedLocalDraftMimeMessage, MessageFlags.Draft).ConfigureAwait(false);
                await remoteDraftFolder.CloseAsync().ConfigureAwait(false);
            }, request, request);
        }

        public override List<IRequestBundle<ImapRequest>> Archive(BatchArchiveRequest request)
        {
            var batchMoveRequest = new BatchMoveRequest(request.Select(item => new MoveRequest(item.Item, item.FromFolder, item.ToFolder)));
            return Move(batchMoveRequest);
        }


        public override List<IRequestBundle<ImapRequest>> EmptyFolder(EmptyFolderRequest request)
            => Delete(new BatchDeleteRequest(request.MailsToDelete.Select(a => new DeleteRequest(a))));

        public override List<IRequestBundle<ImapRequest>> MarkFolderAsRead(MarkFolderAsReadRequest request)
            => MarkRead(new BatchMarkReadRequest(request.MailsToMarkRead.Select(a => new MarkReadRequest(a, true))));

        public override List<IRequestBundle<ImapRequest>> SendDraft(SendDraftRequest request)
        {
            return CreateSingleTaskBundle(async (client, item) =>
            {
                // Batch sending is not supported. It will always be a single request therefore no need for a loop here.

                var singleRequest = request.Request;

                singleRequest.Mime.Prepare(EncodingConstraint.None);

                using var smtpClient = new MailKit.Net.Smtp.SmtpClient();

                if (smtpClient.IsConnected && client.IsAuthenticated) return;

                if (!smtpClient.IsConnected)
                    await smtpClient.ConnectAsync(Account.ServerInformation.OutgoingServer, int.Parse(Account.ServerInformation.OutgoingServerPort), MailKit.Security.SecureSocketOptions.Auto);

                if (!smtpClient.IsAuthenticated)
                    await smtpClient.AuthenticateAsync(Account.ServerInformation.OutgoingServerUsername, Account.ServerInformation.OutgoingServerPassword);

                // TODO: Transfer progress implementation as popup in the UI.
                await smtpClient.SendAsync(singleRequest.Mime, default);
                await smtpClient.DisconnectAsync(true);

                // SMTP sent the message, but we need to remove it from the Draft folder.
                var draftFolder = singleRequest.MailItem.AssignedFolder;

                var folder = await client.GetFolderAsync(draftFolder.RemoteFolderId);

                await folder.OpenAsync(FolderAccess.ReadWrite);

                var notUpdatedIds = await folder.StoreAsync(new UniqueId(MailkitClientExtensions.ResolveUid(singleRequest.MailItem.Id)), new StoreFlagsRequest(StoreAction.Add, MessageFlags.Deleted) { Silent = true });

                await folder.ExpungeAsync();
                await folder.CloseAsync();

                // Check whether we need to create a copy of the message to Sent folder.
                // This comes from the account preferences.

                if (singleRequest.AccountPreferences.ShouldAppendMessagesToSentFolder && singleRequest.SentFolder != null)
                {
                    var sentFolder = await client.GetFolderAsync(singleRequest.SentFolder.RemoteFolderId);

                    await sentFolder.OpenAsync(FolderAccess.ReadWrite);

                    // Delete local Wino draft header. Otherwise mapping will be applied on re-sync.
                    singleRequest.Mime.Headers.Remove(Domain.Constants.WinoLocalDraftHeader);

                    await sentFolder.AppendAsync(singleRequest.Mime, MessageFlags.Seen);
                    await sentFolder.CloseAsync();
                }
            }, request, request);
        }

        public override async Task DownloadMissingMimeMessageAsync(IMailItem mailItem,
                                                               ITransferProgress transferProgress = null,
                                                               CancellationToken cancellationToken = default)
        {
            var folder = mailItem.AssignedFolder;
            var remoteFolderId = folder.RemoteFolderId;

            var client = await _clientPool.GetClientAsync().ConfigureAwait(false);
            var remoteFolder = await client.GetFolderAsync(remoteFolderId, cancellationToken).ConfigureAwait(false);

            var uniqueId = new UniqueId(MailkitClientExtensions.ResolveUid(mailItem.Id));

            await remoteFolder.OpenAsync(FolderAccess.ReadOnly, cancellationToken).ConfigureAwait(false);

            var message = await remoteFolder.GetMessageAsync(uniqueId, cancellationToken, transferProgress).ConfigureAwait(false);

            await _imapChangeProcessor.SaveMimeFileAsync(mailItem.FileId, message, Account.Id).ConfigureAwait(false);
            await remoteFolder.CloseAsync(false, cancellationToken).ConfigureAwait(false);

            _clientPool.Release(client);
        }

        public override List<IRequestBundle<ImapRequest>> RenameFolder(RenameFolderRequest request)
        {
            return CreateSingleTaskBundle(async (client, item) =>
            {
                var folder = await client.GetFolderAsync(request.Folder.RemoteFolderId).ConfigureAwait(false);
                await folder.RenameAsync(folder.ParentFolder, request.NewFolderName).ConfigureAwait(false);
            }, request, request);
        }

        #endregion

        public override async Task<List<NewMailItemPackage>> CreateNewMailPackagesAsync(ImapMessageCreationPackage message, MailItemFolder assignedFolder, CancellationToken cancellationToken = default)
        {
            var imapFolder = message.MailFolder;
            var summary = message.MessageSummary;

            var mimeMessage = await imapFolder.GetMessageAsync(summary.UniqueId, cancellationToken).ConfigureAwait(false);
            var mailCopy = summary.GetMailDetails(assignedFolder, mimeMessage);

            // Draft folder message updates must be updated as IsDraft.
            // I couldn't find it in MimeMessage...

            mailCopy.IsDraft = assignedFolder.SpecialFolderType == SpecialFolderType.Draft;

            // Check draft mapping.
            // This is the same implementation as in the OutlookSynchronizer.

            if (mimeMessage.Headers.Contains(Domain.Constants.WinoLocalDraftHeader)
                && Guid.TryParse(mimeMessage.Headers[Domain.Constants.WinoLocalDraftHeader], out Guid localDraftCopyUniqueId))
            {
                // This message belongs to existing local draft copy.
                // We don't need to create a new mail copy for this message, just update the existing one.

                bool isMappingSuccessful = await _imapChangeProcessor.MapLocalDraftAsync(Account.Id, localDraftCopyUniqueId, mailCopy.Id, mailCopy.DraftId, mailCopy.ThreadId);

                if (isMappingSuccessful) return null;

                // Local copy doesn't exists. Continue execution to insert mail copy.
            }

            var package = new NewMailItemPackage(mailCopy, mimeMessage, assignedFolder.RemoteFolderId);

            return
            [
                package
            ];
        }

        protected override async Task<MailSynchronizationResult> SynchronizeMailsInternalAsync(MailSynchronizationOptions options, CancellationToken cancellationToken = default)
        {
            var downloadedMessageIds = new List<string>();

            _logger.Information("Internal synchronization started for {Name}", Account.Name);
            _logger.Information("Options: {Options}", options);

            PublishSynchronizationProgress(1);

            bool shouldDoFolderSync = options.Type == MailSynchronizationType.FullFolders || options.Type == MailSynchronizationType.FoldersOnly;

            if (shouldDoFolderSync)
            {
                await SynchronizeFoldersAsync(cancellationToken).ConfigureAwait(false);
            }

            if (options.Type != MailSynchronizationType.FoldersOnly)
            {
                var synchronizationFolders = await _imapChangeProcessor.GetSynchronizationFoldersAsync(options).ConfigureAwait(false);

                for (int i = 0; i < synchronizationFolders.Count; i++)
                {
                    var folder = synchronizationFolders[i];
                    var progress = (int)Math.Round((double)(i + 1) / synchronizationFolders.Count * 100);

                    PublishSynchronizationProgress(progress);

                    var folderDownloadedMessageIds = await SynchronizeFolderInternalAsync(folder, cancellationToken).ConfigureAwait(false);
                    downloadedMessageIds.AddRange(folderDownloadedMessageIds);
                }
            }

            PublishSynchronizationProgress(100);

            // Get all unread new downloaded items and return in the result.
            // This is primarily used in notifications.

            var unreadNewItems = await _imapChangeProcessor.GetDownloadedUnreadMailsAsync(Account.Id, downloadedMessageIds).ConfigureAwait(false);

            return MailSynchronizationResult.Completed(unreadNewItems);
        }

        public override async Task ExecuteNativeRequestsAsync(List<IRequestBundle<ImapRequest>> batchedRequests, CancellationToken cancellationToken = default)
        {
            // First apply the UI changes for each bundle.
            // This is important to reflect changes to the UI before the network call is done.

            foreach (var item in batchedRequests)
            {
                item.Request.ApplyUIChanges();
            }

            // All task bundles will execute on the same client.
            // Tasks themselves don't pull the client from the pool
            // because exception handling is easier this way.
            // Also we might parallelize these bundles later on for additional performance.

            foreach (var item in batchedRequests)
            {
                // At this point this client is ready to execute async commands.
                // Each task bundle will await and execution will continue in case of error.

                ImapClient executorClient = null;

                bool isCrashed = false;

                try
                {
                    executorClient = await _clientPool.GetClientAsync();
                }
                catch (ImapClientPoolException)
                {
                    // Client pool failed to get a client.
                    // Requests may not be executed at this point.

                    item.Request.RevertUIChanges();

                    isCrashed = true;
                    throw;
                }
                finally
                {
                    // Make sure that the client is released from the pool for next usages if error occurs.
                    if (isCrashed && executorClient != null)
                    {
                        _clientPool.Release(executorClient);
                    }
                }

                // TODO: Retry pattern.
                // TODO: Error handling.
                try
                {
                    await item.NativeRequest.IntegratorTask(executorClient, item.Request).ConfigureAwait(false);
                }
                catch (Exception)
                {
                    item.Request.RevertUIChanges();
                    throw;
                }
                finally
                {
                    _clientPool.Release(executorClient);
                }
            }
        }

        /// <summary>
        /// Assigns special folder type for the given local folder.
        /// If server doesn't support special folders, we can't determine the type. MailKit will throw for GetFolder.
        /// Default type is Other.
        /// </summary>
        /// <param name="executorClient">ImapClient from the pool</param>
        /// <param name="remoteFolder">Assigning remote folder.</param>
        /// <param name="localFolder">Assigning local folder.</param>
        private void AssignSpecialFolderType(ImapClient executorClient, IMailFolder remoteFolder, MailItemFolder localFolder)
        {
            // Inbox is awlawys available. Don't miss it for assignment even though XList or SpecialUser is not supported.
            if (executorClient.Inbox == remoteFolder)
            {
                localFolder.SpecialFolderType = SpecialFolderType.Inbox;
                return;

            }

            bool isSpecialFoldersSupported = executorClient.Capabilities.HasFlag(ImapCapabilities.SpecialUse) || executorClient.Capabilities.HasFlag(ImapCapabilities.XList);

            if (!isSpecialFoldersSupported)
            {
                localFolder.SpecialFolderType = SpecialFolderType.Other;
                return;
            }

            if (remoteFolder == executorClient.Inbox)
                localFolder.SpecialFolderType = SpecialFolderType.Inbox;
            else if (remoteFolder == executorClient.GetFolder(SpecialFolder.Drafts))
                localFolder.SpecialFolderType = SpecialFolderType.Draft;
            else if (remoteFolder == executorClient.GetFolder(SpecialFolder.Junk))
                localFolder.SpecialFolderType = SpecialFolderType.Junk;
            else if (remoteFolder == executorClient.GetFolder(SpecialFolder.Trash))
                localFolder.SpecialFolderType = SpecialFolderType.Deleted;
            else if (remoteFolder == executorClient.GetFolder(SpecialFolder.Sent))
                localFolder.SpecialFolderType = SpecialFolderType.Sent;
            else if (remoteFolder == executorClient.GetFolder(SpecialFolder.Archive))
                localFolder.SpecialFolderType = SpecialFolderType.Archive;
            else if (remoteFolder == executorClient.GetFolder(SpecialFolder.Important))
                localFolder.SpecialFolderType = SpecialFolderType.Important;
            else if (remoteFolder == executorClient.GetFolder(SpecialFolder.Flagged))
                localFolder.SpecialFolderType = SpecialFolderType.Starred;
        }

        private async Task SynchronizeFoldersAsync(CancellationToken cancellationToken = default)
        {
            // https://www.rfc-editor.org/rfc/rfc4549#section-1.1

            var localFolders = await _imapChangeProcessor.GetLocalFoldersAsync(Account.Id).ConfigureAwait(false);

            ImapClient executorClient = null;

            try
            {
                List<MailItemFolder> insertedFolders = new();
                List<MailItemFolder> updatedFolders = new();
                List<MailItemFolder> deletedFolders = new();

                executorClient = await _clientPool.GetClientAsync().ConfigureAwait(false);

                var remoteFolders = (await executorClient.GetFoldersAsync(executorClient.PersonalNamespaces[0], cancellationToken: cancellationToken)).ToList();

                // 1. First check deleted folders.

                // 1.a If local folder doesn't exists remotely, delete it.
                // 1.b If local folder exists remotely, check if it is still a valid folder. If UidValidity is changed, delete it.

                foreach (var localFolder in localFolders)
                {
                    IMailFolder remoteFolder = null;

                    try
                    {
                        remoteFolder = remoteFolders.FirstOrDefault(a => a.FullName == localFolder.RemoteFolderId);

                        bool shouldDeleteLocalFolder = false;

                        // Check UidValidity of the remote folder if exists.

                        if (remoteFolder != null)
                        {
                            // UidValidity won't be available until it's opened.
                            await remoteFolder.OpenAsync(FolderAccess.ReadOnly, cancellationToken).ConfigureAwait(false);

                            shouldDeleteLocalFolder = remoteFolder.UidValidity != localFolder.UidValidity;
                        }
                        else
                        {
                            // Remote folder doesn't exist. Delete it.
                            shouldDeleteLocalFolder = true;
                        }

                        if (shouldDeleteLocalFolder)
                        {
                            await _imapChangeProcessor.DeleteFolderAsync(Account.Id, localFolder.RemoteFolderId).ConfigureAwait(false);

                            deletedFolders.Add(localFolder);
                        }
                    }
                    catch (Exception)
                    {
                        throw;
                    }
                    finally
                    {
                        if (remoteFolder != null)
                        {
                            await remoteFolder.CloseAsync().ConfigureAwait(false);
                        }
                    }
                }

                deletedFolders.ForEach(a => localFolders.Remove(a));

                // 2. Get all remote folders and insert/update each of them.

                var nameSpace = executorClient.PersonalNamespaces[0];

                IMailFolder inbox = executorClient.Inbox;

                // Sometimes Inbox is the root namespace. We need to check for that.
                if (inbox != null && !remoteFolders.Contains(inbox))
                    remoteFolders.Add(inbox);

                foreach (var remoteFolder in remoteFolders)
                {
                    // Namespaces are not needed as folders.
                    // Non-existed folders don't need to be synchronized.

                    if (remoteFolder.IsNamespace && !remoteFolder.Attributes.HasFlag(FolderAttributes.Inbox) || !remoteFolder.Exists)
                        continue;

                    var existingLocalFolder = localFolders.FirstOrDefault(a => a.RemoteFolderId == remoteFolder.FullName);

                    if (existingLocalFolder == null)
                    {
                        // Folder doesn't exist locally. Insert it.

                        var localFolder = remoteFolder.GetLocalFolder();

                        // Check whether this is a special folder.
                        AssignSpecialFolderType(executorClient, remoteFolder, localFolder);

                        bool isSystemFolder = localFolder.SpecialFolderType != SpecialFolderType.Other;

                        localFolder.IsSynchronizationEnabled = isSystemFolder;
                        localFolder.IsSticky = isSystemFolder;

                        // By default, all special folders update unread count in the UI except Trash.
                        localFolder.ShowUnreadCount = localFolder.SpecialFolderType != SpecialFolderType.Deleted || localFolder.SpecialFolderType != SpecialFolderType.Other;

                        localFolder.MailAccountId = Account.Id;

                        // Sometimes sub folders are parented under Inbox.
                        // Even though this makes sense in server level, in the client it sucks.
                        // That will make sub folders to be parented under Inbox in the client.
                        // Instead, we will mark them as non-parented folders.
                        // This is better. Model allows personalized folder structure anyways
                        // even though we don't have the page/control to adjust it.

                        if (remoteFolder.ParentFolder == executorClient.Inbox)
                            localFolder.ParentRemoteFolderId = string.Empty;

                        // Set UidValidity for cache expiration.
                        // Folder must be opened for this.

                        await remoteFolder.OpenAsync(FolderAccess.ReadOnly, cancellationToken);

                        localFolder.UidValidity = remoteFolder.UidValidity;

                        await remoteFolder.CloseAsync(cancellationToken: cancellationToken);

                        insertedFolders.Add(localFolder);
                    }
                    else
                    {
                        // Update existing folder. Right now we only update the name.

                        // TODO: Moving folders around different parents. This is not supported right now.
                        // We will need more comphrensive folder update mechanism to support this.

                        if (ShouldUpdateFolder(remoteFolder, existingLocalFolder))
                        {
                            existingLocalFolder.FolderName = remoteFolder.Name;
                            updatedFolders.Add(existingLocalFolder);
                        }
                        else
                        {
                            // Remove it from the local folder list to skip additional folder updates.
                            localFolders.Remove(existingLocalFolder);
                        }
                    }
                }

                // Process changes in order-> Insert, Update. Deleted ones are already processed.

                foreach (var folder in insertedFolders)
                {
                    await _imapChangeProcessor.InsertFolderAsync(folder).ConfigureAwait(false);
                }

                foreach (var folder in updatedFolders)
                {
                    await _imapChangeProcessor.UpdateFolderAsync(folder).ConfigureAwait(false);
                }

                if (insertedFolders.Any() || deletedFolders.Any() || updatedFolders.Any())
                {
                    WeakReferenceMessenger.Default.Send(new AccountFolderConfigurationUpdated(Account.Id));
                }
            }
            catch (Exception ex)
            {
                _logger.Error(ex, "Synchronizing IMAP folders failed.");

                throw;
            }
            finally
            {
                if (executorClient != null)
                {
                    _clientPool.Release(executorClient);
                }
            }
        }



        private async Task<IEnumerable<string>> SynchronizeFolderInternalAsync(MailItemFolder folder, CancellationToken cancellationToken = default)
        {
            if (!folder.IsSynchronizationEnabled) return default;

            var downloadedMessageIds = new List<string>();

            // STEP1: Ask for flag changes for older mails.
            // STEP2: Get new mail changes.
            // https://www.rfc-editor.org/rfc/rfc4549 - Section 4.3

            var _synchronizationClient = await _clientPool.GetClientAsync();

            IMailFolder imapFolder = null;

            var knownMailIds = new UniqueIdSet();
            var locallyKnownMailUids = await _imapChangeProcessor.GetKnownUidsForFolderAsync(folder.Id);
            knownMailIds.AddRange(locallyKnownMailUids.Select(a => new UniqueId(a)));

            var highestUniqueId = Math.Max(0, locallyKnownMailUids.Count == 0 ? 0 : locallyKnownMailUids.Max());

            var missingMailIds = new UniqueIdSet();

            var uidValidity = folder.UidValidity;
            var highestModeSeq = folder.HighestModeSeq;

            var logger = Log.ForContext("FolderName", folder.FolderName);

            logger.Verbose("HighestModeSeq: {HighestModeSeq}, HighestUniqueId: {HighestUniqueId}, UIDValidity: {UIDValidity}", highestModeSeq, highestUniqueId, uidValidity);

            // Event handlers are placed here to handle existing MailItemFolder and IIMailFolder from MailKit.
            // MailKit doesn't expose folder data when these events are emitted.

            // Use local folder's UidValidty because cache might've been expired for remote IMAP folder.
            // That will make our mail copy id invalid.

            EventHandler<MessagesVanishedEventArgs> MessageVanishedHandler = async (s, e) =>
            {
                if (imapFolder == null) return;

                foreach (var uniqueId in e.UniqueIds)
                {
                    var localMailCopyId = MailkitClientExtensions.CreateUid(folder.Id, uniqueId.Id);

                    await _imapChangeProcessor.DeleteMailAsync(Account.Id, localMailCopyId);
                }
            };

            EventHandler<MessageFlagsChangedEventArgs> MessageFlagsChangedHandler = async (s, e) =>
            {
                if (imapFolder == null) return;
                if (e.UniqueId == null) return;

                var localMailCopyId = MailkitClientExtensions.CreateUid(folder.Id, e.UniqueId.Value.Id);

                var isFlagged = MailkitClientExtensions.GetIsFlagged(e.Flags);
                var isRead = MailkitClientExtensions.GetIsRead(e.Flags);

                await _imapChangeProcessor.ChangeMailReadStatusAsync(localMailCopyId, isRead);
                await _imapChangeProcessor.ChangeFlagStatusAsync(localMailCopyId, isFlagged);
            };

            EventHandler<MessageEventArgs> MessageExpungedHandler = async (s, e) =>
            {
                if (imapFolder == null) return;
                if (e.UniqueId == null) return;

                var localMailCopyId = MailkitClientExtensions.CreateUid(folder.Id, e.UniqueId.Value.Id);
                await _imapChangeProcessor.DeleteMailAsync(Account.Id, localMailCopyId);
            };

            try
            {
                imapFolder = await _synchronizationClient.GetFolderAsync(folder.RemoteFolderId, cancellationToken);

                imapFolder.MessageFlagsChanged += MessageFlagsChangedHandler;

                // TODO: Bug: Enabling quick re-sync actually doesn't enable it.

                var qsyncEnabled = false; // _synchronizationClient.Capabilities.HasFlag(ImapCapabilities.QuickResync);
                var condStoreEnabled = _synchronizationClient.Capabilities.HasFlag(ImapCapabilities.CondStore);

                if (qsyncEnabled)
                {

                    imapFolder.MessagesVanished += MessageVanishedHandler;

                    await imapFolder.OpenAsync(FolderAccess.ReadWrite, uidValidity, (ulong)highestModeSeq, knownMailIds, cancellationToken);

                    // Check the folder validity.
                    // We'll delete our existing cache if it's not.

                    // Get all messages after the last successful synchronization date.
                    // This is fine for Wino synchronization because we're not really looking to
                    // synchronize all folder.

                    var allMessageIds = await imapFolder.SearchAsync(SearchQuery.All, cancellationToken);

                    if (uidValidity != imapFolder.UidValidity)
                    {
                        // TODO: Cache is invalid. Delete all local cache.
                        //await ChangeProcessor.FolderService.ClearImapFolderCacheAsync(folder.Id);

                        folder.UidValidity = imapFolder.UidValidity;
                        missingMailIds.AddRange(allMessageIds);
                    }
                    else
                    {
                        // Cache is valid.
                        // Add missing mails only.

                        missingMailIds.AddRange(allMessageIds.Except(knownMailIds).Where(a => a.Id > highestUniqueId));
                    }
                }
                else
                {
                    // QSYNC extension is not enabled for the server.
                    // We rely on ConditionalStore.

                    imapFolder.MessageExpunged += MessageExpungedHandler;
                    await imapFolder.OpenAsync(FolderAccess.ReadWrite, cancellationToken);

                    // Get all messages after the last succesful synchronization date.
                    // This is fine for Wino synchronization because we're not really looking to
                    // synchronize all folder.

                    var allMessageIds = await imapFolder.SearchAsync(SearchQuery.All, cancellationToken);

                    if (uidValidity != imapFolder.UidValidity)
                    {
                        // TODO: Cache is invalid. Delete all local cache.
                        // await ChangeProcessor.FolderService.ClearImapFolderCacheAsync(folder.Id);

                        folder.UidValidity = imapFolder.UidValidity;
                        missingMailIds.AddRange(allMessageIds);
                    }
                    else
                    {
                        // Cache is valid.

                        var purgedMessages = knownMailIds.Except(allMessageIds);

                        foreach (var purgedMessage in purgedMessages)
                        {
                            var mailId = MailkitClientExtensions.CreateUid(folder.Id, purgedMessage.Id);

                            await _imapChangeProcessor.DeleteMailAsync(Account.Id, mailId);
                        }

                        IList<IMessageSummary> changed;

                        if (knownMailIds.Count > 0)
                        {
                            // CONDSTORE enabled. Fetch items with highest mode seq for known items
                            // to track flag changes. Otherwise just get changes without the mode seq.

                            if (condStoreEnabled)
                                changed = await imapFolder.FetchAsync(knownMailIds, (ulong)highestModeSeq, MessageSummaryItems.Flags | MessageSummaryItems.ModSeq | MessageSummaryItems.UniqueId);
                            else
                                changed = await imapFolder.FetchAsync(knownMailIds, MessageSummaryItems.Flags | MessageSummaryItems.UniqueId);

                            foreach (var changedItem in changed)
                            {
                                var localMailCopyId = MailkitClientExtensions.CreateUid(folder.Id, changedItem.UniqueId.Id);

                                var isFlagged = changedItem.Flags.GetIsFlagged();
                                var isRead = changedItem.Flags.GetIsRead();

                                await _imapChangeProcessor.ChangeMailReadStatusAsync(localMailCopyId, isRead);
                                await _imapChangeProcessor.ChangeFlagStatusAsync(localMailCopyId, isFlagged);
                            }
                        }

                        // We're only interested in items that has highier known uid than we fetched before.
                        // Others are just older messages.

                        missingMailIds.AddRange(allMessageIds.Except(knownMailIds).Where(a => a.Id > highestUniqueId));
                    }
                }

                // Fetch completely missing new items in the end.

                // Limit check.
                if (missingMailIds.Count > InitialMessageDownloadCountPerFolder)
                {
                    missingMailIds = new UniqueIdSet(missingMailIds.TakeLast((int)InitialMessageDownloadCountPerFolder));
                }

                // In case of the high input, we'll batch them by 50 to reflect changes quickly.
                var batchedMissingMailIds = missingMailIds.Batch(50).Select(a => new UniqueIdSet(a, SortOrder.Ascending));

                foreach (var batchMissingMailIds in batchedMissingMailIds)
                {
                    var summaries = await imapFolder.FetchAsync(batchMissingMailIds, mailSynchronizationFlags, cancellationToken).ConfigureAwait(false);

                    foreach (var summary in summaries)
                    {
                        // We pass the opened folder and summary to retrieve raw MimeMessage.

                        var creationPackage = new ImapMessageCreationPackage(summary, imapFolder);
                        var createdMailPackages = await CreateNewMailPackagesAsync(creationPackage, folder, cancellationToken).ConfigureAwait(false);

                        // Local draft is mapped. We don't need to create a new mail copy.
                        if (createdMailPackages == null)
                            continue;

                        foreach (var mailPackage in createdMailPackages)
                        {
                            bool isCreated = await _imapChangeProcessor.CreateMailAsync(Account.Id, mailPackage).ConfigureAwait(false);

                            if (isCreated)
                            {
                                downloadedMessageIds.Add(mailPackage.Copy.Id);
                            }
                        }
                    }
                }

                if (folder.HighestModeSeq != (long)imapFolder.HighestModSeq)
                {
                    folder.HighestModeSeq = (long)imapFolder.HighestModSeq;

                    await _imapChangeProcessor.UpdateFolderAsync(folder).ConfigureAwait(false);
                }

                // Update last synchronization date for the folder..

                await _imapChangeProcessor.UpdateFolderLastSyncDateAsync(folder.Id).ConfigureAwait(false);

                return downloadedMessageIds;
            }
            catch (FolderNotFoundException)
            {
                await _imapChangeProcessor.DeleteFolderAsync(Account.Id, folder.RemoteFolderId).ConfigureAwait(false);

                return default;
            }
            catch (Exception)
            {
                throw;
            }
            finally
            {
                if (imapFolder != null)
                {
                    imapFolder.MessageFlagsChanged -= MessageFlagsChangedHandler;
                    imapFolder.MessageExpunged -= MessageExpungedHandler;
                    imapFolder.MessagesVanished -= MessageVanishedHandler;

                    if (imapFolder.IsOpen)
                        await imapFolder.CloseAsync();
                }

                _clientPool.Release(_synchronizationClient);
            }
        }


        /// <summary>
        /// Whether the local folder should be updated with the remote folder.
        /// IMAP only compares folder name for now.
        /// </summary>
        /// <param name="remoteFolder">Remote folder</param>
        /// <param name="localFolder">Local folder.</param>
        public bool ShouldUpdateFolder(IMailFolder remoteFolder, MailItemFolder localFolder)
            => !localFolder.FolderName.Equals(remoteFolder.Name, StringComparison.OrdinalIgnoreCase);

        protected override Task<CalendarSynchronizationResult> SynchronizeCalendarEventsInternalAsync(CalendarSynchronizationOptions options, CancellationToken cancellationToken = default)
        {
            throw new NotImplementedException();
        }
    }
}