step3/api/QuestionsApi/Services/RatingStore.cs

using MongoDB.Driver;
using QuestionsApi.Models;

namespace QuestionsApi.Services;

public class RatingStore
{
    private const int BaseElo = 1000;
    private const int MinElo = 100;

    private readonly IMongoCollection<UserRating> _users;
    private readonly IMongoCollection<QuestionRating> _questions;

    public RatingStore(MongoDatabaseProvider mongo)
    {
        _users = mongo.Database.GetCollection<UserRating>("UserRatings");
        _questions = mongo.Database.GetCollection<QuestionRating>("QuestionRatings");

        _users.Indexes.CreateOne(new CreateIndexModel<UserRating>(
            Builders<UserRating>.IndexKeys.Ascending(r => r.UserId),
            new CreateIndexOptions { Unique = true }));
        _questions.Indexes.CreateOne(new CreateIndexModel<QuestionRating>(
            Builders<QuestionRating>.IndexKeys.Ascending(r => r.QuestionExternalId),
            new CreateIndexOptions { Unique = true }));
    }

    public async Task<RatingProfileResponse> GetProfileAsync(string userId)
    {
        var rating = await GetOrCreateUserRatingAsync(userId);
        return new RatingProfileResponse
        {
            OverallElo = rating.OverallElo,
            RatedQuestionCount = rating.RatedQuestionCount,
            CategoryElos = rating.CategoryElos
        };
    }

    public async Task<UserRating> GetOrInitializeHistoryAsync(string userId, DateTime nowUtc)
    {
        var rating = await GetOrCreateUserRatingAsync(userId);
        return await EnsureHistoryBaselineAsync(rating, nowUtc);
    }

    public async Task<Dictionary<string, int>> GetQuestionElosAsync(IEnumerable<string> externalIds)
    {
        var ids = externalIds.Where(id => !string.IsNullOrWhiteSpace(id)).Distinct().ToList();
        if (ids.Count == 0)
            return [];

        var ratings = await _questions.Find(r => ids.Contains(r.QuestionExternalId)).ToListAsync();
        return ids.ToDictionary(
            id => id,
            id => ratings.FirstOrDefault(r => r.QuestionExternalId == id)?.Elo ?? BaseElo);
    }

    public async Task<int> GetQuestionEloAsync(string questionExternalId)
    {
        var rating = await GetOrCreateQuestionRatingAsync(questionExternalId);
        return rating.Elo;
    }

    public async Task<RatingChangeResponse> ApplyFirstResultAsync(string userId, Question question, bool isCorrect, bool shouldRate)
    {
        var userRating = await GetOrCreateUserRatingAsync(userId);
        var questionRating = await GetOrCreateQuestionRatingAsync(question.ExternalId);
        var category = RatingCategory.Normalize(question.PerformanceCategory, question.System, question.Discipline);

        if (!shouldRate)
        {
            return new RatingChangeResponse
            {
                Rated = false,
                Category = category,
                UserOverallElo = userRating.OverallElo,
                UserCategoryElo = userRating.CategoryElos.GetValueOrDefault(category, BaseElo),
                QuestionElo = questionRating.Elo
            };
        }

        userRating = await EnsureHistoryBaselineAsync(userRating, DateTime.UtcNow);
        var actual = isCorrect ? 1.0 : 0.0;
        var userK = userRating.RatedQuestionCount < 20 ? 40 : 24;
        var questionK = questionRating.AttemptCount < 20 ? 32 : 24;

        var oldOverall = userRating.OverallElo;
        var oldCategory = userRating.CategoryElos.GetValueOrDefault(category, BaseElo);
        var oldQuestion = questionRating.Elo;

        var overallExpected = ExpectedScore(oldOverall, oldQuestion);
        var categoryExpected = ExpectedScore(oldCategory, oldQuestion);
        var overallDelta = (int)Math.Round(userK * (actual - overallExpected));
        var categoryDelta = (int)Math.Round(userK * (actual - categoryExpected));
        var questionDelta = (int)Math.Round(questionK * (overallExpected - actual));

        var now = DateTime.UtcNow;
        var categoryPath = $"CategoryElos.{category}";
        var userUpdate = Builders<UserRating>.Update
            .Inc(rating => rating.OverallElo, overallDelta)
            .Inc(categoryPath, categoryDelta)
            .Inc(rating => rating.RatedQuestionCount, 1)
            .Set(rating => rating.UpdatedUtc, now);
        await _users.UpdateOneAsync(rating => rating.UserId == userId, userUpdate);
        await _users.UpdateOneAsync(
            rating => rating.UserId == userId,
            Builders<UserRating>.Update
                .Max(rating => rating.OverallElo, MinElo)
                .Max(categoryPath, MinElo));

        var questionUpdate = Builders<QuestionRating>.Update
            .Inc(rating => rating.Elo, questionDelta)
            .Inc(rating => rating.AttemptCount, 1)
            .Set(rating => rating.UpdatedUtc, now);
        await _questions.UpdateOneAsync(rating => rating.QuestionExternalId == question.ExternalId, questionUpdate);
        await _questions.UpdateOneAsync(
            rating => rating.QuestionExternalId == question.ExternalId,
            Builders<QuestionRating>.Update.Max(rating => rating.Elo, MinElo));

        var updatedUser = await _users.Find(rating => rating.UserId == userId).FirstAsync();
        var updatedQuestion = await _questions.Find(rating => rating.QuestionExternalId == question.ExternalId).FirstAsync();

        return new RatingChangeResponse
        {
            Rated = true,
            Category = category,
            UserOverallElo = updatedUser.OverallElo,
            UserOverallDelta = overallDelta,
            UserCategoryElo = updatedUser.CategoryElos.GetValueOrDefault(category, BaseElo),
            UserCategoryDelta = categoryDelta,
            QuestionElo = updatedQuestion.Elo,
            QuestionEloDelta = questionDelta
        };
    }

    private async Task<UserRating> GetOrCreateUserRatingAsync(string userId)
    {
        var create = Builders<UserRating>.Update
            .SetOnInsert(rating => rating.UserId, userId)
            .SetOnInsert(rating => rating.OverallElo, BaseElo)
            .SetOnInsert(rating => rating.RatedQuestionCount, 0)
            .SetOnInsert(rating => rating.CategoryElos, DefaultCategoryElos())
            .SetOnInsert(rating => rating.UpdatedUtc, DateTime.UtcNow);
        UserRating rating;
        try
        {
            rating = await _users.FindOneAndUpdateAsync(
                item => item.UserId == userId,
                create,
                new FindOneAndUpdateOptions<UserRating>
                {
                    IsUpsert = true,
                    ReturnDocument = ReturnDocument.After
                });
        }
        catch (MongoWriteException exception) when (exception.WriteError.Category == ServerErrorCategory.DuplicateKey)
        {
            rating = await _users.Find(item => item.UserId == userId).FirstAsync();
        }

        var missingCategories = RatingCategory.Categories
            .Where(category => !rating.CategoryElos.ContainsKey(category))
            .ToList();
        if (missingCategories.Count > 0)
        {
            var updates = missingCategories
                .Select(category => Builders<UserRating>.Update.Set($"CategoryElos.{category}", BaseElo))
                .ToList();
            if (updates.Count > 0)
                await _users.UpdateOneAsync(item => item.UserId == userId, Builders<UserRating>.Update.Combine(updates));
            rating = await _users.Find(item => item.UserId == userId).FirstAsync();
        }
        return rating;
    }

    private async Task<UserRating> EnsureHistoryBaselineAsync(UserRating rating, DateTime nowUtc)
    {
        if (rating.HistoryBaselineElo.HasValue && rating.HistoryStartedUtc.HasValue)
            return rating;

        var result = await _users.UpdateOneAsync(
            value => value.UserId == rating.UserId && value.HistoryStartedUtc == null,
            Builders<UserRating>.Update
                .Set(value => value.HistoryBaselineElo, rating.OverallElo)
                .Set(value => value.HistoryStartedUtc, nowUtc));
        if (result.ModifiedCount > 0)
        {
            rating.HistoryBaselineElo = rating.OverallElo;
            rating.HistoryStartedUtc = nowUtc;
            return rating;
        }

        return await _users.Find(value => value.UserId == rating.UserId).FirstAsync();
    }

    private async Task<QuestionRating> GetOrCreateQuestionRatingAsync(string questionExternalId)
    {
        var create = Builders<QuestionRating>.Update
            .SetOnInsert(rating => rating.QuestionExternalId, questionExternalId)
            .SetOnInsert(rating => rating.Elo, BaseElo)
            .SetOnInsert(rating => rating.AttemptCount, 0)
            .SetOnInsert(rating => rating.UpdatedUtc, DateTime.UtcNow);
        try
        {
            return await _questions.FindOneAndUpdateAsync(
                rating => rating.QuestionExternalId == questionExternalId,
                create,
                new FindOneAndUpdateOptions<QuestionRating>
                {
                    IsUpsert = true,
                    ReturnDocument = ReturnDocument.After
                });
        }
        catch (MongoWriteException exception) when (exception.WriteError.Category == ServerErrorCategory.DuplicateKey)
        {
            return await _questions.Find(rating => rating.QuestionExternalId == questionExternalId).FirstAsync();
        }
    }

    private static Dictionary<string, int> DefaultCategoryElos() =>
        RatingCategory.Categories.ToDictionary(category => category, _ => BaseElo);

    private static double ExpectedScore(int playerElo, int questionElo) =>
        1.0 / (1.0 + Math.Pow(10.0, (questionElo - playerElo) / 400.0));
}