Wino-Mail/Wino.Core/Synchronizers/OutlookSynchronizer.cs

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Diagnostics.CodeAnalysis;
using System.IO;
using System.Linq;
using System.Net.Http;
using System.Text;
using System.Text.Json;
using System.Text.Json.Nodes;
using System.Text.Json.Serialization;
using System.Text.RegularExpressions;
using System.Threading;
using System.Threading.Tasks;
using CommunityToolkit.Mvvm.Messaging;
using Microsoft.Graph;
using Microsoft.Graph.Me.MailFolders.Item.Messages.Delta;
using Microsoft.Graph.Models;
using Microsoft.Graph.Models.ODataErrors;
using Microsoft.Kiota.Abstractions;
using Microsoft.Kiota.Abstractions.Authentication;
using Microsoft.Kiota.Abstractions.Serialization;
using MimeKit;
using MoreLinq.Extensions;
using Serilog;
using Wino.Core.Domain.Entities.Calendar;
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.Accounts;
using Wino.Core.Domain.Models.Errors;
using Wino.Core.Domain.Models.Folders;
using Wino.Core.Domain.Models.MailItem;
using Wino.Core.Domain.Models.Synchronization;
using Wino.Core.Extensions;
using Wino.Core.Http;
using Wino.Core.Integration.Processors;
using Wino.Core.Misc;
using Wino.Core.Requests.Bundles;
using Wino.Core.Requests.Folder;
using Wino.Core.Requests.Mail;

namespace Wino.Core.Synchronizers.Mail;

[JsonSerializable(typeof(Microsoft.Graph.Me.Messages.Item.Move.MovePostRequestBody))]
[JsonSerializable(typeof(OutlookFileAttachment))]
public partial class OutlookSynchronizerJsonContext : JsonSerializerContext;

/// <summary>
/// Outlook synchronizer implementation with queue-based metadata-only synchronization.
/// 
/// SYNCHRONIZATION STRATEGY:
/// - Uses per-folder queue system (unlike Gmail's per-account queue)
/// - During sync (initial/delta), only message metadata is downloaded (no MIME content)
/// - Messages are queued by folder using MailItemQueue with RemoteFolderId
/// - MailCopy objects are created from Graph API metadata fields only
/// - MIME files are downloaded on-demand when user explicitly reads a message
/// - This dramatically reduces bandwidth usage and sync time
/// 
/// Key implementation details:
/// - QueueMailIdsForFolderAsync: Queues all mail IDs for a folder using Delta API
/// - ProcessMailQueueForFolderAsync: Downloads metadata in batches from queue
/// - DownloadMessageMetadataBatchAsync: Concurrently downloads metadata for batches
/// - CreateMailCopyFromMessage: Centralized method to create MailCopy from Message (metadata only)
/// - DownloadMissingMimeMessageAsync: Downloads raw MIME only when explicitly requested
/// - CreateNewMailPackagesAsync: Only used for search results and special cases (downloads MIME)
/// </summary>
public class OutlookSynchronizer : WinoSynchronizer<RequestInformation, Message, Event>
{
    public override uint BatchModificationSize => 20;
    public override uint InitialMessageDownloadCountPerFolder => 1000;
    private const uint MaximumAllowedBatchRequestSize = 20;

    private const string INBOX_NAME = "inbox";
    private const string SENT_NAME = "sentitems";
    private const string DELETED_NAME = "deleteditems";
    private const string JUNK_NAME = "junkemail";
    private const string DRAFTS_NAME = "drafts";
    private const string ARCHIVE_NAME = "archive";

    private readonly string[] outlookMessageSelectParameters =
    [
        "InferenceClassification",
        "Flag",
        "Importance",
        "IsRead",
        "IsDraft",
        "ReceivedDateTime",
        "HasAttachments",
        "BodyPreview",
        "Id",
        "ConversationId",
        "From",
        "Subject",
        "ParentFolderId",
        "InternetMessageId",
    ];

    private readonly SemaphoreSlim _handleItemRetrievalSemaphore = new(1);

    private readonly ILogger _logger = Log.ForContext<OutlookSynchronizer>();
    private readonly IOutlookChangeProcessor _outlookChangeProcessor;
    private readonly GraphServiceClient _graphClient;
    private readonly IOutlookSynchronizerErrorHandlerFactory _errorHandlingFactory;

    private readonly SemaphoreSlim _concurrentDownloadSemaphore = new(10); // Limit to 10 concurrent downloads

    public OutlookSynchronizer(MailAccount account,
                               IAuthenticator authenticator,
                               IOutlookChangeProcessor outlookChangeProcessor,
                               IOutlookSynchronizerErrorHandlerFactory errorHandlingFactory) : base(account, WeakReferenceMessenger.Default)
    {
        var tokenProvider = new MicrosoftTokenProvider(Account, authenticator);

        // Update request handlers for Graph client.
        var handlers = GraphClientFactory.CreateDefaultHandlers();

        handlers.Add(GetMicrosoftImmutableIdHandler());

        var httpClient = GraphClientFactory.Create(handlers);
        _graphClient = new GraphServiceClient(httpClient, new BaseBearerTokenAuthenticationProvider(tokenProvider));

        _outlookChangeProcessor = outlookChangeProcessor;
        _errorHandlingFactory = errorHandlingFactory;
    }

    #region MS Graph Handlers

    private MicrosoftImmutableIdHandler GetMicrosoftImmutableIdHandler() => new();

    #endregion


    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);

        try
        {
            // Set indeterminate progress initially
            UpdateSyncProgress(0, 0, "Synchronizing folders...");

            await SynchronizeFoldersAsync(cancellationToken).ConfigureAwait(false);

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

                _logger.Information(string.Format("{1} Folders: {0}", string.Join(",", synchronizationFolders.Select(a => a.FolderName)), synchronizationFolders.Count));

                var totalFolders = synchronizationFolders.Count;

                for (int i = 0; i < totalFolders; i++)
                {
                    var folder = synchronizationFolders[i];

                    // Update progress based on folder completion
                    UpdateSyncProgress(totalFolders, totalFolders - (i + 1), $"Syncing {folder.FolderName}...");

                    var folderDownloadedMessageIds = await SynchronizeFolderAsync(folder, cancellationToken).ConfigureAwait(false);
                    downloadedMessageIds.AddRange(folderDownloadedMessageIds);
                }
            }
        }
        catch (Exception ex)
        {
            _logger.Error(ex, "Synchronizing folders for {Name}", Account.Name);
            Debugger.Break();

            throw;
        }
        finally
        {
            // Reset progress at the end
            ResetSyncProgress();
        }

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

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

        return MailSynchronizationResult.Completed(unreadNewItems);
    }

    public async Task DownloadSearchResultMessageAsync(string messageId, MailItemFolder assignedFolder, CancellationToken cancellationToken = default)
    {
        Log.Information("Downloading search result message {messageId} for {Name} - {FolderName}", messageId, Account.Name, assignedFolder.FolderName);

        // Outlook message handling was a little strange.
        // Instead of changing it from the scratch, we will just download the message and process it.
        // Search results will only return Id for the messages.
        // This method will download the raw mime, get the required enough metadata from the service and create
        // the mail locally. Message ids passed to this method is expected to be non-existent locally.

        var message = await _graphClient.Me.Messages[messageId].GetAsync((config) =>
        {
            config.QueryParameters.Select = outlookMessageSelectParameters;
        }, cancellationToken).ConfigureAwait(false);

        var mailPackages = await CreateNewMailPackagesAsync(message, assignedFolder, cancellationToken).ConfigureAwait(false);

        if (mailPackages == null) return;

        foreach (var package in mailPackages)
        {
            cancellationToken.ThrowIfCancellationRequested();

            await _outlookChangeProcessor.CreateMailRawAsync(Account, assignedFolder, package).ConfigureAwait(false);
        }
    }

    private async Task<IEnumerable<string>> SynchronizeFolderAsync(MailItemFolder folder, CancellationToken cancellationToken = default)
    {
        var downloadedMessageIds = new List<string>();

        cancellationToken.ThrowIfCancellationRequested();

        _logger.Debug("Synchronizing {FolderName} with direct download approach", folder.FolderName);

        // Check if initial sync is completed for this folder
        if (folder.FolderStatus != InitialSynchronizationStatus.Completed)
        {
            _logger.Debug("Initial sync not completed for folder {FolderName}. Starting mail synchronization.", folder.FolderName);

            // Download mails for initial sync
            await DownloadMailsForInitialSyncAsync(folder, downloadedMessageIds, cancellationToken).ConfigureAwait(false);

            // Mark initial sync as completed
            await _outlookChangeProcessor.UpdateFolderInitialSyncCompletedAsync(folder.Id, true).ConfigureAwait(false);
            folder.FolderStatus = InitialSynchronizationStatus.Completed;
        }
        else
        {
            // Initial sync is completed, process delta changes and download new mails
            _logger.Debug("Initial sync completed for folder {FolderName}. Processing delta changes and downloading new mails.", folder.FolderName);

            await ProcessDeltaChangesAndDownloadMailsAsync(folder, downloadedMessageIds, cancellationToken).ConfigureAwait(false);
        }

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

        if (downloadedMessageIds.Any())
        {
            _logger.Information("Downloaded {Count} messages for folder {FolderName}", downloadedMessageIds.Count, folder.FolderName);
        }

        return downloadedMessageIds;
    }

    /// <summary>
    /// Downloads mails for initial synchronization using Delta API and queue-based system.
    /// First, queues all mail IDs, then downloads metadata in batches.
    /// </summary>
    private async Task DownloadMailsForInitialSyncAsync(MailItemFolder folder, List<string> downloadedMessageIds, CancellationToken cancellationToken)
    {
        _logger.Debug("Starting initial mail download for folder {FolderName}", folder.FolderName);

        try
        {
            // Step 1: Queue all mail IDs using Delta API
            await QueueMailIdsForFolderAsync(folder, cancellationToken).ConfigureAwait(false);

            // Step 2: Process queued mail IDs in batches
            await ProcessMailQueueForFolderAsync(folder, downloadedMessageIds, cancellationToken).ConfigureAwait(false);
        }
        catch (ApiException apiException)
        {
            // Try to handle the error using the error handling factory
            var errorContext = new SynchronizerErrorContext
            {
                Account = Account,
                ErrorCode = (int?)apiException.ResponseStatusCode,
                ErrorMessage = $"API error during initial sync: {apiException.Message}",
                Exception = apiException
            };

            var handled = await _errorHandlingFactory.HandleErrorAsync(errorContext).ConfigureAwait(false);

            if (handled)
            {
                // The error handler has processed the error (e.g., DeltaTokenExpiredHandler for 410)
                // Update in-memory folder state if it was a delta token expiration
                if (apiException.ResponseStatusCode == 410)
                {
                    folder.DeltaToken = string.Empty;
                    folder.FolderStatus = InitialSynchronizationStatus.None;
                    _logger.Information("API error handled successfully for folder {FolderName} during initial sync. Error: {ErrorCode}", folder.FolderName, apiException.ResponseStatusCode);
                }
            }
            else
            {
                // No handler could process this error, log and re-throw
                _logger.Error(apiException, "Unhandled API error during initial sync for folder {FolderName}. Error: {ErrorCode}", folder.FolderName, apiException.ResponseStatusCode);
            }

            // Re-throw the exception so the synchronization can be retried
            throw;
        }
        catch (Exception ex)
        {
            _logger.Error(ex, "Error occurred during initial mail download for folder {FolderName}", folder.FolderName);
            throw;
        }
    }

    /// <summary>
    /// Queues all mail IDs for a folder using Delta API.
    /// Only retrieves message IDs to minimize data transfer.
    /// </summary>
    private async Task QueueMailIdsForFolderAsync(MailItemFolder folder, CancellationToken cancellationToken)
    {
        _logger.Debug("Queuing mail IDs for folder {FolderName}", folder.FolderName);

        var mailIds = new List<string>();

        // Always use Delta API for initial sync - this ensures proper delta token setup for future incremental syncs
        DeltaGetResponse messageCollectionPage = null;

        if (string.IsNullOrEmpty(folder.DeltaToken))
        {
            messageCollectionPage = await _graphClient.Me.MailFolders[folder.RemoteFolderId].Messages.Delta.GetAsDeltaGetResponseAsync((config) =>
            {
                config.QueryParameters.Select = ["Id"]; // Only get the message Ids
                config.QueryParameters.Orderby = ["receivedDateTime desc"]; // Sort by received date desc
                config.QueryParameters.Top = (int)InitialMessageDownloadCountPerFolder;
            }, cancellationToken).ConfigureAwait(false);
        }
        else
        {
            var requestInformation = _graphClient.Me.MailFolders[folder.RemoteFolderId].Messages.Delta.ToGetRequestInformation((config) =>
            {
                config.QueryParameters.Select = ["Id"]; // Only get the message Ids
                config.QueryParameters.Orderby = ["receivedDateTime desc"]; // Sort by received date desc
            });

            requestInformation.UrlTemplate = requestInformation.UrlTemplate.Insert(requestInformation.UrlTemplate.Length - 1, ",%24deltatoken");
            requestInformation.QueryParameters.Add("%24deltatoken", folder.DeltaToken);

            messageCollectionPage = await _graphClient.RequestAdapter.SendAsync(requestInformation, DeltaGetResponse.CreateFromDiscriminatorValue, cancellationToken: cancellationToken);
        }

        // Use PageIterator to iterate through all messages and collect IDs
        var messageIterator = PageIterator<Message, DeltaGetResponse>.CreatePageIterator(_graphClient, messageCollectionPage, (message) =>
        {
            if (!IsResourceDeleted(message.AdditionalData))
            {
                mailIds.Add(message.Id);
            }

            // Iterator must continue all the time to receive delta token at the end.
            return true;
        });

        await messageIterator.IterateAsync(cancellationToken).ConfigureAwait(false);

        // Extract delta token from the iterator's delta link
        string deltaToken = null;
        if (!string.IsNullOrEmpty(messageIterator.Deltalink))
        {
            deltaToken = GetDeltaTokenFromDeltaLink(messageIterator.Deltalink);
        }

        // Queue all mail IDs for processing
        if (mailIds.Any())
        {
            var queueEntries = mailIds.Select(id => new MailItemQueue
            {
                Id = Guid.CreateVersion7(),
                AccountId = Account.Id,
                RemoteServerId = id,
                RemoteFolderId = folder.RemoteFolderId,
                IsProcessed = false,
                CreatedAt = DateTime.UtcNow
            });

            await _outlookChangeProcessor.AddMailItemQueueItemsAsync(queueEntries).ConfigureAwait(false);

            _logger.Information("Queued {Count} mail IDs for folder {FolderName}", mailIds.Count, folder.FolderName);
        }
        else
        {
            _logger.Information("No mail ids found to queue for folder {FolderName}", folder.FolderName);
        }

        // Store the delta token for future incremental syncs - always store when available
        if (!string.IsNullOrEmpty(deltaToken))
        {
            await _outlookChangeProcessor.UpdateFolderDeltaSynchronizationIdentifierAsync(folder.Id, deltaToken).ConfigureAwait(false);
            await _outlookChangeProcessor.UpdateFolderLastSyncDateAsync(folder.Id).ConfigureAwait(false);
            folder.DeltaToken = deltaToken;
            _logger.Information("Stored delta token for folder {FolderName} - future syncs will be incremental", folder.FolderName);
        }
        else
        {
            _logger.Warning("No delta token received for folder {FolderName} - future syncs may re-download messages", folder.FolderName);
        }
    }

    /// <summary>
    /// Processes queued mail IDs in batches, downloading metadata only (no MIME).
    /// </summary>
    private async Task ProcessMailQueueForFolderAsync(MailItemFolder folder, List<string> downloadedMessageIds, CancellationToken cancellationToken)
    {
        var totalInQueue = await _outlookChangeProcessor.GetMailItemQueueCountByFolderAsync(Account.Id, folder.RemoteFolderId).ConfigureAwait(false);

        if (totalInQueue == 0)
        {
            _logger.Information("No mails in queue for folder {FolderName}", folder.FolderName);
            return;
        }

        _logger.Information("Processing {Count} queued mails for folder {FolderName}", totalInQueue, folder.FolderName);

        var totalFailed = 0;
        var totalProcessed = 0;

        // Set initial progress for queue processing
        UpdateSyncProgress(totalInQueue, totalInQueue, $"Downloading {folder.FolderName}...");

        // Continue until all emails in queue are processed
        while (true)
        {
            // Get next batch of unprocessed emails from queue
            var mailItemQueue = await _outlookChangeProcessor.GetMailItemQueueByFolderAsync(Account.Id, folder.RemoteFolderId, 100).ConfigureAwait(false);

            if (mailItemQueue.Count == 0)
                break; // No more emails to process

            // Remove the items that should be deleted from queue first
            mailItemQueue.RemoveAll(a => a.ShouldDelete());

            var mailChunks = mailItemQueue.Chunk(20); // Process 20 at a time

            foreach (var chunk in mailChunks)
            {
                cancellationToken.ThrowIfCancellationRequested();

                // Collect message IDs from the chunk
                var messageIdsToDownload = chunk.Select(q => q.RemoteServerId).ToList();

                try
                {
                    // Download all messages in this chunk concurrently
                    var chunkDownloadedIds = await DownloadMessageMetadataBatchAsync(messageIdsToDownload, folder, true, cancellationToken).ConfigureAwait(false);

                    downloadedMessageIds.AddRange(chunkDownloadedIds);

                    // Mark all items in chunk as processed
                    foreach (var queueItem in chunk)
                    {
                        queueItem.IsProcessed = true;
                        queueItem.ProcessedAt = DateTime.UtcNow;
                        totalProcessed++;
                    }

                    // Update progress with remaining items
                    var remainingItems = totalInQueue - totalProcessed;
                    UpdateSyncProgress(totalInQueue, remainingItems, $"Downloading {folder.FolderName} ({totalProcessed}/{totalInQueue})");
                }
                catch (Exception ex)
                {
                    _logger.Error(ex, "Failed to download chunk of messages for folder {FolderName}", folder.FolderName);

                    // Mark all items in chunk as failed
                    foreach (var queueItem in chunk)
                    {
                        queueItem.IsProcessed = false;
                        queueItem.ProcessedAt = null;
                        queueItem.FailedCount++;
                        totalFailed++;
                    }
                }

                await _outlookChangeProcessor.UpdateMailItemQueueAsync(mailItemQueue).ConfigureAwait(false);

                // If too many failures, pause to avoid hitting rate limits
                if (totalFailed > 50)
                {
                    _logger.Warning("Too many failures ({Count}), pausing for 10 seconds", totalFailed);
                    await Task.Delay(TimeSpan.FromSeconds(10), cancellationToken);
                    totalFailed = 0; // Reset counter
                }
            }

            _logger.Debug("Processed batch: {Processed}/{Total} for folder {FolderName}", totalProcessed, totalInQueue, folder.FolderName);
        }

        _logger.Information("Completed processing queue for folder {FolderName}. Processed: {Count}", folder.FolderName, totalProcessed);
    }

    /// <summary>
    /// Downloads metadata for a batch of messages using Graph SDK batch API (no MIME content).
    /// Processes up to 20 messages per batch request as per MaximumAllowedBatchRequestSize.
    /// </summary>
    private async Task<List<string>> DownloadMessageMetadataBatchAsync(List<string> messageIds, MailItemFolder folder, bool retryFailedOnce, CancellationToken cancellationToken)
    {
        if (messageIds == null || messageIds.Count == 0)
            return new List<string>();

        var downloadedIds = new List<string>();

        // Filter out messages that already exist in the database
        var messagesToDownload = new List<string>();
        foreach (var messageId in messageIds)
        {
            bool mailExists = await _outlookChangeProcessor.IsMailExistsInFolderAsync(messageId, folder.Id).ConfigureAwait(false);
            if (!mailExists)
            {
                messagesToDownload.Add(messageId);
            }
            else
            {
                _logger.Debug("Mail {MailId} already exists in folder {FolderName}, skipping download", messageId, folder.FolderName);
            }
        }

        if (messagesToDownload.Count == 0)
        {
            _logger.Debug("All messages already exist in folder {FolderName}", folder.FolderName);
            return downloadedIds;
        }

        // Store failed message ids to retry after.

        List<string> failedMessageIds = new();

        // Process in batches of MaximumAllowedBatchRequestSize (20)
        var batches = messagesToDownload.Batch((int)MaximumAllowedBatchRequestSize);

        foreach (var batch in batches)
        {
            cancellationToken.ThrowIfCancellationRequested();

            try
            {
                var batchContent = new BatchRequestContentCollection(_graphClient);
                var requestIdToMessageIdMap = new Dictionary<string, string>();

                // Add all message requests to the batch
                foreach (var messageId in batch)
                {
                    var requestInfo = _graphClient.Me.Messages[messageId].ToGetRequestInformation((config) =>
                    {
                        config.QueryParameters.Select = outlookMessageSelectParameters;
                    });

                    var batchRequestId = await batchContent.AddBatchRequestStepAsync(requestInfo).ConfigureAwait(false);
                    requestIdToMessageIdMap[batchRequestId] = messageId;
                }

                // Execute the batch request
                var batchResponse = await _graphClient.Batch.PostAsync(batchContent, cancellationToken).ConfigureAwait(false);

                // Process all responses
                foreach (var batchRequestId in requestIdToMessageIdMap.Keys)
                {
                    var messageId = requestIdToMessageIdMap[batchRequestId];

                    try
                    {
                        // Deserialize the Message directly from batch response
                        var message = await batchResponse.GetResponseByIdAsync<Message>(batchRequestId).ConfigureAwait(false);

                        if (message != null)
                        {
                            // Create MailCopy from metadata only
                            var mailCopy = CreateMailCopyFromMessage(message, folder);

                            if (mailCopy != null)
                            {
                                // Create package without MIME
                                var package = new NewMailItemPackage(mailCopy, null, folder.RemoteFolderId);
                                bool isInserted = await _outlookChangeProcessor.CreateMailAsync(Account.Id, package).ConfigureAwait(false);

                                if (isInserted)
                                {
                                    downloadedIds.Add(mailCopy.Id);
                                    _logger.Debug("Downloaded metadata for message {MailId} in folder {FolderName}", messageId, folder.FolderName);
                                }
                                else
                                {
                                    _logger.Warning("Failed to insert mail {MailId} for folder {FolderName}", messageId, folder.FolderName);
                                }
                            }
                        }
                        else
                        {
                            _logger.Warning("Failed to deserialize message {MailId} for folder {FolderName}", messageId, folder.FolderName);
                            failedMessageIds.Add(messageId);
                        }
                    }
                    catch (ODataError odataError)
                    {
                        // Handle OData errors from the batch response
                        if (odataError.ResponseStatusCode == 404)
                        {
                            _logger.Warning("Mail {MailId} not found on server (404) for folder {FolderName}", messageId, folder.FolderName);
                        }
                        else
                        {
                            failedMessageIds.Add(messageId);
                            _logger.Error("OData error while downloading mail {MailId} for folder {FolderName}. Error: {Error}", messageId, folder.FolderName, odataError.Error?.Message);
                        }
                    }
                    catch (ServiceException serviceException)
                    {
                        // Try to handle the error using the error handling factory
                        var errorContext = new SynchronizerErrorContext
                        {
                            Account = Account,
                            ErrorCode = (int?)serviceException.ResponseStatusCode,
                            ErrorMessage = $"Service error during batch mail download: {serviceException.Message}",
                            Exception = serviceException,
                        };

                        var handled = await _errorHandlingFactory.HandleErrorAsync(errorContext).ConfigureAwait(false);

                        if (!handled)
                        {
                            failedMessageIds.Add(messageId);
                            _logger.Error(serviceException, "Unhandled service error while downloading mail {MailId} for folder {FolderName}. Error: {ErrorCode}", messageId, folder.FolderName, serviceException.ResponseStatusCode);
                        }
                    }
                    catch (Exception ex)
                    {
                        failedMessageIds.Add(messageId);
                        _logger.Error(ex, "Error occurred while processing message {MailId} for folder {FolderName}", messageId, folder.FolderName);
                    }
                }
            }
            catch (Exception ex)
            {
                failedMessageIds.AddRange(batch);

                _logger.Error(ex, "Error occurred during batch download for folder {FolderName}", folder.FolderName);
            }
        }

        if (retryFailedOnce && failedMessageIds.Any())
        {
            // For a good cause wait a little bit.

            await Task.Delay(3000);

            // Do not retry here once again.
            var failedDownloadedMessagIds = await DownloadMessageMetadataBatchAsync(failedMessageIds, folder, false, cancellationToken);

            downloadedIds.Concat(failedDownloadedMessagIds);
        }

        return downloadedIds;
    }

    /// <summary>
    /// Creates a MailCopy from an Outlook Message with metadata only (centralized method).
    /// This replaces the scattered CreateMinimalMailCopyAsync and AsMailCopy calls.
    /// </summary>
    private MailCopy CreateMailCopyFromMessage(Message message, MailItemFolder assignedFolder)
    {
        if (message == null) return null;

        var mailCopy = message.AsMailCopy();
        mailCopy.FolderId = assignedFolder.Id;
        mailCopy.UniqueId = Guid.NewGuid();
        mailCopy.FileId = Guid.NewGuid();

        return mailCopy;
    }

    private string GetDeltaTokenFromDeltaLink(string deltaLink)
        => Regex.Split(deltaLink, "deltatoken=")[1];

    protected override async Task QueueMailIdsForInitialSyncAsync(MailItemFolder folder, CancellationToken cancellationToken = default)
    {
        // Queue all mail IDs for the folder
        await QueueMailIdsForFolderAsync(folder, cancellationToken).ConfigureAwait(false);
    }

    protected override Task<MailCopy> CreateMinimalMailCopyAsync(Message message, MailItemFolder assignedFolder, CancellationToken cancellationToken = default)
    {
        // Use centralized method
        return Task.FromResult(CreateMailCopyFromMessage(message, assignedFolder));
    }

    private async Task<Message> GetMessageByIdAsync(string messageId, CancellationToken cancellationToken = default)
    {
        try
        {
            return await _graphClient.Me.Messages[messageId].GetAsync((config) =>
            {
                config.QueryParameters.Select = outlookMessageSelectParameters;
            }, cancellationToken).ConfigureAwait(false);
        }
        catch (ServiceException serviceException)
        {
            // Try to handle the error using the error handling factory first
            var errorContext = new SynchronizerErrorContext
            {
                Account = Account,
                ErrorCode = (int?)serviceException.ResponseStatusCode,
                ErrorMessage = $"Service error during message retrieval: {serviceException.Message}",
                Exception = serviceException
            };

            var handled = await _errorHandlingFactory.HandleErrorAsync(errorContext).ConfigureAwait(false);

            if (!handled)
            {
                // No handler could process this error, log and handle appropriately
                if (serviceException.ResponseStatusCode == 404)
                {
                    // Re-throw 404 errors to be handled by the caller for queue cleanup
                    throw;
                }
                else
                {
                    _logger.Error(serviceException, "Unhandled service error while getting message {MessageId}. Error: {ErrorCode}", messageId, serviceException.ResponseStatusCode);
                    return null;
                }
            }
            else
            {
                _logger.Information("Service error handled successfully during message retrieval. Message: {MessageId}, Error: {ErrorCode}", messageId, serviceException.ResponseStatusCode);
                return null; // Return null since the error was handled but we couldn't get the message
            }
        }
        catch (Exception ex)
        {
            _logger.Error(ex, "Failed to get message {MessageId}", messageId);
            return null;
        }
    }

    private async Task ProcessDeltaChangesAndDownloadMailsAsync(MailItemFolder folder, List<string> downloadedMessageIds, CancellationToken cancellationToken = default)
    {
        // Process delta changes and download new mails with metadata only (no MIME)
        if (string.IsNullOrEmpty(folder.DeltaToken))
        {
            _logger.Debug("No delta token available for folder {FolderName}. Skipping delta sync.", folder.FolderName);
            return;
        }

        try
        {
            var currentDeltaToken = folder.DeltaToken;

            _logger.Debug("Processing delta changes for folder {FolderName} with token {DeltaToken}", folder.FolderName, currentDeltaToken.Substring(0, Math.Min(10, currentDeltaToken.Length)) + "...");

            // Always use Delta endpoint with proper configuration
            var requestInformation = _graphClient.Me.MailFolders[folder.RemoteFolderId].Messages.Delta.ToGetRequestInformation((config) =>
            {
                config.QueryParameters.Select = ["Id"]; // Only get IDs
                config.QueryParameters.Orderby = ["receivedDateTime desc"]; // Sort by received date desc
            });

            requestInformation.UrlTemplate = requestInformation.UrlTemplate.Insert(requestInformation.UrlTemplate.Length - 1, ",%24deltatoken");
            requestInformation.QueryParameters.Add("%24deltatoken", currentDeltaToken);

            var messageCollectionPage = await _graphClient.RequestAdapter.SendAsync(requestInformation,
                DeltaGetResponse.CreateFromDiscriminatorValue,
                cancellationToken: cancellationToken);

            var newMailIds = new List<string>();

            // Use PageIterator<DeltaGetResponse> for iterating through delta changes
            var messageIterator = PageIterator<Message, DeltaGetResponse>
                .CreatePageIterator(_graphClient, messageCollectionPage, (message) =>
                {
                    // Only process new messages, not deleted ones
                    if (!IsResourceDeleted(message.AdditionalData))
                    {
                        newMailIds.Add(message.Id);
                    }
                    return true;
                });

            await messageIterator.IterateAsync(cancellationToken).ConfigureAwait(false);

            // Download new mails with metadata only (no MIME)
            if (newMailIds.Any())
            {
                _logger.Information("Downloading {Count} new mails from delta sync for folder {FolderName} (metadata only)", newMailIds.Count, folder.FolderName);
                var deltaDownloadedIds = await DownloadMessageMetadataBatchAsync(newMailIds, folder, true, cancellationToken).ConfigureAwait(false);
                downloadedMessageIds.AddRange(deltaDownloadedIds);
            }

            // Update delta token for next sync - always store when there are no nextPageToken remaining
            if (!string.IsNullOrEmpty(messageIterator.Deltalink))
            {
                var deltaToken = GetDeltaTokenFromDeltaLink(messageIterator.Deltalink);
                await _outlookChangeProcessor.UpdateFolderDeltaSynchronizationIdentifierAsync(folder.Id, deltaToken).ConfigureAwait(false);
                folder.DeltaToken = deltaToken; // Update in-memory object too
                _logger.Debug("Updated delta token for folder {FolderName} after processing delta changes", folder.FolderName);
            }
        }
        catch (ApiException apiException)
        {
            // Try to handle the error using the error handling factory
            var errorContext = new SynchronizerErrorContext
            {
                Account = Account,
                ErrorCode = (int?)apiException.ResponseStatusCode,
                ErrorMessage = $"API error during delta sync: {apiException.Message}",
                Exception = apiException
            };

            var handled = await _errorHandlingFactory.HandleErrorAsync(errorContext).ConfigureAwait(false);

            if (handled)
            {
                // The error handler has processed the error (e.g., DeltaTokenExpiredHandler for 410)
                // Update in-memory folder state if it was a delta token expiration
                if (apiException.ResponseStatusCode == 410)
                {
                    folder.DeltaToken = string.Empty;
                    folder.FolderStatus = InitialSynchronizationStatus.None;
                    _logger.Information("API error handled successfully for folder {FolderName} during delta sync. Error: {ErrorCode}", folder.FolderName, apiException.ResponseStatusCode);
                }
            }
            else
            {
                // No handler could process this error, log and re-throw
                _logger.Error(apiException, "Unhandled API error during delta sync for folder {FolderName}. Error: {ErrorCode}", folder.FolderName, apiException.ResponseStatusCode);
            }
        }
        catch (Exception ex)
        {
            _logger.Error(ex, "Error processing delta changes for folder {FolderName}", folder.FolderName);
        }
    }

    private async Task ProcessDeltaChangesAsync(MailItemFolder folder, List<string> downloadedMessageIds, CancellationToken cancellationToken = default)
    {
        // Only process delta changes if we have a delta token (not initial sync)
        if (string.IsNullOrEmpty(folder.DeltaToken))
            return;

        try
        {
            var currentDeltaToken = folder.DeltaToken;

            // Always use Delta endpoint with proper configuration
            var requestInformation = _graphClient.Me.MailFolders[folder.RemoteFolderId].Messages.Delta.ToGetRequestInformation((config) =>
            {
                config.QueryParameters.Select = outlookMessageSelectParameters;
                config.QueryParameters.Orderby = ["receivedDateTime desc"]; // Sort by received date desc
            });

            requestInformation.UrlTemplate = requestInformation.UrlTemplate.Insert(requestInformation.UrlTemplate.Length - 1, ",%24deltatoken");
            requestInformation.QueryParameters.Add("%24deltatoken", currentDeltaToken);

            var messageCollectionPage = await _graphClient.RequestAdapter.SendAsync(requestInformation,
                DeltaGetResponse.CreateFromDiscriminatorValue,
                cancellationToken: cancellationToken);

            // Use PageIterator<DeltaGetResponse> for iterating mails
            var messageIterator = PageIterator<Message, DeltaGetResponse>
                .CreatePageIterator(_graphClient, messageCollectionPage, async (item) =>
                {
                    try
                    {
                        await _handleItemRetrievalSemaphore.WaitAsync();
                        return await HandleItemRetrievedAsync(item, folder, downloadedMessageIds, cancellationToken);
                    }
                    catch (Exception ex)
                    {
                        _logger.Error(ex, "Error occurred while handling delta item {Id} for folder {FolderName}", item.Id, folder.FolderName);
                    }
                    finally
                    {
                        _handleItemRetrievalSemaphore.Release();
                    }

                    return true;
                });

            await messageIterator.IterateAsync(cancellationToken).ConfigureAwait(false);

            // Update delta token for next sync - store delta token when there are no nextPageToken remaining
            if (!string.IsNullOrEmpty(messageIterator.Deltalink))
            {
                var deltaToken = GetDeltaTokenFromDeltaLink(messageIterator.Deltalink);
                await _outlookChangeProcessor.UpdateFolderDeltaSynchronizationIdentifierAsync(folder.Id, deltaToken).ConfigureAwait(false);
                _logger.Debug("Updated delta token for folder {FolderName} after processing delta changes", folder.FolderName);
            }
        }
        catch (ApiException apiException)
        {
            // Try to handle the error using the error handling factory
            var errorContext = new SynchronizerErrorContext
            {
                Account = Account,
                ErrorCode = (int?)apiException.ResponseStatusCode,
                ErrorMessage = $"API error during legacy delta sync: {apiException.Message}",
                Exception = apiException
            };

            var handled = await _errorHandlingFactory.HandleErrorAsync(errorContext).ConfigureAwait(false);

            if (!handled)
            {
                // No handler could process this error, log and re-throw
                _logger.Error(apiException, "Unhandled API error during legacy delta sync for folder {FolderName}. Error: {ErrorCode}", folder.FolderName, apiException.ResponseStatusCode);
            }
        }
    }

    private bool IsResourceDeleted(IDictionary<string, object> additionalData)
        => additionalData != null && additionalData.ContainsKey("@removed");

    private async Task<bool> HandleFolderRetrievedAsync(MailFolder folder, OutlookSpecialFolderIdInformation outlookSpecialFolderIdInformation, CancellationToken cancellationToken = default)
    {
        if (IsResourceDeleted(folder.AdditionalData))
        {
            await _outlookChangeProcessor.DeleteFolderAsync(Account.Id, folder.Id).ConfigureAwait(false);
        }
        else
        {
            // New folder created.

            var item = folder.GetLocalFolder(Account.Id);

            if (item.RemoteFolderId.Equals(outlookSpecialFolderIdInformation.InboxId))
                item.SpecialFolderType = SpecialFolderType.Inbox;
            else if (item.RemoteFolderId.Equals(outlookSpecialFolderIdInformation.SentId))
                item.SpecialFolderType = SpecialFolderType.Sent;
            else if (item.RemoteFolderId.Equals(outlookSpecialFolderIdInformation.DraftId))
                item.SpecialFolderType = SpecialFolderType.Draft;
            else if (item.RemoteFolderId.Equals(outlookSpecialFolderIdInformation.TrashId))
                item.SpecialFolderType = SpecialFolderType.Deleted;
            else if (item.RemoteFolderId.Equals(outlookSpecialFolderIdInformation.JunkId))
                item.SpecialFolderType = SpecialFolderType.Junk;
            else if (item.RemoteFolderId.Equals(outlookSpecialFolderIdInformation.ArchiveId))
                item.SpecialFolderType = SpecialFolderType.Archive;
            else
                item.SpecialFolderType = SpecialFolderType.Other;

            // Automatically mark special folders as Sticky for better visibility.
            item.IsSticky = item.SpecialFolderType != SpecialFolderType.Other;

            // By default, all non-others are system folder.
            item.IsSystemFolder = item.SpecialFolderType != SpecialFolderType.Other;

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

            await _outlookChangeProcessor.InsertFolderAsync(item).ConfigureAwait(false);
        }

        return true;
    }

    /// <summary>
    /// Somehow, Graph API returns Message type item for items like TodoTask, EventMessage and Contact.
    /// Basically deleted item retention items are stored as Message object in Deleted Items folder.
    /// Suprisingly, odatatype will also be the same as Message.
    /// In order to differentiate them from regular messages, we need to check the addresses in the message.
    /// </summary>
    /// <param name="item">Retrieved message.</param>
    /// <returns>Whether the item is non-Message type or not.</returns>
    private bool IsNotRealMessageType(Message item)
        => item is EventMessage || item.From?.EmailAddress == null;

    private async Task<bool> HandleItemRetrievedAsync(Message item, MailItemFolder folder, IList<string> downloadedMessageIds, CancellationToken cancellationToken = default)
    {
        if (IsResourceDeleted(item.AdditionalData))
        {
            // Deleting item with this override instead of the other one that deletes all mail copies.
            // Outlook mails have 1 assignment per-folder, unlike Gmail that has one to many.

            await _outlookChangeProcessor.DeleteAssignmentAsync(Account.Id, item.Id, folder.RemoteFolderId).ConfigureAwait(false);
        }
        else
        {
            // If the item exists in the local database, it means that it's already downloaded. Process as an Update.

            var isMailExists = await _outlookChangeProcessor.IsMailExistsInFolderAsync(item.Id, folder.Id);

            if (isMailExists)
            {
                // Some of the properties of the item are updated.

                if (item.IsRead != null)
                {
                    await _outlookChangeProcessor.ChangeMailReadStatusAsync(item.Id, item.IsRead.GetValueOrDefault()).ConfigureAwait(false);
                }

                if (item.Flag?.FlagStatus != null)
                {
                    await _outlookChangeProcessor.ChangeFlagStatusAsync(item.Id, item.Flag.FlagStatus.GetValueOrDefault() == FollowupFlagStatus.Flagged)
                                                 .ConfigureAwait(false);
                }
            }
            else
            {
                if (IsNotRealMessageType(item))
                {
                    if (item is EventMessage eventMessage)
                    {
                        Log.Warning("Recieved event message. This is not supported yet. {Id}", eventMessage.Id);
                    }
                    else
                    {
                        Log.Warning("Recieved either contact or todo item as message This is not supported yet. {Id}", item.Id);
                    }

                    return true;
                }

                // Package may return null on some cases mapping the remote draft to existing local draft.

                var newMailPackages = await CreateNewMailPackagesAsync(item, folder, cancellationToken);

                if (newMailPackages != null)
                {
                    foreach (var package in newMailPackages)
                    {
                        // Only add to downloaded message ids if it's inserted successfuly.
                        // Updates should not be added to the list because they are not new.
                        bool isInserted = await _outlookChangeProcessor.CreateMailAsync(Account.Id, package).ConfigureAwait(false);

                        if (isInserted)
                        {
                            downloadedMessageIds.Add(package.Copy.Id);
                        }
                    }
                }
            }
        }

        return true;
    }

    private async Task SynchronizeFoldersAsync(CancellationToken cancellationToken = default)
    {
        var specialFolderInfo = await GetSpecialFolderIdsAsync(cancellationToken).ConfigureAwait(false);
        var graphFolders = await GetDeltaFoldersAsync(cancellationToken).ConfigureAwait(false);

        var iterator = PageIterator<MailFolder, Microsoft.Graph.Me.MailFolders.Delta.DeltaGetResponse>
            .CreatePageIterator(_graphClient, graphFolders, (folder) =>
                HandleFolderRetrievedAsync(folder, specialFolderInfo, cancellationToken));

        await iterator.IterateAsync();

        await UpdateDeltaSynchronizationIdentifierAsync(iterator.Deltalink).ConfigureAwait(false);
    }

    [RequiresUnreferencedCode("Calls Microsoft.Kiota.Abstractions.Serialization.KiotaJsonSerializer.DeserializeAsync<T>(String, CancellationToken)")]
    private async Task<T> DeserializeGraphBatchResponseAsync<T>(BatchResponseContentCollection collection, string requestId, CancellationToken cancellationToken = default) where T : IParsable, new()
    {
        // This deserialization may throw generalException in case of failure.
        // Bug: https://github.com/microsoftgraph/msgraph-sdk-dotnet/issues/2010
        // This is a workaround for the bug to retrieve the actual exception.
        // All generic batch response deserializations must go under this method.

        try
        {
            return await collection.GetResponseByIdAsync<T>(requestId);
        }
        catch (ODataError)
        {
            throw;
        }
        catch (ServiceException serviceException)
        {
            // Actual exception is hidden inside ServiceException.


            ODataError errorResult = await KiotaJsonSerializer.DeserializeAsync<ODataError>(serviceException.RawResponseBody, cancellationToken);

            throw new SynchronizerException("Outlook Error", errorResult);
        }
    }

    private async Task<OutlookSpecialFolderIdInformation> GetSpecialFolderIdsAsync(CancellationToken cancellationToken)
    {
        var wellKnownFolderIdBatch = new BatchRequestContentCollection(_graphClient);
        var folderRequests = new Dictionary<string, RequestInformation>
        {
            { INBOX_NAME, _graphClient.Me.MailFolders[INBOX_NAME].ToGetRequestInformation((t) => { t.QueryParameters.Select = ["id"]; }) },
            { SENT_NAME, _graphClient.Me.MailFolders[SENT_NAME].ToGetRequestInformation((t) => { t.QueryParameters.Select = ["id"]; }) },
            { DELETED_NAME, _graphClient.Me.MailFolders[DELETED_NAME].ToGetRequestInformation((t) => { t.QueryParameters.Select = ["id"]; }) },
            { JUNK_NAME, _graphClient.Me.MailFolders[JUNK_NAME].ToGetRequestInformation((t) => { t.QueryParameters.Select = ["id"]; }) },
            { DRAFTS_NAME, _graphClient.Me.MailFolders[DRAFTS_NAME].ToGetRequestInformation((t) => { t.QueryParameters.Select = ["id"]; }) },
            { ARCHIVE_NAME, _graphClient.Me.MailFolders[ARCHIVE_NAME].ToGetRequestInformation((t) => { t.QueryParameters.Select = ["id"]; }) }
        };

        var batchIds = new Dictionary<string, string>();
        foreach (var request in folderRequests)
        {
            batchIds[request.Key] = await wellKnownFolderIdBatch.AddBatchRequestStepAsync(request.Value);
        }

        var returnedResponse = await _graphClient.Batch.PostAsync(wellKnownFolderIdBatch, cancellationToken).ConfigureAwait(false);

        var folderIds = new Dictionary<string, string>();
        foreach (var batchId in batchIds)
        {
            folderIds[batchId.Key] = (await DeserializeGraphBatchResponseAsync<MailFolder>(returnedResponse, batchId.Value, cancellationToken)).Id;
        }

        return new OutlookSpecialFolderIdInformation(
            folderIds[INBOX_NAME],
            folderIds[DELETED_NAME],
            folderIds[JUNK_NAME],
            folderIds[DRAFTS_NAME],
            folderIds[SENT_NAME],
            folderIds[ARCHIVE_NAME]);
    }

    private async Task<Microsoft.Graph.Me.MailFolders.Delta.DeltaGetResponse> GetDeltaFoldersAsync(CancellationToken cancellationToken)
    {
        if (string.IsNullOrEmpty(Account.SynchronizationDeltaIdentifier))
        {
            var deltaRequest = _graphClient.Me.MailFolders.Delta.ToGetRequestInformation();
            deltaRequest.UrlTemplate = deltaRequest.UrlTemplate.Insert(deltaRequest.UrlTemplate.Length - 1, ",includehiddenfolders");
            deltaRequest.QueryParameters.Add("includehiddenfolders", "true");

            return await _graphClient.RequestAdapter.SendAsync(deltaRequest,
                Microsoft.Graph.Me.MailFolders.Delta.DeltaGetResponse.CreateFromDiscriminatorValue,
                cancellationToken: cancellationToken).ConfigureAwait(false);
        }

        try
        {
            var deltaRequest = _graphClient.Me.MailFolders.Delta.ToGetRequestInformation();
            deltaRequest.UrlTemplate = deltaRequest.UrlTemplate.Insert(deltaRequest.UrlTemplate.Length - 1, ",%24deltaToken");
            deltaRequest.QueryParameters.Add("%24deltaToken", Account.SynchronizationDeltaIdentifier);

            return await _graphClient.RequestAdapter.SendAsync(deltaRequest,
                Microsoft.Graph.Me.MailFolders.Delta.DeltaGetResponse.CreateFromDiscriminatorValue,
                cancellationToken: cancellationToken).ConfigureAwait(false);
        }
        catch (ApiException apiException)
        {
            // Try to handle the error using the error handling factory
            var errorContext = new SynchronizerErrorContext
            {
                Account = Account,
                ErrorCode = (int?)apiException.ResponseStatusCode,
                ErrorMessage = $"API error during folder synchronization: {apiException.Message}",
                Exception = apiException
            };

            var handled = await _errorHandlingFactory.HandleErrorAsync(errorContext).ConfigureAwait(false);

            if (handled)
            {
                // The error handler has processed the error (e.g., DeltaTokenExpiredHandler for 410)
                // Update in-memory account state if it was a delta token expiration
                if (apiException.ResponseStatusCode == 410)
                {
                    Account.SynchronizationDeltaIdentifier = string.Empty;
                    _logger.Information("API error handled successfully for account {AccountName} during folder sync. Error: {ErrorCode}", Account.Name, apiException.ResponseStatusCode);
                }
            }
            else
            {
                // No handler could process this error, log and re-throw
                _logger.Error(apiException, "Unhandled API error during folder synchronization for account {AccountName}. Error: {ErrorCode}", Account.Name, apiException.ResponseStatusCode);
                throw;
            }

            // If a handler processed the error and it was 410, retry with fresh token
            if (apiException.ResponseStatusCode == 410)
            {
                return await GetDeltaFoldersAsync(cancellationToken);
            }

            // For other handled errors, we still need to throw since we can't return a meaningful response
            throw;
        }
    }

    private async Task UpdateDeltaSynchronizationIdentifierAsync(string deltalink)
    {
        if (string.IsNullOrEmpty(deltalink)) return;

        var deltaToken = deltalink.Split('=')[1];
        var latestAccountDeltaToken = await _outlookChangeProcessor
            .UpdateAccountDeltaSynchronizationIdentifierAsync(Account.Id, deltaToken);

        if (!string.IsNullOrEmpty(latestAccountDeltaToken))
        {
            Account.SynchronizationDeltaIdentifier = latestAccountDeltaToken;
        }
    }

    /// <summary>
    /// Get the user's profile picture
    /// </summary>
    /// <returns>Base64 encoded profile picture.</returns>
    private async Task<string> GetUserProfilePictureAsync()
    {
        try
        {
            var photoStream = await _graphClient.Me.Photos["48x48"].Content.GetAsync();

            using var memoryStream = new MemoryStream();
            await photoStream.CopyToAsync(memoryStream);
            var byteArray = memoryStream.ToArray();

            return Convert.ToBase64String(byteArray);
        }
        catch (ODataError odataError) when (odataError.Error.Code == "ImageNotFound")
        {
            // Accounts without profile picture will throw this error.
            // At this point nothing we can do. Just return empty string.

            return string.Empty;
        }
        catch (Exception)
        {
            // Don't throw for profile picture.
            // Office 365 apps require different permissions for profile picture.
            // This permission requires admin consent.
            // We avoid those permissions for now.

            return string.Empty;
        }
    }

    /// <summary>
    /// Get the user's display name.
    /// </summary>
    /// <returns>Display name and address of the user.</returns>
    private async Task<Tuple<string, string>> GetDisplayNameAndAddressAsync()
    {
        var userInfo = await _graphClient.Me.GetAsync();

        return new Tuple<string, string>(userInfo.DisplayName, userInfo.Mail);
    }

    public override async Task<ProfileInformation> GetProfileInformationAsync()
    {
        var profilePictureData = await GetUserProfilePictureAsync().ConfigureAwait(false);
        var displayNameAndAddress = await GetDisplayNameAndAddressAsync().ConfigureAwait(false);

        return new ProfileInformation(displayNameAndAddress.Item1, profilePictureData, displayNameAndAddress.Item2);
    }

    /// <summary>
    /// POST requests are handled differently in batches in Graph SDK.
    /// Batch basically ignores the step's coontent-type and body.
    /// Manually create a POST request with empty body and send it.
    /// </summary>
    /// <param name="requestInformation">Post request information.</param>
    /// <param name="content">Content object to serialize.</param>
    /// <returns>Updated post request information.</returns>
    private RequestInformation PreparePostRequestInformation(RequestInformation requestInformation, Microsoft.Graph.Me.Messages.Item.Move.MovePostRequestBody content = null)
    {
        requestInformation.Headers.Clear();

        string contentJson = content == null ? "{}" : JsonSerializer.Serialize(content, OutlookSynchronizerJsonContext.Default.MovePostRequestBody);

        requestInformation.Content = new MemoryStream(Encoding.UTF8.GetBytes(contentJson));
        requestInformation.HttpMethod = Method.POST;
        requestInformation.Headers.Add("Content-Type", "application/json");

        return requestInformation;
    }

    #region Mail Integration

    public override bool DelaySendOperationSynchronization() => true;

    public override List<IRequestBundle<RequestInformation>> Move(BatchMoveRequest request)
    {
        return ForEachRequest(request, (item) =>
        {
            var requestBody = new Microsoft.Graph.Me.Messages.Item.Move.MovePostRequestBody()
            {
                DestinationId = item.ToFolder.RemoteFolderId
            };

            return PreparePostRequestInformation(_graphClient.Me.Messages[item.Item.Id].Move.ToPostRequestInformation(requestBody),
                                                                 requestBody);
        });
    }

    public override List<IRequestBundle<RequestInformation>> ChangeFlag(BatchChangeFlagRequest request)
    {
        return ForEachRequest(request, (item) =>
        {
            var message = new Message()
            {
                Flag = new FollowupFlag() { FlagStatus = item.IsFlagged ? FollowupFlagStatus.Flagged : FollowupFlagStatus.NotFlagged }
            };

            return _graphClient.Me.Messages[item.Item.Id].ToPatchRequestInformation(message);
        });
    }

    public override List<IRequestBundle<RequestInformation>> MarkRead(BatchMarkReadRequest request)
    {
        return ForEachRequest(request, (item) =>
        {
            var message = new Message()
            {
                IsRead = item.IsRead
            };

            return _graphClient.Me.Messages[item.Item.Id].ToPatchRequestInformation(message);
        });
    }

    public override List<IRequestBundle<RequestInformation>> Delete(BatchDeleteRequest request)
    {
        return ForEachRequest(request, (item) =>
        {
            return _graphClient.Me.Messages[item.Item.Id].ToDeleteRequestInformation();
        });
    }

    public override List<IRequestBundle<RequestInformation>> MoveToFocused(BatchMoveToFocusedRequest request)
    {
        return ForEachRequest(request, (item) =>
        {
            if (item is MoveToFocusedRequest moveToFocusedRequest)
            {
                var message = new Message()
                {
                    InferenceClassification = moveToFocusedRequest.MoveToFocused ? InferenceClassificationType.Focused : InferenceClassificationType.Other
                };

                return _graphClient.Me.Messages[moveToFocusedRequest.Item.Id].ToPatchRequestInformation(message);
            }

            throw new Exception("Invalid request type.");
        });
    }

    public override List<IRequestBundle<RequestInformation>> AlwaysMoveTo(BatchAlwaysMoveToRequest request)
    {
        return ForEachRequest(request, (item) =>
        {
            var inferenceClassificationOverride = new InferenceClassificationOverride
            {
                ClassifyAs = item.MoveToFocused ? InferenceClassificationType.Focused : InferenceClassificationType.Other,
                SenderEmailAddress = new EmailAddress
                {
                    Name = item.Item.FromName,
                    Address = item.Item.FromAddress
                }
            };

            return _graphClient.Me.InferenceClassification.Overrides.ToPostRequestInformation(inferenceClassificationOverride);
        });
    }

    public override List<IRequestBundle<RequestInformation>> CreateDraft(CreateDraftRequest createDraftRequest)
    {
        var reason = createDraftRequest.DraftPreperationRequest.Reason;
        var message = createDraftRequest.DraftPreperationRequest.CreatedLocalDraftMimeMessage.AsOutlookMessage(true);

        if (reason == DraftCreationReason.Empty)
        {
            return [new HttpRequestBundle<RequestInformation>(_graphClient.Me.Messages.ToPostRequestInformation(message), createDraftRequest)];
        }
        else if (reason == DraftCreationReason.Reply)
        {
            return [new HttpRequestBundle<RequestInformation>(_graphClient.Me.Messages[createDraftRequest.DraftPreperationRequest.ReferenceMailCopy.Id].CreateReply.ToPostRequestInformation(new Microsoft.Graph.Me.Messages.Item.CreateReply.CreateReplyPostRequestBody()
            {
                Message = message
            }), createDraftRequest)];

        }
        else if (reason == DraftCreationReason.ReplyAll)
        {
            return [new HttpRequestBundle<RequestInformation>(_graphClient.Me.Messages[createDraftRequest.DraftPreperationRequest.ReferenceMailCopy.Id].CreateReplyAll.ToPostRequestInformation(new Microsoft.Graph.Me.Messages.Item.CreateReplyAll.CreateReplyAllPostRequestBody()
            {
                Message = message
            }), createDraftRequest)];
        }
        else if (reason == DraftCreationReason.Forward)
        {
            return [new HttpRequestBundle<RequestInformation>( _graphClient.Me.Messages[createDraftRequest.DraftPreperationRequest.ReferenceMailCopy.Id].CreateForward.ToPostRequestInformation(new Microsoft.Graph.Me.Messages.Item.CreateForward.CreateForwardPostRequestBody()
            {
                Message = message
            }), createDraftRequest)];
        }
        else
        {
            throw new NotImplementedException("Draft creation reason is not implemented.");
        }
    }

    public override List<IRequestBundle<RequestInformation>> SendDraft(SendDraftRequest request)
    {
        var sendDraftPreparationRequest = request.Request;

        // 1. Delete draft
        // 2. Create new Message with new MIME.
        // 3. Make sure that conversation id is tagged correctly for replies.

        var mailCopyId = sendDraftPreparationRequest.MailItem.Id;
        var mimeMessage = sendDraftPreparationRequest.Mime;

        // Convert mime message to Outlook message.
        // Outlook synchronizer does not send MIME messages directly anymore.
        // Alias support is lacking with direct MIMEs.
        // Therefore we convert the MIME message to Outlook message and use proper APIs.

        var outlookMessage = mimeMessage.AsOutlookMessage(false);

        // Create attachment requests.
        // TODO: We need to support large file attachments with sessioned upload at some point.

        var attachmentRequestList = CreateAttachmentUploadBundles(mimeMessage, mailCopyId, request).ToList();

        // Update draft.

        var patchDraftRequest = _graphClient.Me.Messages[mailCopyId].ToPatchRequestInformation(outlookMessage);
        var patchDraftRequestBundle = new HttpRequestBundle<RequestInformation>(patchDraftRequest, request);

        // Send draft.

        var sendDraftRequest = PreparePostRequestInformation(_graphClient.Me.Messages[mailCopyId].Send.ToPostRequestInformation());
        var sendDraftRequestBundle = new HttpRequestBundle<RequestInformation>(sendDraftRequest, request);

        return [.. attachmentRequestList, patchDraftRequestBundle, sendDraftRequestBundle];
    }

    private List<IRequestBundle<RequestInformation>> CreateAttachmentUploadBundles(MimeMessage mime, string mailCopyId, IRequestBase sourceRequest)
    {
        var allAttachments = new List<OutlookFileAttachment>();

        foreach (var part in mime.BodyParts)
        {
            var isAttachmentOrInline = part.IsAttachment ? true : part.ContentDisposition?.Disposition == "inline";

            if (!isAttachmentOrInline) continue;

            using var memory = new MemoryStream();
            ((MimePart)part).Content.DecodeTo(memory);

            var base64String = Convert.ToBase64String(memory.ToArray());

            var attachment = new OutlookFileAttachment()
            {
                Base64EncodedContentBytes = base64String,
                FileName = part.ContentDisposition?.FileName ?? part.ContentType.Name,
                ContentId = part.ContentId,
                ContentType = part.ContentType.MimeType,
                IsInline = part.ContentDisposition?.Disposition == "inline"
            };

            allAttachments.Add(attachment);
        }

        static RequestInformation PrepareUploadAttachmentRequest(RequestInformation requestInformation, OutlookFileAttachment outlookFileAttachment)
        {
            requestInformation.Headers.Clear();

            string contentJson = JsonSerializer.Serialize(outlookFileAttachment, OutlookSynchronizerJsonContext.Default.OutlookFileAttachment);

            requestInformation.Content = new MemoryStream(Encoding.UTF8.GetBytes(contentJson));
            requestInformation.HttpMethod = Method.POST;
            requestInformation.Headers.Add("Content-Type", "application/json");

            return requestInformation;
        }

        var retList = new List<IRequestBundle<RequestInformation>>();

        // Prepare attachment upload requests.

        foreach (var attachment in allAttachments)
        {
            var emptyPostRequest = _graphClient.Me.Messages[mailCopyId].Attachments.ToPostRequestInformation(new Attachment());
            var modifiedAttachmentUploadRequest = PrepareUploadAttachmentRequest(emptyPostRequest, attachment);

            var bundle = new HttpRequestBundle<RequestInformation>(modifiedAttachmentUploadRequest, null);

            retList.Add(bundle);
        }

        return retList;
    }

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

        return Move(batchMoveRequest);
    }

    public override async Task DownloadMissingMimeMessageAsync(MailCopy mailItem,
                                                           MailKit.ITransferProgress transferProgress = null,
                                                           CancellationToken cancellationToken = default)
    {
        var mimeMessage = await DownloadMimeMessageAsync(mailItem.Id, cancellationToken).ConfigureAwait(false);
        await _outlookChangeProcessor.SaveMimeFileAsync(mailItem.FileId, mimeMessage, Account.Id).ConfigureAwait(false);
    }

    public override List<IRequestBundle<RequestInformation>> RenameFolder(RenameFolderRequest request)
    {
        var requestBody = new MailFolder
        {
            DisplayName = request.NewFolderName,
        };

        var networkCall = _graphClient.Me.MailFolders[request.Folder.RemoteFolderId].ToPatchRequestInformation(requestBody);

        return [new HttpRequestBundle<RequestInformation>(networkCall, request)];
    }

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

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

    #endregion

    public override async Task ExecuteNativeRequestsAsync(List<IRequestBundle<RequestInformation>> batchedRequests, CancellationToken cancellationToken = default)
    {
        var batchedGroups = batchedRequests.Batch((int)MaximumAllowedBatchRequestSize);

        foreach (var batch in batchedGroups)
        {
            await ExecuteBatchRequestsAsync(batch, cancellationToken);
        }
    }

    private async Task ExecuteBatchRequestsAsync(IEnumerable<IRequestBundle<RequestInformation>> batch, CancellationToken cancellationToken)
    {
        var batchContent = new BatchRequestContentCollection(_graphClient);
        var itemCount = batch.Count();

        if (itemCount == 0) return;

        var bundleIdMap = await PrepareBatchContentAsync(batch, batchContent, itemCount);

        // Execute batch to collect responses from network call
        var batchRequestResponse = await _graphClient.Batch.PostAsync(batchContent, cancellationToken);

        await ProcessBatchResponsesAsync(batch, batchRequestResponse, bundleIdMap);
    }

    private async Task<Dictionary<string, IRequestBundle<RequestInformation>>> PrepareBatchContentAsync(
        IEnumerable<IRequestBundle<RequestInformation>> batch,
        BatchRequestContentCollection batchContent,
        int itemCount)
    {
        var bundleIdMap = new Dictionary<string, IRequestBundle<RequestInformation>>();
        bool requiresSerial = false;

        for (int i = 0; i < itemCount; i++)
        {
            var bundle = batch.ElementAt(i);
            requiresSerial |= bundle.UIChangeRequest is SendDraftRequest;

            bundle.UIChangeRequest?.ApplyUIChanges();
            var batchRequestId = await batchContent.AddBatchRequestStepAsync(bundle.NativeRequest);
            bundle.BundleId = batchRequestId;
            bundleIdMap[batchRequestId] = bundle;
        }

        if (requiresSerial)
        {
            ConfigureSerialExecution(batchContent);
        }

        return bundleIdMap;
    }

    private void ConfigureSerialExecution(BatchRequestContentCollection batchContent)
    {
        // Set each step to depend on previous one for serial execution
        var steps = batchContent.BatchRequestSteps.ToList();
        for (int i = 1; i < steps.Count; i++)
        {
            var currentStep = steps[i].Value;
            var previousStepKey = steps[i - 1].Key;
            currentStep.DependsOn = [previousStepKey];
        }
    }

    private async Task ProcessBatchResponsesAsync(
        IEnumerable<IRequestBundle<RequestInformation>> batch,
        BatchResponseContentCollection batchResponse,
        Dictionary<string, IRequestBundle<RequestInformation>> bundleIdMap)
    {
        var errors = new List<string>();

        foreach (var bundleId in bundleIdMap.Keys)
        {
            var bundle = bundleIdMap[bundleId];
            var response = await batchResponse.GetResponseByIdAsync(bundleId);

            if (response == null) continue;

            using (response)
            {
                if (!response.IsSuccessStatusCode)
                {
                    await HandleFailedResponseAsync(bundle, response, errors);
                }
            }
        }

        if (errors.Any())
        {
            ThrowBatchExecutionException(errors);
        }
    }

    private async Task HandleFailedResponseAsync(
        IRequestBundle<RequestInformation> bundle,
        HttpResponseMessage response,
        List<string> errors)
    {
        var content = await response.Content.ReadAsStringAsync();
        var errorJson = JsonNode.Parse(content);
        var errorCode = errorJson["error"]["code"].GetValue<string>();
        var errorMessage = errorJson["error"]["message"].GetValue<string>();
        var errorString = $"[{response.StatusCode}] {errorCode} - {errorMessage}\n";

        // Create error context
        var errorContext = new SynchronizerErrorContext
        {
            Account = Account,
            ErrorCode = (int)response.StatusCode,
            ErrorMessage = errorMessage,
            RequestBundle = bundle,
            AdditionalData = new Dictionary<string, object>
            {
                { "ErrorCode", errorCode },
                { "HttpResponse", response },
                { "Content", content }
            }
        };

        // Try to handle the error with registered handlers
        var handled = await _errorHandlingFactory.HandleErrorAsync(errorContext);

        // If not handled by any specific handler, revert UI changes and add to error list
        if (!handled)
        {
            bundle.UIChangeRequest?.RevertUIChanges();
            Debug.WriteLine(errorString);
            errors.Add(errorString);
        }
    }

    private void ThrowBatchExecutionException(List<string> errors)
    {
        var formattedErrorString = string.Join("\n",
            errors.Select((item, index) => $"{index + 1}. {item}"));
        throw new SynchronizerException(formattedErrorString);
    }

    public override async Task<List<MailCopy>> OnlineSearchAsync(string queryText, List<IMailItemFolder> folders, CancellationToken cancellationToken = default)
    {
        List<Message> messagesReturnedByApi = [];

        // Perform search for each folder separately.
        if (folders?.Count > 0)
        {
            var folderIds = folders.Select(a => a.RemoteFolderId);

            var tasks = folderIds.Select(async folderId =>
            {
                var mailQuery = _graphClient.Me.MailFolders[folderId].Messages
                    .GetAsync(requestConfig =>
                    {
                        requestConfig.QueryParameters.Search = $"\"{queryText}\"";
                        requestConfig.QueryParameters.Select = ["Id, ParentFolderId"];
                        requestConfig.QueryParameters.Top = 1000;
                    });

                var result = await mailQuery;

                if (result?.Value != null)
                {
                    lock (messagesReturnedByApi)
                    {
                        messagesReturnedByApi.AddRange(result.Value);
                    }
                }
            });

            await Task.WhenAll(tasks);
        }
        else
        {
            // Perform search for all messages without folder data.
            var mailQuery = _graphClient.Me.Messages
                .GetAsync(requestConfig =>
                {
                    requestConfig.QueryParameters.Search = $"\"{queryText}\"";
                    requestConfig.QueryParameters.Select = ["Id, ParentFolderId"];
                    requestConfig.QueryParameters.Top = 1000;
                }, cancellationToken);

            var result = await mailQuery;

            if (result?.Value != null)
            {
                messagesReturnedByApi.AddRange(result.Value);
            }
        }

        if (messagesReturnedByApi.Count == 0) return [];

        var localFolders = (await _outlookChangeProcessor.GetLocalFoldersAsync(Account.Id).ConfigureAwait(false))
            .ToDictionary(x => x.RemoteFolderId);

        var messagesDictionary = messagesReturnedByApi.ToDictionary(a => a.Id);

        // Contains a list of message ids that potentially can be downloaded.
        List<string> messageIdsWithKnownFolder = [];

        // Validate that all messages are in a known folder.
        foreach (var message in messagesReturnedByApi)
        {
            if (!localFolders.ContainsKey(message.ParentFolderId))
            {
                Log.Warning("Search result returned a message from a folder that is not synchronized.");
                continue;
            }

            messageIdsWithKnownFolder.Add(message.Id);
        }

        var locallyExistingMails = await _outlookChangeProcessor.AreMailsExistsAsync(messageIdsWithKnownFolder).ConfigureAwait(false);

        // Find messages that are not downloaded yet.
        List<Message> messagesToDownload = [];
        foreach (var id in messagesDictionary.Keys.Except(locallyExistingMails))
        {
            messagesToDownload.Add(messagesDictionary[id]);
        }

        foreach (var message in messagesToDownload)
        {
            await DownloadSearchResultMessageAsync(message.Id, localFolders[message.ParentFolderId], cancellationToken).ConfigureAwait(false);
        }

        // Get results from database and return.
        return await _outlookChangeProcessor.GetMailCopiesAsync(messageIdsWithKnownFolder).ConfigureAwait(false);
    }

    private async Task<MimeMessage> DownloadMimeMessageAsync(string messageId, CancellationToken cancellationToken = default)
    {
        var mimeContentStream = await _graphClient.Me.Messages[messageId].Content.GetAsync(null, cancellationToken).ConfigureAwait(false);
        return await MimeMessage.LoadAsync(mimeContentStream, cancellationToken).ConfigureAwait(false);
    }

    public override async Task<List<NewMailItemPackage>> CreateNewMailPackagesAsync(Message message, MailItemFolder assignedFolder, CancellationToken cancellationToken = default)
    {
        // Download MIME message for specific scenarios (e.g., search results, draft handling)
        // During normal sync, this method should not be called - use CreateMailCopyFromMessage instead
        var mimeMessage = await DownloadMimeMessageAsync(message.Id, cancellationToken).ConfigureAwait(false);
        var mailCopy = CreateMailCopyFromMessage(message, assignedFolder);

        if (message.IsDraft.GetValueOrDefault()
            && 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 _outlookChangeProcessor.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.
        }

        // Outlook messages can only be assigned to 1 folder at a time.
        // Therefore we don't need to create multiple copies of the same message for different folders.
        var package = new NewMailItemPackage(mailCopy, mimeMessage, assignedFolder.RemoteFolderId);

        return [package];
    }

    protected override async Task<CalendarSynchronizationResult> SynchronizeCalendarEventsInternalAsync(CalendarSynchronizationOptions options, CancellationToken cancellationToken = default)
    {
        _logger.Information("Internal calendar synchronization started for {Name}", Account.Name);

        cancellationToken.ThrowIfCancellationRequested();

        await SynchronizeCalendarsAsync(cancellationToken).ConfigureAwait(false);

        var localCalendars = await _outlookChangeProcessor.GetAccountCalendarsAsync(Account.Id).ConfigureAwait(false);

        Microsoft.Graph.Me.Calendars.Item.CalendarView.Delta.DeltaGetResponse eventsDeltaResponse = null;

        // TODO: Maybe we can batch each calendar?

        foreach (var calendar in localCalendars)
        {
            bool isInitialSync = string.IsNullOrEmpty(calendar.SynchronizationDeltaToken);

            if (isInitialSync)
            {
                _logger.Information("No calendar sync identifier for calendar {Name}. Performing initial sync.", calendar.Name);

                var startDate = DateTime.UtcNow.AddYears(-2).ToString("u");
                var endDate = DateTime.UtcNow.ToString("u");

                eventsDeltaResponse = await _graphClient.Me.Calendars[calendar.RemoteCalendarId].CalendarView.Delta.GetAsDeltaGetResponseAsync((requestConfiguration) =>
                {
                    requestConfiguration.QueryParameters.StartDateTime = startDate;
                    requestConfiguration.QueryParameters.EndDateTime = endDate;
                }, cancellationToken: cancellationToken);

                // No delta link. Performing initial sync.
                //eventsDeltaResponse = await _graphClient.Me.CalendarView.Delta.GetAsDeltaGetResponseAsync((requestConfiguration) =>
                //{
                //    requestConfiguration.QueryParameters.StartDateTime = startDate;
                //    requestConfiguration.QueryParameters.EndDateTime = endDate;

                //    // TODO: Expand does not work.
                //    // https://github.com/microsoftgraph/msgraph-sdk-dotnet/issues/2358

                //    requestConfiguration.QueryParameters.Expand = new string[] { "calendar($select=name,id)" }; // Expand the calendar and select name and id. Customize as needed.
                //}, cancellationToken: cancellationToken);
            }
            else
            {
                var currentDeltaToken = calendar.SynchronizationDeltaToken;

                _logger.Information("Performing delta sync for calendar {Name}.", calendar.Name);

                var requestInformation = _graphClient.Me.Calendars[calendar.RemoteCalendarId].CalendarView.Delta.ToGetRequestInformation((requestConfiguration) =>
                {

                    //requestConfiguration.QueryParameters.StartDateTime = startDate;
                    //requestConfiguration.QueryParameters.EndDateTime = endDate;
                });

                //var requestInformation = _graphClient.Me.Calendars[calendar.RemoteCalendarId].CalendarView.Delta.ToGetRequestInformation((config) =>
                //{
                //    config.QueryParameters.Top = (int)InitialMessageDownloadCountPerFolder;
                //    config.QueryParameters.Select = outlookMessageSelectParameters;
                //    config.QueryParameters.Orderby = ["receivedDateTime desc"];
                //});


                requestInformation.UrlTemplate = requestInformation.UrlTemplate.Insert(requestInformation.UrlTemplate.Length - 1, ",%24deltatoken");
                requestInformation.QueryParameters.Add("%24deltatoken", currentDeltaToken);

                eventsDeltaResponse = await _graphClient.RequestAdapter.SendAsync(requestInformation, Microsoft.Graph.Me.Calendars.Item.CalendarView.Delta.DeltaGetResponse.CreateFromDiscriminatorValue);
            }

            List<Event> events = new();

            // We must first save the parent recurring events to not lose exceptions.
            // Therefore, order the existing items by their type and save the parent recurring events first.

            var messageIteratorAsync = PageIterator<Event, Microsoft.Graph.Me.Calendars.Item.CalendarView.Delta.DeltaGetResponse>.CreatePageIterator(_graphClient, eventsDeltaResponse, (item) =>
            {
                events.Add(item);

                return true;
            });

            await messageIteratorAsync
                .IterateAsync(cancellationToken)
                .ConfigureAwait(false);

            // Desc-order will move parent recurring events to the top.
            events = events.OrderByDescending(a => a.Type).ToList();

            _logger.Information("Found {Count} events in total.", events.Count);

            foreach (var item in events)
            {
                try
                {
                    await _handleItemRetrievalSemaphore.WaitAsync();
                    await _outlookChangeProcessor.ManageCalendarEventAsync(item, calendar, Account).ConfigureAwait(false);
                }
                catch (Exception)
                {
                    // _logger.Error(ex, "Error occurred while handling item {Id} for calendar {Name}", item.Id, calendar.Name);
                }
                finally
                {
                    _handleItemRetrievalSemaphore.Release();
                }
            }

            var latestDeltaLink = messageIteratorAsync.Deltalink;

            //Store delta link for tracking new changes.
            if (!string.IsNullOrEmpty(latestDeltaLink))
            {
                // Parse Delta Token from Delta Link since v5 of Graph SDK works based on the token, not the link.

                var deltaToken = GetDeltaTokenFromDeltaLink(latestDeltaLink);

                await _outlookChangeProcessor.UpdateCalendarDeltaSynchronizationToken(calendar.Id, deltaToken).ConfigureAwait(false);
            }
        }

        return default;
    }

    private async Task SynchronizeCalendarsAsync(CancellationToken cancellationToken = default)
    {
        var calendars = await _graphClient.Me.Calendars.GetAsync(cancellationToken: cancellationToken).ConfigureAwait(false);

        var localCalendars = await _outlookChangeProcessor.GetAccountCalendarsAsync(Account.Id).ConfigureAwait(false);

        List<AccountCalendar> insertedCalendars = new();
        List<AccountCalendar> updatedCalendars = new();
        List<AccountCalendar> deletedCalendars = new();

        // 1. Handle deleted calendars.

        foreach (var calendar in localCalendars)
        {
            var remoteCalendar = calendars.Value.FirstOrDefault(a => a.Id == calendar.RemoteCalendarId);
            if (remoteCalendar == null)
            {
                // Local calendar doesn't exists remotely. Delete local copy.

                await _outlookChangeProcessor.DeleteAccountCalendarAsync(calendar).ConfigureAwait(false);
                deletedCalendars.Add(calendar);
            }
        }

        // Delete the deleted folders from local list.
        deletedCalendars.ForEach(a => localCalendars.Remove(a));

        // 2. Handle update/insert based on remote calendars.
        foreach (var calendar in calendars.Value)
        {
            var existingLocalCalendar = localCalendars.FirstOrDefault(a => a.RemoteCalendarId == calendar.Id);
            if (existingLocalCalendar == null)
            {
                // Insert new calendar.
                var localCalendar = calendar.AsCalendar(Account);
                insertedCalendars.Add(localCalendar);
            }
            else
            {
                // Update existing calendar. Right now we only update the name.
                if (ShouldUpdateCalendar(calendar, existingLocalCalendar))
                {
                    existingLocalCalendar.Name = calendar.Name;

                    updatedCalendars.Add(existingLocalCalendar);
                }
                else
                {
                    // Remove it from the local folder list to skip additional calendar updates.
                    localCalendars.Remove(existingLocalCalendar);
                }
            }
        }

        // 3.Process changes in order-> Insert, Update. Deleted ones are already processed.
        foreach (var calendar in insertedCalendars)
        {
            await _outlookChangeProcessor.InsertAccountCalendarAsync(calendar).ConfigureAwait(false);
        }

        foreach (var calendar in updatedCalendars)
        {
            await _outlookChangeProcessor.UpdateAccountCalendarAsync(calendar).ConfigureAwait(false);
        }

        if (insertedCalendars.Any() || deletedCalendars.Any() || updatedCalendars.Any())
        {
            // TODO: Notify calendar updates.
            // WeakReferenceMessenger.Default.Send(new AccountFolderConfigurationUpdated(Account.Id));
        }
    }

    private bool ShouldUpdateCalendar(Calendar calendar, AccountCalendar accountCalendar)
    {
        // TODO: Only calendar name is updated for now. We can add more checks here.

        var remoteCalendarName = calendar.Name;
        var localCalendarName = accountCalendar.Name;

        return !localCalendarName.Equals(remoteCalendarName, StringComparison.OrdinalIgnoreCase);
    }

    public override async Task KillSynchronizerAsync()
    {
        await base.KillSynchronizerAsync();

        _graphClient.Dispose();
    }
}