step3/api/QuestionsApi/Services/QuestionAttemptStore.cs

using MongoDB.Driver;
using QuestionsApi.Models;
using System.Security.Cryptography;
using System.Text;

namespace QuestionsApi.Services;

public class QuestionAttemptStore
{
    private readonly IMongoCollection<QuestionAttempt> _col;

    public const int CounterBucketCount = 32;

    public QuestionAttemptStore(MongoDatabaseProvider mongo)
    {
        _col = mongo.Database.GetCollection<QuestionAttempt>("QuestionAttempts");

        try
        {
            _col.Indexes.DropOne("QuestionExternalId_1_UserId_1");
        }
        catch (MongoCommandException exception) when (exception.Code is 26 or 27)
        {
        }
        var firstAttemptIndex = Builders<QuestionAttempt>.IndexKeys
            .Ascending(a => a.QuestionExternalId)
            .Ascending(a => a.UserId)
            .Ascending(a => a.BankCycle);
        _col.Indexes.CreateOne(new CreateIndexModel<QuestionAttempt>(
            firstAttemptIndex,
            new CreateIndexOptions { Unique = true, Name = "uq_question_user_cycle" }));

        var questionIndex = Builders<QuestionAttempt>.IndexKeys.Ascending(a => a.QuestionExternalId);
        _col.Indexes.CreateOne(new CreateIndexModel<QuestionAttempt>(questionIndex));

        var createdIndex = Builders<QuestionAttempt>.IndexKeys.Ascending(a => a.CreatedUtc);
        _col.Indexes.CreateOne(new CreateIndexModel<QuestionAttempt>(createdIndex));
        _col.Indexes.CreateOne(new CreateIndexModel<QuestionAttempt>(
            Builders<QuestionAttempt>.IndexKeys.Ascending(a => a.UserId).Descending(a => a.CreatedUtc)));
    }

    public async Task<QuestionAttemptRecordResult> RecordFirstAttemptAsync(
        string questionExternalId,
        string userId,
        string choiceId,
        bool isCorrect,
        bool isSkip = false,
        IReadOnlyCollection<string>? presentedChoiceIds = null,
        int durationSeconds = 0,
        int bankCycle = 0,
        string completionId = "")
    {
        var attempt = new QuestionAttempt
        {
            QuestionExternalId = questionExternalId,
            UserId = userId,
            BankCycle = bankCycle,
            CompletionId = completionId,
            ChoiceId = choiceId,
            PresentedChoiceIds = presentedChoiceIds?.ToList() ?? [],
            TracksPresentation = presentedChoiceIds is { Count: > 0 },
            IsCorrect = isCorrect,
            IsSkip = isSkip,
            DurationSeconds = Math.Clamp(durationSeconds, 0, 86_400),
            CounterBucket = GetCounterBucket(userId),
            RatingApplied = false,
            CreatedUtc = DateTime.UtcNow
        };

        try
        {
            await _col.InsertOneAsync(attempt);
            return new QuestionAttemptRecordResult { Inserted = true, Attempt = attempt };
        }
        catch (MongoWriteException ex) when (ex.WriteError.Category == ServerErrorCategory.DuplicateKey)
        {
            var existing = await _col
                .Find(a => a.QuestionExternalId == questionExternalId && a.UserId == userId && a.BankCycle == bankCycle)
                .FirstAsync();
            return new QuestionAttemptRecordResult { Inserted = false, Attempt = existing };
        }
    }

    public Task<List<string>> GetActiveQuestionIdsAsync(DateTime sinceUtc) =>
        _col.Distinct(
            attempt => attempt.QuestionExternalId,
            Builders<QuestionAttempt>.Filter.Gte(attempt => attempt.CreatedUtc, sinceUtc))
            .ToListAsync();

    public Task<List<string>> GetUserQuestionIdsAsync(string userId, int? bankCycle = null) =>
        _col.Distinct(
            attempt => attempt.QuestionExternalId,
            Builders<QuestionAttempt>.Filter.Eq(attempt => attempt.UserId, userId) &
            (bankCycle is null
                ? FilterDefinition<QuestionAttempt>.Empty
                : Builders<QuestionAttempt>.Filter.Eq(attempt => attempt.BankCycle, bankCycle.Value)))
            .ToListAsync();

    public IMongoCollection<QuestionAttempt> Collection => _col;

    public async Task<bool> TryAcquireRatingLeaseAsync(string attemptId, string owner, TimeSpan duration)
    {
        var now = DateTime.UtcNow;
        var filter = Builders<QuestionAttempt>.Filter.And(
            Builders<QuestionAttempt>.Filter.Eq(attempt => attempt.Id, attemptId),
            Builders<QuestionAttempt>.Filter.Exists(attempt => attempt.RatingApplied, true),
            Builders<QuestionAttempt>.Filter.Eq(attempt => attempt.RatingApplied, false),
            Builders<QuestionAttempt>.Filter.Or(
                Builders<QuestionAttempt>.Filter.Lt(attempt => attempt.RatingLeaseUntilUtc, now),
                Builders<QuestionAttempt>.Filter.Eq(attempt => attempt.RatingLeaseOwner, owner)));
        var update = Builders<QuestionAttempt>.Update
            .Set(attempt => attempt.RatingLeaseOwner, owner)
            .Set(attempt => attempt.RatingLeaseUntilUtc, now.Add(duration));
        var leased = await _col.FindOneAndUpdateAsync(
            filter,
            update,
            new FindOneAndUpdateOptions<QuestionAttempt> { ReturnDocument = ReturnDocument.After });
        return leased?.RatingLeaseOwner == owner;
    }

    public Task MarkRatingAppliedAsync(string attemptId, string owner, int userOverallElo) =>
        _col.UpdateOneAsync(
            attempt => attempt.Id == attemptId && attempt.RatingLeaseOwner == owner,
            Builders<QuestionAttempt>.Update
                .Set(attempt => attempt.RatingApplied, true)
                .Set(attempt => attempt.UserOverallEloAfter, userOverallElo)
                .Set(attempt => attempt.RatingAppliedUtc, DateTime.UtcNow)
                .Set(attempt => attempt.RatingLeaseUntilUtc, DateTime.MinValue));

    public Task ReleaseRatingLeaseAsync(string attemptId, string owner) =>
        _col.UpdateOneAsync(
            attempt => attempt.Id == attemptId && attempt.RatingLeaseOwner == owner && attempt.RatingApplied == false,
            Builders<QuestionAttempt>.Update.Set(attempt => attempt.RatingLeaseUntilUtc, DateTime.MinValue));

    public static int GetCounterBucket(string userId)
    {
        var hash = SHA256.HashData(Encoding.UTF8.GetBytes(userId));
        return (int)(BitConverter.ToUInt32(hash, 0) % CounterBucketCount);
    }
}