using AutoMapper;
using Marco.Pms.DataAccess.Data;
using Marco.Pms.Model.Dtos.Expenses;
using Marco.Pms.Model.Entitlements;
using Marco.Pms.Model.Expenses;
using Marco.Pms.Model.Utilities;
using Marco.Pms.Model.ViewModels.Expanses;
using Marco.Pms.Model.ViewModels.Master;
using Marco.Pms.Model.ViewModels.Projects;
using Marco.Pms.Services.Service;
using MarcoBMS.Services.Helpers;
using MarcoBMS.Services.Service;
using Microsoft.AspNetCore.Authorization;
using Microsoft.AspNetCore.Mvc;
using Microsoft.EntityFrameworkCore;
using System.Text.Json;
using Document = Marco.Pms.Model.DocumentManager.Document;


namespace Marco.Pms.Services.Controllers
{
    [Route("api/[controller]")]
    [ApiController]
    [Authorize]
    public class ExpenseController : ControllerBase
    {
        private readonly IDbContextFactory<ApplicationDbContext> _dbContextFactory;
        private readonly ApplicationDbContext _context;
        private readonly UserHelper _userHelper;
        private readonly PermissionServices _permission;
        private readonly ILoggingService _logger;
        private readonly S3UploadService _s3Service;
        private readonly IServiceScopeFactory _serviceScopeFactory;
        private readonly IMapper _mapper;
        private readonly Guid tenantId;
        private static readonly Guid Draft = Guid.Parse("297e0d8f-f668-41b5-bfea-e03b354251c8");
        public ExpenseController(
            IDbContextFactory<ApplicationDbContext> dbContextFactory,
            ApplicationDbContext context,
            UserHelper userHelper,
            PermissionServices permission,
            IServiceScopeFactory serviceScopeFactory,
            ILoggingService logger,
            S3UploadService s3Service,
            IMapper mapper)
        {
            _dbContextFactory = dbContextFactory;
            _context = context;
            _userHelper = userHelper;
            _permission = permission;
            _logger = logger;
            _serviceScopeFactory = serviceScopeFactory;
            _s3Service = s3Service;
            _mapper = mapper;
            tenantId = userHelper.GetTenantId();
        }


        /// <summary>
        /// Retrieves a paginated list of expenses based on user permissions and optional filters.
        /// </summary>
        /// <param name="filter">A URL-encoded JSON string containing filter criteria. See <see cref="ExpensesFilter"/>.</param>
        /// <param name="pageSize">The number of records to return per page.</param>
        /// <param name="pageNumber">The page number to retrieve.</param>
        /// <returns>A paginated list of expenses.</returns>
        [HttpGet("list")] // Assuming this is a GET endpoint
        public async Task<IActionResult> GetExpensesList(string? filter, int pageSize = 20, int pageNumber = 1)
        {
            try
            {
                _logger.LogInfo(
                    "Attempting to fetch expenses list for PageNumber: {PageNumber}, PageSize: {PageSize} with Filter: {Filter}",
                    pageNumber, pageSize, filter ?? "");

                // 1. --- Get User and Permissions ---
                var loggedInEmployee = await _userHelper.GetCurrentEmployeeAsync();
                if (loggedInEmployee == null)
                {
                    // This is an authentication/authorization issue. The user should be logged in.
                    _logger.LogWarning("Could not find an employee for the current logged-in user.");
                    return Unauthorized(ApiResponse<object>.ErrorResponse("User not found or not authenticated.", 401));
                }
                Guid loggedInEmployeeId = loggedInEmployee.Id;

                var hasViewSelfPermissionTask = Task.Run(async () =>
                {
                    using var scope = _serviceScopeFactory.CreateScope();
                    var permissionService = scope.ServiceProvider.GetRequiredService<PermissionServices>();
                    return await permissionService.HasPermission(PermissionsMaster.ExpenseViewSelf, loggedInEmployeeId);
                });

                var hasViewAllPermissionTask = Task.Run(async () =>
                {
                    using var scope = _serviceScopeFactory.CreateScope();
                    var permissionService = scope.ServiceProvider.GetRequiredService<PermissionServices>();
                    return await permissionService.HasPermission(PermissionsMaster.ExpenseViewAll, loggedInEmployeeId);
                });

                await Task.WhenAll(hasViewSelfPermissionTask, hasViewAllPermissionTask);

                // 2. --- Build Base Query and Apply Permissions ---
                // Start with a base IQueryable. Filters will be chained onto this.
                var expensesQuery = _context.Expenses
                    .Include(e => e.ExpensesType)
                    .Include(e => e.Project)
                    .Include(e => e.PaidBy)
                        .ThenInclude(e => e!.JobRole)
                    .Include(e => e.PaymentMode)
                    .Include(e => e.Status)
                    .Include(e => e.CreatedBy)
                    .Where(e => e.TenantId == tenantId); // Always filter by TenantId first.

                // Apply permission-based filtering BEFORE any other filters or pagination.
                if (hasViewAllPermissionTask.Result)
                {
                    // User has 'View All' permission, no initial restriction on who created the expense.
                    _logger.LogInfo("User {EmployeeId} has 'View All' permission.", loggedInEmployeeId);
                }
                else if (hasViewSelfPermissionTask.Result)
                {
                    // User only has 'View Self' permission, so restrict the query to their own expenses.
                    _logger.LogInfo("User {EmployeeId} has 'View Self' permission. Restricting query to their expenses.", loggedInEmployeeId);
                    expensesQuery = expensesQuery.Where(e => e.CreatedById == loggedInEmployeeId);
                }
                else
                {
                    // User has neither required permission. Deny access.
                    _logger.LogWarning("Access DENIED for employee {EmployeeId} attempting to get expenses list.", loggedInEmployeeId);
                    return Ok(ApiResponse<object>.SuccessResponse(new List<ExpenseList>(), "You do not have permission to view any expenses.", 200));
                }

                // 3. --- Deserialize Filter and Apply ---
                ExpensesFilter? expenseFilter = TryDeserializeFilter(filter);

                if (expenseFilter != null)
                {
                    // CRITICAL FIX: Apply filters cumulatively using multiple `if` statements, not `if-else if`.
                    if (expenseFilter.StartDate.HasValue && expenseFilter.EndDate.HasValue)
                    {
                        expensesQuery = expensesQuery.Where(e => e.CreatedAt.Date >= expenseFilter.StartDate.Value.Date && e.CreatedAt.Date <= expenseFilter.EndDate.Value.Date);
                    }

                    if (expenseFilter.ProjectIds?.Any() == true)
                    {
                        expensesQuery = expensesQuery.Where(e => expenseFilter.ProjectIds.Contains(e.ProjectId));
                    }

                    if (expenseFilter.StatusIds?.Any() == true)
                    {
                        expensesQuery = expensesQuery.Where(e => expenseFilter.StatusIds.Contains(e.StatusId));
                    }

                    if (expenseFilter.PaidById?.Any() == true)
                    {
                        expensesQuery = expensesQuery.Where(e => expenseFilter.PaidById.Contains(e.PaidById));
                    }

                    // Only allow filtering by 'CreatedBy' if the user has 'View All' permission.
                    if (expenseFilter.CreatedByIds?.Any() == true && hasViewAllPermissionTask.Result)
                    {
                        expensesQuery = expensesQuery.Where(e => expenseFilter.CreatedByIds.Contains(e.CreatedById));
                    }
                }

                // 4. --- Apply Ordering and Pagination ---
                // This should be the last step before executing the query.
                var paginatedQuery = expensesQuery
                    .OrderByDescending(e => e.CreatedAt)
                    .Skip((pageNumber - 1) * pageSize)
                    .Take(pageSize);

                // 5. --- Execute Query and Map Results ---
                var expensesList = await paginatedQuery.ToListAsync();

                if (!expensesList.Any())
                {
                    _logger.LogInfo("No expenses found matching the criteria for employee {EmployeeId}.", loggedInEmployeeId);
                    return Ok(ApiResponse<object>.SuccessResponse(new List<ExpenseList>(), "No expenses found for the given criteria.", 200));
                }

                var response = _mapper.Map<List<ExpenseList>>(expensesList);

                // 6. --- Efficiently Fetch and Append 'Next Status' Information ---
                var statusIds = expensesList.Select(e => e.StatusId).Distinct().ToList();

                var statusMappings = await _context.ExpensesStatusMapping
                    .Include(sm => sm.NextStatus)
                    .Where(sm => statusIds.Contains(sm.StatusId))
                    .ToListAsync();

                // Use a Lookup for efficient O(1) mapping. This is much better than repeated `.Where()` in a loop.
                var statusMapLookup = statusMappings.ToLookup(sm => sm.StatusId);

                foreach (var expense in response)
                {
                    if (expense.Status?.Id != null && statusMapLookup.Contains(expense.Status.Id))
                    {
                        expense.NextStatus = statusMapLookup[expense.Status.Id]
                            .Select(sm => _mapper.Map<ExpensesStatusMasterVM>(sm.NextStatus))
                            .ToList();
                    }
                    else
                    {
                        expense.NextStatus = new List<ExpensesStatusMasterVM>(); // Ensure it's never null
                    }
                }

                // 7. --- Return Final Success Response ---
                var message = $"{response.Count} expense records fetched successfully.";
                _logger.LogInfo(message);
                return StatusCode(200, ApiResponse<object>.SuccessResponse(response, message, 200));
            }
            catch (DbUpdateException dbEx)
            {
                _logger.LogError(dbEx, "Databsae Exception occured while fetching list expenses");
                return BadRequest(ApiResponse<object>.ErrorResponse("Databsae Exception", new
                {
                    Message = dbEx.Message,
                    StackTrace = dbEx.StackTrace,
                    Source = dbEx.Source,
                    innerexcption = new
                    {
                        Message = dbEx.InnerException?.Message,
                        StackTrace = dbEx.InnerException?.StackTrace,
                        Source = dbEx.InnerException?.Source,
                    }
                }, 400));
            }
            catch (Exception ex)
            {
                _logger.LogError(ex, "Error occured while fetching list expenses");
                return BadRequest(ApiResponse<object>.ErrorResponse("Error Occured", new
                {
                    Message = ex.Message,
                    StackTrace = ex.StackTrace,
                    Source = ex.Source,
                    innerexcption = new
                    {
                        Message = ex.InnerException?.Message,
                        StackTrace = ex.InnerException?.StackTrace,
                        Source = ex.InnerException?.Source,
                    }
                }, 400));
            }
        }

        [HttpGet("details/{id}")]
        public string Get(int id)
        {
            return "value";
        }

        [HttpPost("create")]
        public async Task<IActionResult> CreateExpense1([FromBody] CreateExpensesDto dto)
        {
            var loggedInEmployee = await _userHelper.GetCurrentEmployeeAsync();
            var hasUploadPermission = await _permission.HasPermission(PermissionsMaster.ExpenseUpload, loggedInEmployee.Id);
            var hasProjectPermission = await _permission.HasProjectPermission(loggedInEmployee, dto.ProjectId);
            if (!hasUploadPermission || !hasProjectPermission)
            {
                _logger.LogWarning("Access DENIED for employee {EmployeeId} for uploading expense on project {ProjectId}.", loggedInEmployee.Id, dto.ProjectId);
                return StatusCode(403, ApiResponse<object>.ErrorResponse("Access Denied.", "You do not have permission to Upload expenses for this project", 403));
            }
            var isExpensesTypeExist = await _context.ExpensesTypeMaster.AnyAsync(et => et.Id == dto.ExpensesTypeId);
            if (!isExpensesTypeExist)
            {
                _logger.LogWarning("Expenses type not for ID: {ExpensesTypeId} when creating new expense", dto.ExpensesTypeId);
                return NotFound(ApiResponse<object>.ErrorResponse("Expanses Type not found", "Expanses Type not found", 404));
            }
            var isPaymentModeExist = await _context.PaymentModeMatser.AnyAsync(et => et.Id == dto.PaymentModeId);
            if (!isPaymentModeExist)
            {
                _logger.LogWarning("Payment Mode not for ID: {PaymentModeId} when creating new expense", dto.PaymentModeId);
                return NotFound(ApiResponse<object>.ErrorResponse("Payment Mode not found", "Payment Mode not found", 404));
            }
            var isStatusExist = await _context.ExpensesStatusMaster.AnyAsync(et => et.Id == Draft);
            if (!isStatusExist)
            {
                _logger.LogWarning("Status not for ID: {PaymentModeId} when creating new expense", dto.PaymentModeId);
                return NotFound(ApiResponse<object>.ErrorResponse("Status not found", "Status not found", 404));
            }

            var expense = _mapper.Map<Expenses>(dto);

            expense.CreatedById = loggedInEmployee.Id;
            expense.CreatedAt = DateTime.UtcNow;
            expense.TenantId = tenantId;
            expense.IsActive = true;
            expense.StatusId = Draft;

            _context.Expenses.Add(expense);

            Guid batchId = Guid.NewGuid();
            foreach (var attachment in dto.BillAttachments)
            {
                if (!_s3Service.IsBase64String(attachment.Base64Data))
                {
                    _logger.LogWarning("Image upload failed: Base64 data is missing While creating new expense entity for project {ProjectId} by employee {EmployeeId}", expense.ProjectId, expense.PaidById);
                    return BadRequest(ApiResponse<object>.ErrorResponse("Base64 data is missing", "Base64 data is missing", 400));
                }
                var base64 = attachment.Base64Data!.Contains(',')
                        ? attachment.Base64Data[(attachment.Base64Data.IndexOf(",") + 1)..]
                        : attachment.Base64Data;

                var fileType = _s3Service.GetContentTypeFromBase64(base64);
                var fileName = _s3Service.GenerateFileName(fileType, expense.Id, "Expense");
                var objectKey = $"tenant-{tenantId}/project-{expense.ProjectId}/Expenses/{fileName}";
                try
                {
                    await _s3Service.UploadFileAsync(base64, fileType, objectKey);
                    _logger.LogInfo("Image uploaded to S3 with key: {ObjectKey}", objectKey);
                }
                catch (Exception ex)
                {
                    _logger.LogError(ex, "Error occured while saving image to S3");
                    return BadRequest(ApiResponse<object>.ErrorResponse("Cannot upload attachment to S3", new
                    {
                        message = ex.Message,
                        innerexcption = ex.InnerException?.Message,
                        stackTrace = ex.StackTrace,
                        source = ex.Source
                    }, 400));
                }

                var document = new Document
                {
                    BatchId = batchId,
                    UploadedById = loggedInEmployee.Id,
                    FileName = attachment.FileName ?? "",
                    ContentType = attachment.ContentType ?? "",
                    S3Key = objectKey,
                    FileSize = attachment.FileSize,
                    UploadedAt = DateTime.UtcNow,
                    TenantId = tenantId
                };
                _context.Documents.Add(document);

                var billAttachement = new BillAttachments
                {
                    DocumentId = document.Id,
                    ExpensesId = expense.Id,
                    TenantId = tenantId
                };
                _context.BillAttachments.Add(billAttachement);
            }
            try
            {
                await _context.SaveChangesAsync();
            }
            catch (DbUpdateException dbEx)
            {
                _logger.LogError(dbEx, "Error occured while saving Expense, Document and bill attachment entity");
                return BadRequest(ApiResponse<object>.ErrorResponse("Databsae Exception", new
                {
                    Message = dbEx.Message,
                    StackTrace = dbEx.StackTrace,
                    Source = dbEx.Source,
                    innerexcption = new
                    {
                        Message = dbEx.InnerException?.Message,
                        StackTrace = dbEx.InnerException?.StackTrace,
                        Source = dbEx.InnerException?.Source,
                    }
                }, 400));
            }
            _logger.LogInfo("Documents and attachments saved for Expense: {ExpenseId}", expense.Id);

            return StatusCode(201, ApiResponse<object>.SuccessResponse(expense, "Expense created Successfully", 201));
        }

        /// <summary>
        /// Creates a new expense entry along with its bill attachments.
        /// This operation is transactional and performs validations and file uploads in parallel for optimal performance.
        /// Permission checks are also run in parallel using IServiceScopeFactory to ensure thread safety.
        /// </summary>
        /// <param name="dto">The data transfer object containing expense details and attachments.</param>
        /// <returns>An IActionResult indicating the result of the creation operation.</returns>
        [HttpPost]
        public async Task<IActionResult> CreateExpense([FromBody] CreateExpensesDto dto)
        {
            _logger.LogDebug("CreateExpense for Project {ProjectId}", dto.ProjectId);
            // The entire operation is wrapped in a transaction to ensure data consistency.
            await using var transaction = await _context.Database.BeginTransactionAsync();

            try
            {
                var loggedInEmployee = await _userHelper.GetCurrentEmployeeAsync();

                // 1. Authorization: Run permission checks in parallel using a service scope for each task.
                // This is crucial for thread-safety as IPermissionService is a scoped service.
                var hasUploadPermissionTask = Task.Run(async () =>
                {
                    using var scope = _serviceScopeFactory.CreateScope();
                    var permissionService = scope.ServiceProvider.GetRequiredService<PermissionServices>();
                    return await permissionService.HasPermission(PermissionsMaster.ExpenseUpload, loggedInEmployee.Id);
                });

                var hasProjectPermissionTask = Task.Run(async () =>
                {
                    using var scope = _serviceScopeFactory.CreateScope();
                    var permissionService = scope.ServiceProvider.GetRequiredService<PermissionServices>();
                    return await permissionService.HasProjectPermission(loggedInEmployee, dto.ProjectId);
                });

                await Task.WhenAll(hasUploadPermissionTask, hasProjectPermissionTask);

                if (!hasUploadPermissionTask.Result || !hasProjectPermissionTask.Result)
                {
                    _logger.LogWarning("Access DENIED for employee {EmployeeId} on project {ProjectId}.", loggedInEmployee.Id, dto.ProjectId);
                    return StatusCode(403, ApiResponse<object>.ErrorResponse("Access Denied.", "You do not have permission to upload expenses for this project.", 403));
                }


                // 2. Validation: Check if prerequisite entities exist.
                // The method now returns a tuple indicating success or failure.
                // Each task creates its own DbContext instance from the factory, making the parallel calls thread-safe.
                var projectGetTask = Task.Run(async () =>
                {
                    await using var dbContext = await _dbContextFactory.CreateDbContextAsync();
                    return await dbContext.Projects.FirstOrDefaultAsync(p => p.Id == dto.ProjectId);
                });

                var expenseTypeGetTask = Task.Run(async () =>
                {
                    await using var dbContext = await _dbContextFactory.CreateDbContextAsync();
                    return await dbContext.ExpensesTypeMaster.FirstOrDefaultAsync(et => et.Id == dto.ExpensesTypeId);
                });

                var paymentModeGetTask = Task.Run(async () =>
                {
                    await using var dbContext = await _dbContextFactory.CreateDbContextAsync();
                    return await dbContext.PaymentModeMatser.FirstOrDefaultAsync(pm => pm.Id == dto.PaymentModeId);
                });

                var statusGetTask = Task.Run(async () =>
                {
                    await using var dbContext = await _dbContextFactory.CreateDbContextAsync();
                    return await dbContext.ExpensesStatusMaster.FirstOrDefaultAsync(es => es.Id == Draft);
                });

                await Task.WhenAll(projectGetTask, expenseTypeGetTask, paymentModeGetTask, statusGetTask);

                var project = await projectGetTask;
                var expenseType = await expenseTypeGetTask;
                var paymentMode = await paymentModeGetTask;
                var status = await statusGetTask;

                if (project == null)
                {
                    await transaction.RollbackAsync(); // Ensure transaction is terminated before returning.
                    _logger.LogWarning("Expense creation failed due to validation: Project with ID {ProjectId} not found.", dto.ProjectId);
                    return NotFound(ApiResponse<object>.ErrorResponse("Project not found.", "Project not found.", 404));
                }
                else if (expenseType == null)
                {
                    await transaction.RollbackAsync(); // Ensure transaction is terminated before returning.
                    _logger.LogWarning("Expense creation failed due to validation: Expense Type with ID {ExpensesTypeId} not found.", dto.ExpensesTypeId);
                    return NotFound(ApiResponse<object>.ErrorResponse("Expense Type not found.", "Expense Type not found.", 404));
                }
                else if (paymentMode == null)
                {
                    await transaction.RollbackAsync(); // Ensure transaction is terminated before returning.
                    _logger.LogWarning("Expense creation failed due to validation: Payment Mode with ID {PaymentModeId} not found.", dto.PaymentModeId);
                    return NotFound(ApiResponse<object>.ErrorResponse("Payment Mode not found.", "Payment Mode not found.", 404));
                }
                else if (status == null)
                {
                    await transaction.RollbackAsync(); // Ensure transaction is terminated before returning.
                    _logger.LogWarning("Expense creation failed due to validation: Status with ID {StatusId} not found.", Draft);
                    return NotFound(ApiResponse<object>.ErrorResponse("Status not found.", "Status not found.", 404));
                }

                // 3. Entity Creation
                var expense = _mapper.Map<Expenses>(dto);
                expense.CreatedById = loggedInEmployee.Id;
                expense.CreatedAt = DateTime.UtcNow;
                expense.TenantId = tenantId;
                expense.IsActive = true;
                expense.StatusId = Draft;

                _context.Expenses.Add(expense);

                // 4. Process Attachments
                if (dto.BillAttachments?.Any() ?? false)
                {
                    await ProcessAndUploadAttachmentsAsync(dto.BillAttachments, expense, loggedInEmployee.Id, tenantId);
                }

                // 5. Database Commit
                await _context.SaveChangesAsync();


                // 6. Transaction Commit
                await transaction.CommitAsync();

                var response = _mapper.Map<ExpenseList>(expense);

                response.Project = _mapper.Map<ProjectInfoVM>(project);
                response.Status = _mapper.Map<ExpensesStatusMasterVM>(status);
                response.PaymentMode = _mapper.Map<PaymentModeMatserVM>(paymentMode);
                response.ExpensesType = _mapper.Map<ExpensesTypeMasterVM>(expenseType);

                _logger.LogInfo("Successfully created Expense {ExpenseId} for Project {ProjectId}.", expense.Id, expense.ProjectId);
                return StatusCode(201, ApiResponse<object>.SuccessResponse(response, "Expense created successfully.", 201));
            }
            catch (ArgumentException ex) // For invalid Base64 or other bad arguments.
            {
                await transaction.RollbackAsync();
                _logger.LogError(ex, "Invalid argument provided during expense creation for project {ProjectId}.", dto.ProjectId);
                return BadRequest(ApiResponse<object>.ErrorResponse("Invalid Request Data", new
                {
                    Message = ex.Message,
                    StackTrace = ex.StackTrace,
                    Source = ex.Source,
                    innerexcption = new
                    {
                        Message = ex.InnerException?.Message,
                        StackTrace = ex.InnerException?.StackTrace,
                        Source = ex.InnerException?.Source,
                    }
                }, 400));
            }
            catch (Exception ex) // General-purpose catch for unexpected errors (e.g., S3 failure).
            {
                await transaction.RollbackAsync();
                _logger.LogError(ex, "An unhandled exception occurred while creating an expense for project {ProjectId}.", dto.ProjectId);
                return StatusCode(500, ApiResponse<object>.ErrorResponse("An internal server error occurred.", new
                {
                    Message = ex.Message,
                    StackTrace = ex.StackTrace,
                    Source = ex.Source,
                    innerexcption = new
                    {
                        Message = ex.InnerException?.Message,
                        StackTrace = ex.InnerException?.StackTrace,
                        Source = ex.InnerException?.Source,
                    }
                }, 500));
            }
        }

        /// <summary>
        /// Processes and uploads attachments in parallel, then adds the resulting entities to the main DbContext.
        /// </summary>
        private async Task ProcessAndUploadAttachmentsAsync(IEnumerable<FileUploadModel> attachments, Expenses expense, Guid employeeId, Guid tenantId)
        {
            var batchId = Guid.NewGuid();

            var processingTasks = attachments.Select(attachment => Task.Run(async () =>
            {
                if (string.IsNullOrWhiteSpace(attachment.Base64Data) || !_s3Service.IsBase64String(attachment.Base64Data))
                    throw new ArgumentException("Invalid or missing Base64 data for an attachment.");

                var base64Data = attachment.Base64Data!.Contains(',') ? attachment.Base64Data[(attachment.Base64Data.IndexOf(",") + 1)..] : attachment.Base64Data;
                var fileType = _s3Service.GetContentTypeFromBase64(base64Data);
                var fileName = _s3Service.GenerateFileName(fileType, expense.Id, "Expense");
                var objectKey = $"tenant-{tenantId}/project-{expense.ProjectId}/Expenses/{fileName}";

                // Upload and create entities
                await _s3Service.UploadFileAsync(base64Data, fileType, objectKey);
                _logger.LogInfo("Uploaded file to S3 with key: {ObjectKey}", objectKey);

                return CreateAttachmentEntities(batchId, expense.Id, employeeId, tenantId, objectKey, attachment);
            })).ToList();

            var results = await Task.WhenAll(processingTasks);

            // This part is thread-safe as it runs after all parallel tasks are complete.
            foreach (var (document, billAttachment) in results)
            {
                _context.Documents.Add(document);
                _context.BillAttachments.Add(billAttachment);
            }
            _logger.LogInfo("{AttachmentCount} attachments processed and staged for saving.", results.Length);
        }

        /// <summary>
        /// A private static helper method to create Document and BillAttachment entities.
        /// </summary>
        private static (Document document, BillAttachments billAttachment) CreateAttachmentEntities(
            Guid batchId, Guid expenseId, Guid uploadedById, Guid tenantId, string s3Key, FileUploadModel attachmentDto)
        {
            var document = new Document
            {
                BatchId = batchId,
                UploadedById = uploadedById,
                FileName = attachmentDto.FileName ?? "",
                ContentType = attachmentDto.ContentType ?? "",
                S3Key = s3Key,
                FileSize = attachmentDto.FileSize,
                UploadedAt = DateTime.UtcNow,
                TenantId = tenantId
            };
            var billAttachment = new BillAttachments { Document = document, ExpensesId = expenseId, TenantId = tenantId };
            return (document, billAttachment);
        }

        [HttpPost("action")]
        public async Task<IActionResult> ChangeStatus([FromBody] ExpenseRecordDto model)
        {
            var loggedInEmployee = await _userHelper.GetCurrentEmployeeAsync();
            var exsitingExpenses = await _context.Expenses
                .FirstOrDefaultAsync(e => e.Id == model.ExpenseId && e.TenantId == tenantId);

            if (exsitingExpenses == null)
            {
                return NotFound(ApiResponse<object>.ErrorResponse("Expense not found", "Expense not found", 404));
            }

            exsitingExpenses.StatusId = model.StatusId;

            try
            {
                await _context.SaveChangesAsync();
            }
            catch (DbUpdateConcurrencyException dbEx)
            {
                // --- Step 3: Handle Concurrency Conflicts ---
                // This happens if another user modified the project after we fetched it.
                _logger.LogError(dbEx, "Error occured while update status of expanse.");
                return StatusCode(500, ApiResponse<object>.ErrorResponse("Error occured while update status of expanse.", new
                {
                    Message = dbEx.Message,
                    StackTrace = dbEx.StackTrace,
                    Source = dbEx.Source,
                    innerexcption = new
                    {
                        Message = dbEx.InnerException?.Message,
                        StackTrace = dbEx.InnerException?.StackTrace,
                        Source = dbEx.InnerException?.Source,
                    }
                }, 500));
            }
            var response = _mapper.Map<ExpenseList>(exsitingExpenses);
            return Ok(ApiResponse<object>.SuccessResponse(response));
        }

        [HttpPut("edit/{id}")]
        public async Task<IActionResult> UpdateExpanse(Guid id, [FromBody] UpdateExpensesDto model)
        {
            var loggedInEmployee = await _userHelper.GetCurrentEmployeeAsync();
            var exsitingExpense = await _context.Expenses.FirstOrDefaultAsync(e => e.Id == model.Id && e.TenantId == tenantId);


            if (exsitingExpense == null)
            {
                return NotFound(ApiResponse<object>.ErrorResponse("Expense not found", "Expense not found", 404));
            }
            _mapper.Map(model, exsitingExpense);
            _context.Entry(exsitingExpense).State = EntityState.Modified;

            try
            {
                await _context.SaveChangesAsync();
                _logger.LogInfo("Successfully updated project {ProjectId} by user {UserId}.", id, loggedInEmployee.Id);
            }
            catch (DbUpdateConcurrencyException ex)
            {
                // --- Step 3: Handle Concurrency Conflicts ---
                // This happens if another user modified the project after we fetched it.
                _logger.LogError(ex, "Concurrency conflict while updating project {ProjectId} ", id);
                return StatusCode(409, ApiResponse<object>.ErrorResponse("Conflict occurred.", "This project has been modified by someone else. Please refresh and try again.", 409));
            }
            var response = _mapper.Map<ExpenseList>(exsitingExpense);
            return Ok(ApiResponse<object>.SuccessResponse(response));
        }

        [HttpDelete("delete/{id}")]
        public void Delete(int id)
        {
        }
        #region =================================================================== Helper Functions ===================================================================

        /// <summary>
        /// Deserializes the filter string, handling multiple potential formats (e.g., direct JSON vs. escaped JSON string).
        /// </summary>
        /// <param name="filter">The JSON filter string from the request.</param>
        /// <returns>An <see cref="ExpensesFilter"/> object or null if deserialization fails.</returns>
        private ExpensesFilter? TryDeserializeFilter(string? filter)
        {
            if (string.IsNullOrWhiteSpace(filter))
            {
                return null;
            }

            var options = new JsonSerializerOptions { PropertyNameCaseInsensitive = true };
            ExpensesFilter? expenseFilter = null;

            try
            {
                // First, try to deserialize directly. This is the expected case (e.g., from a web client).
                expenseFilter = JsonSerializer.Deserialize<ExpensesFilter>(filter, options);
            }
            catch (JsonException ex)
            {
                _logger.LogError(ex, "[{MethodName}] Failed to directly deserialize filter. Attempting to unescape and re-parse. Filter: {Filter}", nameof(TryDeserializeFilter), filter);

                // If direct deserialization fails, it might be an escaped string (common with tools like Postman or some mobile clients).
                try
                {
                    // Unescape the string first, then deserialize the result.
                    string unescapedJsonString = JsonSerializer.Deserialize<string>(filter, options) ?? "";
                    if (!string.IsNullOrWhiteSpace(unescapedJsonString))
                    {
                        expenseFilter = JsonSerializer.Deserialize<ExpensesFilter>(unescapedJsonString, options);
                    }
                }
                catch (JsonException ex1)
                {
                    // If both attempts fail, log the final error and return null.
                    _logger.LogError(ex1, "[{MethodName}] All attempts to deserialize the filter failed. Filter will be ignored. Filter: {Filter}", nameof(TryDeserializeFilter), filter);
                    return null;
                }
            }
            return expenseFilter;
        }

        #endregion
    }
}