using BirthList.Domain.Entities;
using BirthList.Infrastructure.Persistence;
using BirthList.Web.Data;
using Microsoft.EntityFrameworkCore;
using System.Security.Cryptography;
using System.Text;
using System.Text.Json;

namespace BirthList.Web.Features.Registries;

internal sealed class RegistryService(RegistryDbContext registryDbContext, ApplicationDbContext applicationDbContext)
{
    private const string DefaultCategoryName = "General";

    public async Task<Guid> CreateRegistryAsync(string userId, RegistryCreateModel model, CancellationToken cancellationToken)
    {
        ArgumentException.ThrowIfNullOrWhiteSpace(userId);
        ArgumentNullException.ThrowIfNull(model);
        if (string.IsNullOrWhiteSpace(model.Title))
        {
            throw new ArgumentException("Title is required.", nameof(model));
        }

        var publicCode = await CreateUniquePublicCodeAsync(cancellationToken).ConfigureAwait(false);

        var registry = new Registry
        {
            Id = Guid.NewGuid(),
            Title = model.Title.Trim(),
            RegistryType = model.RegistryType,
            ThemeKey = string.IsNullOrWhiteSpace(model.ThemeKey) ? "default" : model.ThemeKey.Trim(),
            PublicLinkCode = publicCode,
            CreatedAtUtc = DateTimeOffset.UtcNow,
            CurrencyCode = "EUR"
        };

        registryDbContext.Registries.Add(registry);
        registryDbContext.RegistryAdmins.Add(new RegistryAdmin
        {
            RegistryId = registry.Id,
            UserId = userId,
            AddedAtUtc = DateTimeOffset.UtcNow
        });
        registryDbContext.RegistrySettings.Add(new RegistrySettings
        {
            RegistryId = registry.Id
        });
        registryDbContext.RegistryItemCategories.Add(new RegistryItemCategory
        {
            Id = Guid.NewGuid(),
            RegistryId = registry.Id,
            Name = DefaultCategoryName,
            SortOrder = 0,
            CreatedAtUtc = DateTimeOffset.UtcNow
        });

        await registryDbContext.SaveChangesAsync(cancellationToken).ConfigureAwait(false);
        return registry.Id;
    }

    public async Task<IReadOnlyList<RegistrySummaryViewModel>> GetVisitedRegistriesAsync(string userId, CancellationToken cancellationToken)
    {
        ArgumentException.ThrowIfNullOrWhiteSpace(userId);

        return await registryDbContext.RegistryVisits
            .Where(x => x.UserId == userId)
            .OrderByDescending(x => x.LastVisitedAtUtc)
            .Select(x => new RegistrySummaryViewModel
            {
                Id = x.RegistryId,
                Title = x.Registry.Title,
                PublicLinkCode = x.Registry.PublicLinkCode,
                RegistryType = x.Registry.RegistryType,
                ThemeKey = x.Registry.ThemeKey
            })
            .ToListAsync(cancellationToken)
            .ConfigureAwait(false);
    }

    public async Task<IReadOnlyList<RegistrySummaryViewModel>> GetAdminRegistriesAsync(string userId, CancellationToken cancellationToken)
    {
        ArgumentException.ThrowIfNullOrWhiteSpace(userId);

        return await registryDbContext.RegistryAdmins
            .Where(x => x.UserId == userId)
            .OrderByDescending(x => x.AddedAtUtc)
            .Select(x => new RegistrySummaryViewModel
            {
                Id = x.RegistryId,
                Title = x.Registry.Title,
                PublicLinkCode = x.Registry.PublicLinkCode,
                RegistryType = x.Registry.RegistryType,
                ThemeKey = x.Registry.ThemeKey
            })
            .ToListAsync(cancellationToken)
            .ConfigureAwait(false);
    }

    public async Task<IReadOnlyList<RegistryAdminDisplayModel>> GetRegistryAdminsAsync(Guid registryId, CancellationToken cancellationToken)
    {
        var admins = await registryDbContext.RegistryAdmins
            .Where(x => x.RegistryId == registryId)
            .OrderBy(x => x.AddedAtUtc)
            .Select(x => x.UserId)
            .ToListAsync(cancellationToken)
            .ConfigureAwait(false);

        var users = await applicationDbContext.Users
            .Where(x => admins.Contains(x.Id))
            .Select(x => new { x.Id, x.Email, x.FirstName, x.LastName })
            .ToListAsync(cancellationToken)
            .ConfigureAwait(false);

        var usersById = users.ToDictionary(
            x => x.Id,
            x => BuildUserDisplayName(x.FirstName, x.LastName, x.Email, x.Id));

        return admins
            .Select(userId => new RegistryAdminDisplayModel
            {
                UserId = userId,
                DisplayName = usersById.TryGetValue(userId, out var displayName)
                    ? displayName
                    : userId
            })
            .ToList();
    }

    public async Task<RegistryPublicViewModel?> GetPublicRegistryByCodeAsync(string code, string? userId, CancellationToken cancellationToken)
    {
        if (string.IsNullOrWhiteSpace(code))
        {
            return null;
        }

        var registry = await registryDbContext.Registries
            .Include(x => x.Items)
                .ThenInclude(x => x.Category)
            .Include(x => x.ItemCategories)
            .Include(x => x.Visits)
            .Include(x => x.Admins)
            .FirstOrDefaultAsync(x => x.PublicLinkCode == code.Trim(), cancellationToken)
            .ConfigureAwait(false);

        if (registry is null)
        {
            return null;
        }

        var settings = await registryDbContext.RegistrySettings
            .FirstOrDefaultAsync(x => x.RegistryId == registry.Id, cancellationToken)
            .ConfigureAwait(false);

        if (!string.IsNullOrWhiteSpace(userId))
        {
            var existingVisit = await registryDbContext.RegistryVisits
                .FirstOrDefaultAsync(x => x.RegistryId == registry.Id && x.UserId == userId, cancellationToken)
                .ConfigureAwait(false);

            if (existingVisit is null)
            {
                registryDbContext.RegistryVisits.Add(new RegistryVisit
                {
                    RegistryId = registry.Id,
                    UserId = userId,
                    LastVisitedAtUtc = DateTimeOffset.UtcNow
                });
            }
            else
            {
                existingVisit.LastVisitedAtUtc = DateTimeOffset.UtcNow;
            }

            await registryDbContext.SaveChangesAsync(cancellationToken).ConfigureAwait(false);
        }

        var itemIds = registry.Items.Select(x => x.Id).ToList();
        var userIds = new HashSet<string>();

        var purchases = await registryDbContext.ItemPurchases
            .Where(x => itemIds.Contains(x.RegistryItemId))
            .ToListAsync(cancellationToken)
            .ConfigureAwait(false);

        var contributions = await registryDbContext.ItemContributions
            .Where(x => itemIds.Contains(x.RegistryItemId))
            .ToListAsync(cancellationToken)
            .ConfigureAwait(false);

        foreach (var purchase in purchases)
        {
            userIds.Add(purchase.UserId);
        }
        foreach (var contribution in contributions)
        {
            userIds.Add(contribution.UserId);
        }

        var users = await applicationDbContext.Users
            .Where(x => userIds.Contains(x.Id))
            .Select(x => new { x.Id, x.Email, x.FirstName, x.LastName })
            .ToListAsync(cancellationToken)
            .ConfigureAwait(false);

        var usersById = users.ToDictionary(
            x => x.Id,
            x => BuildUserDisplayName(x.FirstName, x.LastName, x.Email, x.Id));

        var purchasesByItemId = purchases
            .GroupBy(x => x.RegistryItemId)
            .ToDictionary(g => g.Key, g => g.ToList());

        var contributionsByItemId = contributions
            .GroupBy(x => x.RegistryItemId)
            .ToDictionary(g => g.Key, g => g.ToList());

        var isAdmin = !string.IsNullOrWhiteSpace(userId) && registry.Admins.Any(x => x.UserId == userId);

        var mappedItems = registry.Items
            .OrderBy(x => x.Category.SortOrder)
            .ThenBy(x => x.SortOrder)
            .ThenBy(x => x.Name)
            .Select(x => new RegistryPublicItemViewModel
            {
                Id = x.Id,
                CategoryId = x.CategoryId,
                CategoryName = x.Category.Name,
                CategorySortOrder = x.Category.SortOrder,
                SortOrder = x.SortOrder,
                Name = x.Name,
                PictureUrl = x.PictureUrl,
                ProductUrl = x.ProductUrl,
                Description = x.Description,
                PriceAmount = x.PriceAmount,
                CurrencyCode = x.CurrencyCode,
                DesiredQuantity = x.DesiredQuantity,
                PurchasedQuantity = x.PurchasedQuantity,
                ParticipationAllowed = x.ParticipationAllowed,
                ParticipationTargetAmount = x.ParticipationTargetAmount,
                MoneyFulfilledAmount = x.MoneyFulfilledAmount,
                PreferSecondHand = x.PreferSecondHand,
                IsGiven = x.IsGiven,
                CanViewPurchasers = isAdmin || (purchasesByItemId.TryGetValue(x.Id, out var itemPurchases) && itemPurchases.Any(p => p.UserId == userId)),
                Purchasers = purchasesByItemId.TryGetValue(x.Id, out var itemPurchases2)
                    ? itemPurchases2
                        .GroupBy(p => p.UserId)
                        .Select(g => new ItemContributorViewModel
                        {
                            UserId = g.Key,
                            DisplayName = usersById.TryGetValue(g.Key, out var displayName) ? displayName : g.Key,
                            Quantity = g.Sum(p => p.Quantity)
                        })
                        .ToList()
                    : [],
                Contributors = contributionsByItemId.TryGetValue(x.Id, out var itemContributions)
                    ? itemContributions
                        .GroupBy(c => c.UserId)
                        .Select(g => new ItemContributorViewModel
                        {
                            UserId = g.Key,
                            DisplayName = usersById.TryGetValue(g.Key, out var displayName) ? displayName : g.Key,
                            Amount = g.Sum(c => c.Amount)
                        })
                        .ToList()
                    : [],
                CurrentUserPurchasedQuantity = purchasesByItemId.TryGetValue(x.Id, out var itemPurchases3)
                    ? itemPurchases3
                        .Where(p => p.UserId == userId)
                        .Sum(p => p.Quantity)
                    : 0
            })
            .ToList();

        var mappedCategories = registry.ItemCategories
            .OrderBy(x => x.SortOrder)
            .ThenBy(x => x.Name)
            .Select(category => new RegistryPublicCategoryViewModel
            {
                Id = category.Id,
                Name = category.Name,
                SortOrder = category.SortOrder,
                Items = mappedItems
                    .Where(item => item.CategoryId == category.Id)
                    .OrderBy(item => item.SortOrder)
                    .ThenBy(item => item.Name)
                    .ToList()
            })
            .ToList();

        return new RegistryPublicViewModel
        {
            Id = registry.Id,
            Title = registry.Title,
            PublicLinkCode = registry.PublicLinkCode,
            BabyName = registry.BabyName,
            HeaderContentHtml = registry.HeaderContentHtml,
            ShippingAddress = registry.ShippingAddress,
            CurrencyCode = registry.CurrencyCode,
            RegistryType = registry.RegistryType,
            ThemeKey = registry.ThemeKey,
            BankAccountIban = settings?.BankAccountIban,
            BankAccountBic = settings?.BankAccountBic,
            BankAccountDisplayName = settings?.BankAccountDisplayName,
            ShowBankAccountName = settings?.ShowBankAccountName ?? false,
            ContributionQrCodeUrl = settings?.ContributionQrCodeUrl,
            ContributionAmountQrCodes = ParseContributionAmountQrCodes(settings?.ContributionAmountQrCodesJson),
            CurrentUserId = userId,
            IsAdmin = isAdmin,
            Categories = mappedCategories,
            Items = mappedItems
        };
    }

    public async Task<RegistrySettingsEditModel?> GetRegistrySettingsAsync(Guid registryId, CancellationToken cancellationToken)
    {
        var registry = await registryDbContext.Registries
            .FirstOrDefaultAsync(x => x.Id == registryId, cancellationToken)
            .ConfigureAwait(false);

        if (registry is null)
        {
            return null;
        }

        var settings = await registryDbContext.RegistrySettings
            .FirstOrDefaultAsync(x => x.RegistryId == registryId, cancellationToken)
            .ConfigureAwait(false);

        return new RegistrySettingsEditModel
        {
            BabyName = registry.BabyName,
            BirthDate = registry.BirthDate,
            HeaderContentHtml = registry.HeaderContentHtml,
            ShippingAddress = registry.ShippingAddress,
            CurrencyCode = registry.CurrencyCode,
            ThemeKey = registry.ThemeKey,
            BankAccountIban = settings?.BankAccountIban,
            BankAccountBic = settings?.BankAccountBic,
            BankAccountDisplayName = settings?.BankAccountDisplayName,
            ShowBankAccountName = settings?.ShowBankAccountName ?? false,
            ContributionQrCodeUrl = settings?.ContributionQrCodeUrl,
            ContributionAmountQrCodes = ParseContributionAmountQrCodes(settings?.ContributionAmountQrCodesJson).ToList()
        };
    }

    public async Task UpdateRegistrySettingsAsync(Guid registryId, RegistrySettingsEditModel model, CancellationToken cancellationToken)
    {
        ArgumentNullException.ThrowIfNull(model);

        var registry = await registryDbContext.Registries
            .FirstOrDefaultAsync(x => x.Id == registryId, cancellationToken)
            .ConfigureAwait(false) ?? throw new InvalidOperationException("Registry not found.");

        var settings = await registryDbContext.RegistrySettings
            .FirstOrDefaultAsync(x => x.RegistryId == registryId, cancellationToken)
            .ConfigureAwait(false);

        if (settings is null)
        {
            settings = new RegistrySettings
            {
                RegistryId = registryId
            };
            registryDbContext.RegistrySettings.Add(settings);
        }

        registry.BabyName = string.IsNullOrWhiteSpace(model.BabyName) ? null : model.BabyName.Trim();
        registry.BirthDate = model.BirthDate;
        registry.HeaderContentHtml = string.IsNullOrWhiteSpace(model.HeaderContentHtml) ? null : model.HeaderContentHtml;
        registry.ShippingAddress = string.IsNullOrWhiteSpace(model.ShippingAddress) ? null : model.ShippingAddress.Trim();
        registry.CurrencyCode = string.IsNullOrWhiteSpace(model.CurrencyCode) ? "EUR" : model.CurrencyCode.Trim().ToUpperInvariant();
        registry.ThemeKey = string.IsNullOrWhiteSpace(model.ThemeKey) ? "default" : model.ThemeKey.Trim();

        settings.BankAccountIban = string.IsNullOrWhiteSpace(model.BankAccountIban) ? null : model.BankAccountIban.Trim();
        settings.BankAccountBic = string.IsNullOrWhiteSpace(model.BankAccountBic) ? null : model.BankAccountBic.Trim();
        settings.BankAccountDisplayName = string.IsNullOrWhiteSpace(model.BankAccountDisplayName) ? null : model.BankAccountDisplayName.Trim();
        settings.ShowBankAccountName = model.ShowBankAccountName;
        settings.ContributionQrCodeUrl = string.IsNullOrWhiteSpace(model.ContributionQrCodeUrl) ? null : model.ContributionQrCodeUrl.Trim();
        settings.ContributionAmountQrCodesJson = SerializeContributionAmountQrCodes(model.ContributionAmountQrCodes);

        await registryDbContext.SaveChangesAsync(cancellationToken).ConfigureAwait(false);
    }

    private static IReadOnlyList<ContributionAmountQrCodeModel> ParseContributionAmountQrCodes(string? json)
    {
        if (string.IsNullOrWhiteSpace(json))
        {
            return [];
        }

        try
        {
            var items = JsonSerializer.Deserialize<List<ContributionAmountQrCodeModel>>(json);
            if (items is null)
            {
                return [];
            }

            return items
                .Where(x => x.Amount > 0 && !string.IsNullOrWhiteSpace(x.QrCodeUrl))
                .Select(x => new ContributionAmountQrCodeModel
                {
                    Amount = x.Amount,
                    QrCodeUrl = x.QrCodeUrl.Trim()
                })
                .OrderBy(x => x.Amount)
                .ToList();
        }
        catch (JsonException)
        {
            return [];
        }
    }

    private static string? SerializeContributionAmountQrCodes(IEnumerable<ContributionAmountQrCodeModel>? amountQrCodes)
    {
        if (amountQrCodes is null)
        {
            return null;
        }

        var normalized = amountQrCodes
            .Where(x => x.Amount > 0 && !string.IsNullOrWhiteSpace(x.QrCodeUrl))
            .Select(x => new ContributionAmountQrCodeModel
            {
                Amount = x.Amount,
                QrCodeUrl = x.QrCodeUrl.Trim()
            })
            .OrderBy(x => x.Amount)
            .ToList();

        if (normalized.Count == 0)
        {
            return null;
        }

        return JsonSerializer.Serialize(normalized);
    }

    public async Task<IReadOnlyList<RegistryItemEditModel>> GetRegistryItemsAsync(Guid registryId, CancellationToken cancellationToken)
    {
        var defaultCategory = await EnsureDefaultCategoryAsync(registryId, cancellationToken).ConfigureAwait(false);

        var items = await registryDbContext.RegistryItems
            .Where(x => x.RegistryId == registryId)
            .Include(x => x.Category)
            .OrderBy(x => x.Category.SortOrder)
            .ThenBy(x => x.SortOrder)
            .ThenBy(x => x.Name)
            .ToListAsync(cancellationToken)
            .ConfigureAwait(false);

        foreach (var item in items.Where(x => x.CategoryId == Guid.Empty))
        {
            item.CategoryId = defaultCategory.Id;
        }

        var itemIds = items.Select(x => x.Id).ToList();
        var userIds = new HashSet<string>();

        var purchases = await registryDbContext.ItemPurchases
            .Where(x => itemIds.Contains(x.RegistryItemId))
            .ToListAsync(cancellationToken)
            .ConfigureAwait(false);

        var contributions = await registryDbContext.ItemContributions
            .Where(x => itemIds.Contains(x.RegistryItemId))
            .ToListAsync(cancellationToken)
            .ConfigureAwait(false);

        foreach (var purchase in purchases)
        {
            userIds.Add(purchase.UserId);
        }
        foreach (var contribution in contributions)
        {
            userIds.Add(contribution.UserId);
        }

        var users = await applicationDbContext.Users
            .Where(x => userIds.Contains(x.Id))
            .Select(x => new { x.Id, x.Email, x.FirstName, x.LastName })
            .ToListAsync(cancellationToken)
            .ConfigureAwait(false);

        var usersById = users.ToDictionary(
            x => x.Id,
            x => BuildUserDisplayName(x.FirstName, x.LastName, x.Email, x.Id));

        var purchasesByItemId = purchases
            .GroupBy(x => x.RegistryItemId)
            .ToDictionary(g => g.Key, g => g.ToList());

        var contributionsByItemId = contributions
            .GroupBy(x => x.RegistryItemId)
            .ToDictionary(g => g.Key, g => g.ToList());

        return items
            .Select(x => new RegistryItemEditModel
            {
                Id = x.Id,
                CategoryId = x.CategoryId,
                Name = x.Name,
                PictureUrl = x.PictureUrl,
                ProductUrl = x.ProductUrl,
                Description = x.Description,
                PriceAmount = x.PriceAmount,
                CurrencyCode = x.CurrencyCode,
                DesiredQuantity = x.DesiredQuantity,
                ParticipationAllowed = x.ParticipationAllowed,
                ParticipationTargetAmount = x.ParticipationTargetAmount,
                PreferSecondHand = x.PreferSecondHand,
                IsGiven = x.IsGiven,
                Purchasers = purchasesByItemId.TryGetValue(x.Id, out var itemPurchases)
                    ? itemPurchases
                        .GroupBy(p => p.UserId)
                        .Select(g => new ItemContributorViewModel
                        {
                            UserId = g.Key,
                            DisplayName = usersById.TryGetValue(g.Key, out var displayName) ? displayName : g.Key,
                            Quantity = g.Sum(p => p.Quantity)
                        })
                        .ToList()
                    : [],
                Contributors = contributionsByItemId.TryGetValue(x.Id, out var itemContributions)
                    ? itemContributions
                        .GroupBy(c => c.UserId)
                        .Select(g => new ItemContributorViewModel
                        {
                            UserId = g.Key,
                            DisplayName = usersById.TryGetValue(g.Key, out var displayName) ? displayName : g.Key,
                            Amount = g.Sum(c => c.Amount)
                        })
                        .ToList()
                    : []
            })
            .ToList();
    }

    public async Task<RegistryItemEditModel?> GetRegistryItemAsync(Guid registryId, Guid itemId, CancellationToken cancellationToken)
    {
        var item = await registryDbContext.RegistryItems
            .FirstOrDefaultAsync(x => x.RegistryId == registryId && x.Id == itemId, cancellationToken)
            .ConfigureAwait(false);

        if (item is null)
        {
            return null;
        }

        var purchases = await registryDbContext.ItemPurchases
            .Where(x => x.RegistryItemId == itemId)
            .ToListAsync(cancellationToken)
            .ConfigureAwait(false);

        var contributions = await registryDbContext.ItemContributions
            .Where(x => x.RegistryItemId == itemId)
            .ToListAsync(cancellationToken)
            .ConfigureAwait(false);

        var userIds = new HashSet<string>();
        foreach (var purchase in purchases)
        {
            userIds.Add(purchase.UserId);
        }
        foreach (var contribution in contributions)
        {
            userIds.Add(contribution.UserId);
        }

        var users = await applicationDbContext.Users
            .Where(x => userIds.Contains(x.Id))
            .Select(x => new { x.Id, x.Email, x.FirstName, x.LastName })
            .ToListAsync(cancellationToken)
            .ConfigureAwait(false);

        var usersById = users.ToDictionary(
            x => x.Id,
            x => BuildUserDisplayName(x.FirstName, x.LastName, x.Email, x.Id));

        return new RegistryItemEditModel
        {
            Id = item.Id,
            CategoryId = item.CategoryId,
            Name = item.Name,
            PictureUrl = item.PictureUrl,
            ProductUrl = item.ProductUrl,
            Description = item.Description,
            PriceAmount = item.PriceAmount,
            CurrencyCode = item.CurrencyCode,
            DesiredQuantity = item.DesiredQuantity,
            ParticipationAllowed = item.ParticipationAllowed,
            ParticipationTargetAmount = item.ParticipationTargetAmount,
            PreferSecondHand = item.PreferSecondHand,
            IsGiven = item.IsGiven,
            Purchasers = purchases
                .GroupBy(p => p.UserId)
                .Select(g => new ItemContributorViewModel
                {
                    UserId = g.Key,
                    DisplayName = usersById.TryGetValue(g.Key, out var displayName) ? displayName : g.Key,
                    Quantity = g.Sum(p => p.Quantity)
                })
                .ToList(),
            Contributors = contributions
                .GroupBy(c => c.UserId)
                .Select(g => new ItemContributorViewModel
                {
                    UserId = g.Key,
                    DisplayName = usersById.TryGetValue(g.Key, out var displayName) ? displayName : g.Key,
                    Amount = g.Sum(c => c.Amount)
                })
                .ToList()
        };
    }

    public async Task UpsertRegistryItemAsync(Guid registryId, RegistryItemEditModel model, CancellationToken cancellationToken)
    {
        ArgumentNullException.ThrowIfNull(model);
        if (string.IsNullOrWhiteSpace(model.Name))
        {
            throw new ArgumentException("Item name is required.", nameof(model));
        }

        var defaultCategory = await EnsureDefaultCategoryAsync(registryId, cancellationToken).ConfigureAwait(false);

        RegistryItem entity;
        if (model.Id is { } itemId)
        {
            entity = await registryDbContext.RegistryItems
                .FirstOrDefaultAsync(x => x.RegistryId == registryId && x.Id == itemId, cancellationToken)
                .ConfigureAwait(false) ?? throw new InvalidOperationException("Item not found.");
        }
        else
        {
            var categoryId = model.CategoryId ?? defaultCategory.Id;
            var nextSortOrder = await registryDbContext.RegistryItems
                .Where(x => x.RegistryId == registryId && x.CategoryId == categoryId)
                .Select(x => (int?)x.SortOrder)
                .MaxAsync(cancellationToken)
                .ConfigureAwait(false);

            entity = new RegistryItem
            {
                Id = Guid.NewGuid(),
                RegistryId = registryId,
                CategoryId = categoryId,
                SortOrder = (nextSortOrder ?? -1) + 1,
                CreatedAtUtc = DateTimeOffset.UtcNow
            };
            registryDbContext.RegistryItems.Add(entity);
        }

        if (model.CategoryId.HasValue && model.CategoryId.Value != entity.CategoryId)
        {
            var categoryExists = await registryDbContext.RegistryItemCategories
                .AnyAsync(x => x.RegistryId == registryId && x.Id == model.CategoryId.Value, cancellationToken)
                .ConfigureAwait(false);

            if (!categoryExists)
            {
                throw new InvalidOperationException("Category not found.");
            }

            var nextSortOrder = await registryDbContext.RegistryItems
                .Where(x => x.RegistryId == registryId && x.CategoryId == model.CategoryId.Value && x.Id != entity.Id)
                .Select(x => (int?)x.SortOrder)
                .MaxAsync(cancellationToken)
                .ConfigureAwait(false);

            entity.CategoryId = model.CategoryId.Value;
            entity.SortOrder = (nextSortOrder ?? -1) + 1;
        }

        entity.Name = model.Name.Trim();
        entity.PictureUrl = string.IsNullOrWhiteSpace(model.PictureUrl) ? null : model.PictureUrl.Trim();
        entity.ProductUrl = string.IsNullOrWhiteSpace(model.ProductUrl) ? null : model.ProductUrl.Trim();
        entity.Description = string.IsNullOrWhiteSpace(model.Description) ? null : model.Description.Trim();
        entity.PriceAmount = model.PriceAmount;
        entity.CurrencyCode = string.IsNullOrWhiteSpace(model.CurrencyCode) ? "EUR" : model.CurrencyCode.Trim().ToUpperInvariant();
        entity.DesiredQuantity = model.DesiredQuantity < 1 ? 1 : model.DesiredQuantity;
        entity.ParticipationAllowed = model.ParticipationAllowed;
        entity.ParticipationTargetAmount = model.ParticipationAllowed ? model.ParticipationTargetAmount : null;
        entity.PreferSecondHand = model.PreferSecondHand;
        entity.IsGiven = model.IsGiven;

        await registryDbContext.SaveChangesAsync(cancellationToken).ConfigureAwait(false);
    }

    public async Task DeleteRegistryItemAsync(Guid registryId, Guid itemId, CancellationToken cancellationToken)
    {
        var entity = await registryDbContext.RegistryItems
            .FirstOrDefaultAsync(x => x.RegistryId == registryId && x.Id == itemId, cancellationToken)
            .ConfigureAwait(false);

        if (entity is null)
        {
            return;
        }

        registryDbContext.RegistryItems.Remove(entity);
        await registryDbContext.SaveChangesAsync(cancellationToken).ConfigureAwait(false);
    }

    public async Task AddPurchaseAsync(Guid itemId, string userId, int quantity, CancellationToken cancellationToken)
    {
        ArgumentException.ThrowIfNullOrWhiteSpace(userId);

        var item = await registryDbContext.RegistryItems
            .FirstOrDefaultAsync(x => x.Id == itemId, cancellationToken)
            .ConfigureAwait(false) ?? throw new InvalidOperationException("Item not found.");

        var normalizedQuantity = quantity < 1 ? 1 : quantity;

        registryDbContext.ItemPurchases.Add(new ItemPurchase
        {
            Id = Guid.NewGuid(),
            RegistryItemId = itemId,
            UserId = userId,
            Quantity = normalizedQuantity,
            PurchasedAtUtc = DateTimeOffset.UtcNow
        });

        item.PurchasedQuantity += normalizedQuantity;
        item.IsGiven = item.PurchasedQuantity >= item.DesiredQuantity;

        registryDbContext.UserActionLogs.Add(new UserActionLog
        {
            Id = Guid.NewGuid(),
            RegistryId = item.RegistryId,
            UserId = userId,
            RegistryItemId = itemId,
            ActionType = UserActionType.MarkPurchased,
            Quantity = normalizedQuantity,
            CreatedAtUtc = DateTimeOffset.UtcNow
        });

        await registryDbContext.SaveChangesAsync(cancellationToken).ConfigureAwait(false);
    }

    public async Task AddContributionAsync(Guid itemId, string userId, decimal amount, string transferMessage, CancellationToken cancellationToken)
    {
        ArgumentException.ThrowIfNullOrWhiteSpace(userId);
        if (amount <= 0)
        {
            throw new ArgumentException("Amount must be positive.", nameof(amount));
        }

        var item = await registryDbContext.RegistryItems
            .FirstOrDefaultAsync(x => x.Id == itemId, cancellationToken)
            .ConfigureAwait(false) ?? throw new InvalidOperationException("Item not found.");

        registryDbContext.ItemContributions.Add(new ItemContribution
        {
            Id = Guid.NewGuid(),
            RegistryItemId = itemId,
            UserId = userId,
            Amount = amount,
            CurrencyCode = item.CurrencyCode,
            TransferMessage = transferMessage,
            ContributedAtUtc = DateTimeOffset.UtcNow
        });

        item.MoneyFulfilledAmount += amount;

        registryDbContext.UserActionLogs.Add(new UserActionLog
        {
            Id = Guid.NewGuid(),
            RegistryId = item.RegistryId,
            UserId = userId,
            RegistryItemId = itemId,
            ActionType = UserActionType.LogContribution,
            Amount = amount,
            Details = transferMessage,
            CreatedAtUtc = DateTimeOffset.UtcNow
        });

        await registryDbContext.SaveChangesAsync(cancellationToken).ConfigureAwait(false);
    }

    public async Task UnmarkPurchaseAsync(Guid itemId, string userId, CancellationToken cancellationToken)
    {
        ArgumentException.ThrowIfNullOrWhiteSpace(userId);

        var item = await registryDbContext.RegistryItems
            .FirstOrDefaultAsync(x => x.Id == itemId, cancellationToken)
            .ConfigureAwait(false) ?? throw new InvalidOperationException("Item not found.");

        var purchase = await registryDbContext.ItemPurchases
            .FirstOrDefaultAsync(x => x.RegistryItemId == itemId && x.UserId == userId, cancellationToken)
            .ConfigureAwait(false);

        if (purchase is null)
        {
            return;
        }

        item.PurchasedQuantity -= purchase.Quantity;
        item.IsGiven = item.PurchasedQuantity >= item.DesiredQuantity;

        registryDbContext.ItemPurchases.Remove(purchase);

        registryDbContext.UserActionLogs.Add(new UserActionLog
        {
            Id = Guid.NewGuid(),
            RegistryId = item.RegistryId,
            UserId = userId,
            RegistryItemId = itemId,
            ActionType = UserActionType.UnmarkPurchased,
            Quantity = purchase.Quantity,
            CreatedAtUtc = DateTimeOffset.UtcNow
        });

        await registryDbContext.SaveChangesAsync(cancellationToken).ConfigureAwait(false);
    }

    public async Task UnmarkPurchaseByAdminAsync(Guid itemId, string purchaserUserId, string adminUserId, CancellationToken cancellationToken)
    {
        ArgumentException.ThrowIfNullOrWhiteSpace(purchaserUserId);
        ArgumentException.ThrowIfNullOrWhiteSpace(adminUserId);

        var item = await registryDbContext.RegistryItems
            .FirstOrDefaultAsync(x => x.Id == itemId, cancellationToken)
            .ConfigureAwait(false) ?? throw new InvalidOperationException("Item not found.");

        var purchase = await registryDbContext.ItemPurchases
            .FirstOrDefaultAsync(x => x.RegistryItemId == itemId && x.UserId == purchaserUserId, cancellationToken)
            .ConfigureAwait(false);

        if (purchase is null)
        {
            return;
        }

        item.PurchasedQuantity -= purchase.Quantity;
        item.IsGiven = item.PurchasedQuantity >= item.DesiredQuantity;

        registryDbContext.ItemPurchases.Remove(purchase);

        registryDbContext.UserActionLogs.Add(new UserActionLog
        {
            Id = Guid.NewGuid(),
            RegistryId = item.RegistryId,
            UserId = adminUserId,
            RegistryItemId = itemId,
            ActionType = UserActionType.UnmarkPurchased,
            Quantity = purchase.Quantity,
            Details = $"Unmarked purchase by {purchaserUserId}",
            CreatedAtUtc = DateTimeOffset.UtcNow
        });

        await registryDbContext.SaveChangesAsync(cancellationToken).ConfigureAwait(false);
    }

    public async Task<IReadOnlyList<RegistryActionLogViewModel>> GetRegistryActionLogsAsync(Guid registryId, CancellationToken cancellationToken)
    {
        var actionLogs = await registryDbContext.UserActionLogs
            .Where(x => x.RegistryId == registryId)
            .OrderByDescending(x => x.CreatedAtUtc)
            .ToListAsync(cancellationToken)
            .ConfigureAwait(false);

        var userIds = actionLogs.Select(x => x.UserId).Distinct().ToList();
        var usersById = await applicationDbContext.Users
            .Where(x => userIds.Contains(x.Id))
            .Select(x => new { x.Id, x.Email })
            .ToDictionaryAsync(x => x.Id, x => x.Email ?? x.Id, cancellationToken)
            .ConfigureAwait(false);

        var itemIds = actionLogs
            .Where(x => x.RegistryItemId.HasValue)
            .Select(x => x.RegistryItemId!.Value)
            .Distinct()
            .ToList();

        var itemsById = await registryDbContext.RegistryItems
            .Where(x => itemIds.Contains(x.Id))
            .Select(x => new { x.Id, x.Name })
            .ToDictionaryAsync(x => x.Id, x => x.Name, cancellationToken)
            .ConfigureAwait(false);

        return actionLogs
            .Select(log => new RegistryActionLogViewModel
            {
                Id = log.Id,
                UserDisplayName = usersById.TryGetValue(log.UserId, out var displayName) ? displayName : log.UserId,
                ActionType = log.ActionType.ToString(),
                ItemName = log.RegistryItemId.HasValue && itemsById.TryGetValue(log.RegistryItemId.Value, out var itemName) ? itemName : null,
                Quantity = log.Quantity,
                Amount = log.Amount,
                Details = log.Details,
                CreatedAtUtc = log.CreatedAtUtc
            })
            .ToList();
    }

    public async Task LogUserActionAsync(Guid registryId, string userId, UserActionType actionType, Guid? itemId, string? details, CancellationToken cancellationToken)
    {
        ArgumentException.ThrowIfNullOrWhiteSpace(userId);

        registryDbContext.UserActionLogs.Add(new UserActionLog
        {
            Id = Guid.NewGuid(),
            RegistryId = registryId,
            UserId = userId,
            RegistryItemId = itemId,
            ActionType = actionType,
            Details = details,
            CreatedAtUtc = DateTimeOffset.UtcNow
        });

        await registryDbContext.SaveChangesAsync(cancellationToken).ConfigureAwait(false);
    }

    public async Task<Guid> CreateAdminInviteAsync(Guid registryId, string? email, TimeSpan validFor, CancellationToken cancellationToken)
    {
        var invite = new RegistryAdminInvite
        {
            Id = Guid.NewGuid(),
            RegistryId = registryId,
            Token = Convert.ToBase64String(RandomNumberGenerator.GetBytes(36)).Replace('+', '-').Replace('/', '_').TrimEnd('='),
            SentToEmail = string.IsNullOrWhiteSpace(email) ? null : email.Trim(),
            ExpiresAtUtc = DateTimeOffset.UtcNow.Add(validFor)
        };

        registryDbContext.RegistryAdminInvites.Add(invite);
        await registryDbContext.SaveChangesAsync(cancellationToken).ConfigureAwait(false);
        return invite.Id;
    }

    public async Task<string?> GetInviteTokenAsync(Guid inviteId, CancellationToken cancellationToken)
    {
        return await registryDbContext.RegistryAdminInvites
            .Where(x => x.Id == inviteId)
            .Select(x => x.Token)
            .FirstOrDefaultAsync(cancellationToken)
            .ConfigureAwait(false);
    }

    public async Task<bool> RedeemAdminInviteAsync(string token, string userId, CancellationToken cancellationToken)
    {
        ArgumentException.ThrowIfNullOrWhiteSpace(token);
        ArgumentException.ThrowIfNullOrWhiteSpace(userId);

        var invite = await registryDbContext.RegistryAdminInvites
            .FirstOrDefaultAsync(x => x.Token == token, cancellationToken)
            .ConfigureAwait(false);

        if (invite is null || invite.RedeemedAtUtc.HasValue || invite.ExpiresAtUtc < DateTimeOffset.UtcNow)
        {
            return false;
        }

        var isAdmin = await registryDbContext.RegistryAdmins
            .AnyAsync(x => x.RegistryId == invite.RegistryId && x.UserId == userId, cancellationToken)
            .ConfigureAwait(false);

        if (!isAdmin)
        {
            registryDbContext.RegistryAdmins.Add(new RegistryAdmin
            {
                RegistryId = invite.RegistryId,
                UserId = userId,
                AddedAtUtc = DateTimeOffset.UtcNow
            });
        }

        invite.RedeemedAtUtc = DateTimeOffset.UtcNow;
        await registryDbContext.SaveChangesAsync(cancellationToken).ConfigureAwait(false);
        return true;
    }

    public async Task RemoveRegistryAdminAsync(Guid registryId, string userId, CancellationToken cancellationToken)
    {
        ArgumentException.ThrowIfNullOrWhiteSpace(userId);

        var admin = await registryDbContext.RegistryAdmins
            .FirstOrDefaultAsync(x => x.RegistryId == registryId && x.UserId == userId, cancellationToken)
            .ConfigureAwait(false);

        if (admin is not null)
        {
            registryDbContext.RegistryAdmins.Remove(admin);
            await registryDbContext.SaveChangesAsync(cancellationToken).ConfigureAwait(false);
        }
    }

    private async Task<string> CreateUniquePublicCodeAsync(CancellationToken cancellationToken)
    {
        for (var attempt = 0; attempt < 10; attempt++)
        {
            var codeBytes = RandomNumberGenerator.GetBytes(6);
            var code = Convert.ToHexString(codeBytes).ToLowerInvariant();
            var exists = await registryDbContext.Registries
                .AnyAsync(x => x.PublicLinkCode == code, cancellationToken)
                .ConfigureAwait(false);

            if (!exists)
            {
                return code;
            }
        }

        var fallback = Convert.ToHexString(SHA256.HashData(Encoding.UTF8.GetBytes(Guid.NewGuid().ToString("N")))).Substring(0, 16).ToLowerInvariant();
        return fallback;
    }

    private static string BuildUserDisplayName(string? firstName, string? lastName, string? email, string userId)
    {
        var fullName = $"{firstName} {lastName}".Trim();
        if (!string.IsNullOrWhiteSpace(fullName))
        {
            return fullName;
        }

        if (!string.IsNullOrWhiteSpace(email))
        {
            return email;
        }

        return userId;
    }

    public async Task<IReadOnlyList<RegistryAccessibleUserAddressViewModel>> GetRegistryAccessibleUserAddressesAsync(Guid registryId, CancellationToken cancellationToken)
    {
        var userIds = new HashSet<string>(StringComparer.Ordinal);

        var adminIds = await registryDbContext.RegistryAdmins
            .Where(x => x.RegistryId == registryId)
            .Select(x => x.UserId)
            .ToListAsync(cancellationToken)
            .ConfigureAwait(false);

        foreach (var adminId in adminIds)
        {
            userIds.Add(adminId);
        }

        var visitorIds = await registryDbContext.RegistryVisits
            .Where(x => x.RegistryId == registryId)
            .Select(x => x.UserId)
            .ToListAsync(cancellationToken)
            .ConfigureAwait(false);

        foreach (var visitorId in visitorIds)
        {
            userIds.Add(visitorId);
        }

        var itemIds = await registryDbContext.RegistryItems
            .Where(x => x.RegistryId == registryId)
            .Select(x => x.Id)
            .ToListAsync(cancellationToken)
            .ConfigureAwait(false);

        if (itemIds.Count > 0)
        {
            var purchaserIds = await registryDbContext.ItemPurchases
                .Where(x => itemIds.Contains(x.RegistryItemId))
                .Select(x => x.UserId)
                .Distinct()
                .ToListAsync(cancellationToken)
                .ConfigureAwait(false);

            foreach (var purchaserId in purchaserIds)
            {
                userIds.Add(purchaserId);
            }

            var contributorIds = await registryDbContext.ItemContributions
                .Where(x => itemIds.Contains(x.RegistryItemId))
                .Select(x => x.UserId)
                .Distinct()
                .ToListAsync(cancellationToken)
                .ConfigureAwait(false);

            foreach (var contributorId in contributorIds)
            {
                userIds.Add(contributorId);
            }
        }

        if (userIds.Count == 0)
        {
            return [];
        }

        var users = await applicationDbContext.Users
            .Where(x => userIds.Contains(x.Id))
            .Select(x => new
            {
                x.Id,
                x.Email,
                x.FirstName,
                x.LastName,
                x.Address
            })
            .ToListAsync(cancellationToken)
            .ConfigureAwait(false);

        return users
            .Select(x => new RegistryAccessibleUserAddressViewModel
            {
                UserId = x.Id,
                DisplayName = BuildUserDisplayName(x.FirstName, x.LastName, x.Email, x.Id),
                Email = x.Email,
                Address = string.IsNullOrWhiteSpace(x.Address) ? null : x.Address.Trim()
            })
            .OrderBy(x => x.DisplayName)
            .ToList();
    }

    public async Task<IReadOnlyList<RegistryItemCategoryEditModel>> GetRegistryItemCategoriesAsync(Guid registryId, CancellationToken cancellationToken)
    {
        var categories = await registryDbContext.RegistryItemCategories
            .Where(x => x.RegistryId == registryId)
            .OrderBy(x => x.SortOrder)
            .ThenBy(x => x.Name)
            .Select(x => new RegistryItemCategoryEditModel
            {
                Id = x.Id,
                Name = x.Name,
                SortOrder = x.SortOrder
            })
            .ToListAsync(cancellationToken)
            .ConfigureAwait(false);

        if (categories.Count > 0)
        {
            return categories;
        }

        var createdCategory = await EnsureDefaultCategoryAsync(registryId, cancellationToken).ConfigureAwait(false);

        return
        [
            new RegistryItemCategoryEditModel
            {
                Id = createdCategory.Id,
                Name = createdCategory.Name,
                SortOrder = createdCategory.SortOrder,
                Items = []
            }
        ];
    }

    public async Task AddRegistryItemCategoryAsync(Guid registryId, string categoryName, CancellationToken cancellationToken)
    {
        if (string.IsNullOrWhiteSpace(categoryName))
        {
            throw new ArgumentException("Category name is required.", nameof(categoryName));
        }

        var normalizedName = categoryName.Trim();

        var exists = await registryDbContext.RegistryItemCategories
            .AnyAsync(x => x.RegistryId == registryId && x.Name == normalizedName, cancellationToken)
            .ConfigureAwait(false);

        if (exists)
        {
            throw new InvalidOperationException("A category with this name already exists.");
        }

        var sortOrder = await registryDbContext.RegistryItemCategories
            .Where(x => x.RegistryId == registryId)
            .Select(x => (int?)x.SortOrder)
            .MaxAsync(cancellationToken)
            .ConfigureAwait(false);

        registryDbContext.RegistryItemCategories.Add(new RegistryItemCategory
        {
            Id = Guid.NewGuid(),
            RegistryId = registryId,
            Name = normalizedName,
            SortOrder = (sortOrder ?? -1) + 1,
            CreatedAtUtc = DateTimeOffset.UtcNow
        });

        await registryDbContext.SaveChangesAsync(cancellationToken).ConfigureAwait(false);
    }

    public async Task RenameRegistryItemCategoryAsync(Guid registryId, Guid categoryId, string categoryName, CancellationToken cancellationToken)
    {
        if (string.IsNullOrWhiteSpace(categoryName))
        {
            throw new ArgumentException("Category name is required.", nameof(categoryName));
        }

        var normalizedName = categoryName.Trim();

        var category = await registryDbContext.RegistryItemCategories
            .FirstOrDefaultAsync(x => x.RegistryId == registryId && x.Id == categoryId, cancellationToken)
            .ConfigureAwait(false) ?? throw new InvalidOperationException("Category not found.");

        var exists = await registryDbContext.RegistryItemCategories
            .AnyAsync(x => x.RegistryId == registryId && x.Id != categoryId && x.Name == normalizedName, cancellationToken)
            .ConfigureAwait(false);

        if (exists)
        {
            throw new InvalidOperationException("A category with this name already exists.");
        }

        category.Name = normalizedName;
        await registryDbContext.SaveChangesAsync(cancellationToken).ConfigureAwait(false);
    }

    public async Task RemoveRegistryItemCategoryAsync(Guid registryId, Guid categoryId, CancellationToken cancellationToken)
    {
        var categoryCount = await registryDbContext.RegistryItemCategories
            .Where(x => x.RegistryId == registryId)
            .CountAsync(cancellationToken)
            .ConfigureAwait(false);

        if (categoryCount <= 1)
        {
            throw new InvalidOperationException("At least one category is required.");
        }

        var category = await registryDbContext.RegistryItemCategories
            .FirstOrDefaultAsync(x => x.RegistryId == registryId && x.Id == categoryId, cancellationToken)
            .ConfigureAwait(false);

        if (category is null)
        {
            return;
        }

        var fallbackCategory = await registryDbContext.RegistryItemCategories
            .Where(x => x.RegistryId == registryId && x.Id != categoryId)
            .OrderBy(x => x.SortOrder)
            .FirstOrDefaultAsync(cancellationToken)
            .ConfigureAwait(false);

        if (fallbackCategory is null)
        {
            throw new InvalidOperationException("At least one category is required.");
        }

        var categoryItems = await registryDbContext.RegistryItems
            .Where(x => x.RegistryId == registryId && x.CategoryId == categoryId)
            .OrderBy(x => x.SortOrder)
            .ToListAsync(cancellationToken)
            .ConfigureAwait(false);

        var destinationSortOrder = await registryDbContext.RegistryItems
            .Where(x => x.RegistryId == registryId && x.CategoryId == fallbackCategory.Id)
            .Select(x => (int?)x.SortOrder)
            .MaxAsync(cancellationToken)
            .ConfigureAwait(false);

        var nextSort = (destinationSortOrder ?? -1) + 1;
        foreach (var item in categoryItems)
        {
            item.CategoryId = fallbackCategory.Id;
            item.SortOrder = nextSort;
            nextSort++;
        }

        registryDbContext.RegistryItemCategories.Remove(category);

        await NormalizeCategoryOrderAsync(registryId, cancellationToken).ConfigureAwait(false);
        await NormalizeItemOrderAsync(registryId, fallbackCategory.Id, cancellationToken).ConfigureAwait(false);

        await registryDbContext.SaveChangesAsync(cancellationToken).ConfigureAwait(false);
    }

    public async Task MoveRegistryItemAsync(Guid registryId, Guid itemId, Guid categoryId, int targetIndex, CancellationToken cancellationToken)
    {
        var item = await registryDbContext.RegistryItems
            .FirstOrDefaultAsync(x => x.RegistryId == registryId && x.Id == itemId, cancellationToken)
            .ConfigureAwait(false) ?? throw new InvalidOperationException("Item not found.");

        var categoryExists = await registryDbContext.RegistryItemCategories
            .AnyAsync(x => x.RegistryId == registryId && x.Id == categoryId, cancellationToken)
            .ConfigureAwait(false);

        if (!categoryExists)
        {
            throw new InvalidOperationException("Category not found.");
        }

        var sourceCategoryId = item.CategoryId;

        var targetItems = await registryDbContext.RegistryItems
            .Where(x => x.RegistryId == registryId && x.CategoryId == categoryId && x.Id != itemId)
            .OrderBy(x => x.SortOrder)
            .ToListAsync(cancellationToken)
            .ConfigureAwait(false);

        if (targetIndex < 0)
        {
            targetIndex = 0;
        }

        if (targetIndex > targetItems.Count)
        {
            targetIndex = targetItems.Count;
        }

        targetItems.Insert(targetIndex, item);

        for (var i = 0; i < targetItems.Count; i++)
        {
            targetItems[i].CategoryId = categoryId;
            targetItems[i].SortOrder = i;
        }

        if (sourceCategoryId != categoryId)
        {
            await NormalizeItemOrderAsync(registryId, sourceCategoryId, cancellationToken).ConfigureAwait(false);
        }

        await registryDbContext.SaveChangesAsync(cancellationToken).ConfigureAwait(false);
    }

    public async Task MoveRegistryCategoryAsync(Guid registryId, Guid categoryId, int targetIndex, CancellationToken cancellationToken)
    {
        var categories = await registryDbContext.RegistryItemCategories
            .Where(x => x.RegistryId == registryId)
            .OrderBy(x => x.SortOrder)
            .ToListAsync(cancellationToken)
            .ConfigureAwait(false);

        var category = categories.FirstOrDefault(x => x.Id == categoryId);
        if (category is null)
        {
            return;
        }

        categories.Remove(category);

        if (targetIndex < 0)
        {
            targetIndex = 0;
        }

        if (targetIndex > categories.Count)
        {
            targetIndex = categories.Count;
        }

        categories.Insert(targetIndex, category);

        for (var i = 0; i < categories.Count; i++)
        {
            categories[i].SortOrder = i;
        }

        await registryDbContext.SaveChangesAsync(cancellationToken).ConfigureAwait(false);
    }

    private async Task<RegistryItemCategory> EnsureDefaultCategoryAsync(Guid registryId, CancellationToken cancellationToken)
    {
        var categories = await registryDbContext.RegistryItemCategories
            .Where(x => x.RegistryId == registryId)
            .OrderBy(x => x.SortOrder)
            .ToListAsync(cancellationToken)
            .ConfigureAwait(false);

        RegistryItemCategory defaultCategory;
        if (categories.Count == 0)
        {
            defaultCategory = new RegistryItemCategory
            {
                Id = Guid.NewGuid(),
                RegistryId = registryId,
                Name = DefaultCategoryName,
                SortOrder = 0,
                CreatedAtUtc = DateTimeOffset.UtcNow
            };
            registryDbContext.RegistryItemCategories.Add(defaultCategory);
        }
        else
        {
            defaultCategory = categories.FirstOrDefault(x => x.Name == DefaultCategoryName) ?? categories[0];
        }

        var unassignedItems = await registryDbContext.RegistryItems
            .Where(x => x.RegistryId == registryId && x.CategoryId == Guid.Empty)
            .OrderBy(x => x.SortOrder)
            .ThenBy(x => x.Name)
            .ToListAsync(cancellationToken)
            .ConfigureAwait(false);

        if (unassignedItems.Count > 0)
        {
            var nextSortOrder = await registryDbContext.RegistryItems
                .Where(x => x.RegistryId == registryId && x.CategoryId == defaultCategory.Id)
                .Select(x => (int?)x.SortOrder)
                .MaxAsync(cancellationToken)
                .ConfigureAwait(false);

            var next = (nextSortOrder ?? -1) + 1;
            foreach (var item in unassignedItems)
            {
                item.CategoryId = defaultCategory.Id;
                item.SortOrder = next;
                next++;
            }
        }

        await NormalizeCategoryOrderAsync(registryId, cancellationToken).ConfigureAwait(false);
        await NormalizeItemOrderAsync(registryId, defaultCategory.Id, cancellationToken).ConfigureAwait(false);

        if (registryDbContext.ChangeTracker.HasChanges())
        {
            await registryDbContext.SaveChangesAsync(cancellationToken).ConfigureAwait(false);
        }

        return defaultCategory;
    }

    private async Task NormalizeCategoryOrderAsync(Guid registryId, CancellationToken cancellationToken)
    {
        var categories = await registryDbContext.RegistryItemCategories
            .Where(x => x.RegistryId == registryId)
            .OrderBy(x => x.SortOrder)
            .ThenBy(x => x.CreatedAtUtc)
            .ToListAsync(cancellationToken)
            .ConfigureAwait(false);

        for (var i = 0; i < categories.Count; i++)
        {
            categories[i].SortOrder = i;
        }
    }

    private async Task NormalizeItemOrderAsync(Guid registryId, Guid categoryId, CancellationToken cancellationToken)
    {
        var items = await registryDbContext.RegistryItems
            .Where(x => x.RegistryId == registryId && x.CategoryId == categoryId)
            .OrderBy(x => x.SortOrder)
            .ThenBy(x => x.CreatedAtUtc)
            .ToListAsync(cancellationToken)
            .ConfigureAwait(false);

        for (var i = 0; i < items.Count; i++)
        {
            items[i].SortOrder = i;
        }
    }
}