step3/api/QuestionsApi/Services/Flashcards/FlashcardDeckStore.cs

using MongoDB.Driver;
using QuestionsApi.Models;

namespace QuestionsApi.Services.Flashcards;

public sealed record FlashcardDeckUpdateResult(FlashcardDeck? Deck, FlashcardDeck? Current)
{
    public bool Succeeded => Deck is not null;
}

public sealed class FlashcardDeckStore
{
    private readonly IMongoCollection<FlashcardDeck> _decks;

    public FlashcardDeckStore(MongoDatabaseProvider mongo)
    {
        _decks = mongo.Database.GetCollection<FlashcardDeck>("FlashcardDecks");
        _decks.Indexes.CreateMany([
            new CreateIndexModel<FlashcardDeck>(
                Builders<FlashcardDeck>.IndexKeys.Ascending(v => v.UserId).Ascending(v => v.NormalizedName),
                new CreateIndexOptions { Unique = true, Name = "flashcard_deck_unique_user_name" }),
            new CreateIndexModel<FlashcardDeck>(
                Builders<FlashcardDeck>.IndexKeys.Ascending(v => v.UserId).Ascending(v => v.StableId),
                new CreateIndexOptions { Unique = true, Name = "flashcard_deck_unique_user_stable" })
        ]);
    }

    public static string Normalize(string name) => name.Trim().ToLowerInvariant();

    public Task<List<FlashcardDeck>> ListAsync(string userId) =>
        _decks.Find(v => v.UserId == userId && v.ArchivedUtc == null).SortBy(v => v.Name).ToListAsync();

    public async Task<FlashcardDeck?> GetAsync(string userId, string deckId) =>
        await _decks.Find(v => v.UserId == userId && v.Id == deckId).FirstOrDefaultAsync();

    // Read-only: the caller's own decks plus every system (shared, admin-curated) deck. Never
    // used to back a write path - mutations must stay strictly own-user via ListAsync/GetAsync.
    public Task<List<FlashcardDeck>> ListVisibleAsync(string userId) =>
        _decks.Find(v => (v.UserId == userId || v.UserId == FlashcardOwners.System) && v.ArchivedUtc == null)
            .SortBy(v => v.Name).ToListAsync();

    public async Task<FlashcardDeck?> GetVisibleAsync(string userId, string deckId) =>
        await _decks.Find(v => (v.UserId == userId || v.UserId == FlashcardOwners.System) && v.Id == deckId)
            .FirstOrDefaultAsync();

    // No owner filter at all - used only by the controller's mutation guard to check IsSystem
    // regardless of who's asking.
    public async Task<FlashcardDeck?> GetAnyAsync(string deckId) =>
        await _decks.Find(v => v.Id == deckId).FirstOrDefaultAsync();

    public async Task<FlashcardDeck> GetOrCreateDefaultDeckAsync(string userId)
    {
        var existing = await _decks.Find(v => v.UserId == userId && v.IsDefault).FirstOrDefaultAsync();
        if (existing is not null) return existing;

        var deck = new FlashcardDeck
        {
            UserId = userId,
            Name = "Default",
            NormalizedName = Normalize("Default"),
            IsDefault = true
        };
        try
        {
            await _decks.InsertOneAsync(deck);
            return deck;
        }
        catch (MongoWriteException exception) when (exception.WriteError?.Category == ServerErrorCategory.DuplicateKey)
        {
            return await _decks.Find(v => v.UserId == userId && v.IsDefault).FirstOrDefaultAsync()
                ?? throw new InvalidOperationException("default deck race with no winner");
        }
    }

    public async Task<FlashcardDeckUpdateResult> UpsertAsync(string userId, UpsertFlashcardDeckRequest request, string? deckId)
    {
        var now = DateTime.UtcNow;
        var normalized = Normalize(request.Name);

        if (string.IsNullOrWhiteSpace(deckId))
        {
            var created = new FlashcardDeck
            {
                UserId = userId,
                Name = request.Name.Trim(),
                NormalizedName = normalized,
                Description = request.Description.Trim(),
                NewCardsPerDay = request.NewCardsPerDay,
                ReviewsPerDay = request.ReviewsPerDay,
                DesiredRetention = request.DesiredRetention,
                CreatedUtc = now,
                UpdatedUtc = now
            };
            try
            {
                await _decks.InsertOneAsync(created);
                return new FlashcardDeckUpdateResult(created, null);
            }
            catch (MongoWriteException exception) when (exception.WriteError?.Category == ServerErrorCategory.DuplicateKey)
            {
                return new FlashcardDeckUpdateResult(null, await GetByNameAsync(userId, normalized));
            }
        }

        var expectedRevision = request.ExpectedRevision ?? 0;
        var updated = await _decks.FindOneAndUpdateAsync(
            v => v.UserId == userId && v.Id == deckId && v.Revision == expectedRevision,
            Builders<FlashcardDeck>.Update
                .Set(v => v.Name, request.Name.Trim())
                .Set(v => v.NormalizedName, normalized)
                .Set(v => v.Description, request.Description.Trim())
                .Set(v => v.NewCardsPerDay, request.NewCardsPerDay)
                .Set(v => v.ReviewsPerDay, request.ReviewsPerDay)
                .Set(v => v.DesiredRetention, request.DesiredRetention)
                .Set(v => v.UpdatedUtc, now)
                .Inc(v => v.Revision, 1),
            new FindOneAndUpdateOptions<FlashcardDeck> { ReturnDocument = ReturnDocument.After });

        return updated is not null
            ? new FlashcardDeckUpdateResult(updated, null)
            : new FlashcardDeckUpdateResult(null, await GetAsync(userId, deckId));
    }

    public async Task<FlashcardDeck?> GetByNameAsync(string userId, string normalizedName) =>
        await _decks.Find(v => v.UserId == userId && v.NormalizedName == normalizedName).FirstOrDefaultAsync();

    public async Task<FlashcardDeckUpdateResult> ArchiveAsync(string userId, string deckId, bool archived)
    {
        var deck = await GetAsync(userId, deckId);
        if (deck is not null && deck.IsDefault) return new FlashcardDeckUpdateResult(null, deck);

        var updated = await _decks.FindOneAndUpdateAsync(
            v => v.UserId == userId && v.Id == deckId && !v.IsDefault,
            Builders<FlashcardDeck>.Update
                .Set(v => v.ArchivedUtc, archived ? DateTime.UtcNow : null)
                .Set(v => v.UpdatedUtc, DateTime.UtcNow)
                .Inc(v => v.Revision, 1),
            new FindOneAndUpdateOptions<FlashcardDeck> { ReturnDocument = ReturnDocument.After });
        return updated is not null
            ? new FlashcardDeckUpdateResult(updated, null)
            : new FlashcardDeckUpdateResult(null, deck);
    }
}