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(IConfiguration cfg)
    {
        var cs = cfg["MongoDB:ConnectionString"] ?? "mongodb://127.0.0.1:27017";
        var dbName = cfg["MongoDB:DatabaseName"] ?? "GameDb";
        var client = new MongoClient(cs);
        var db = client.GetDatabase(dbName);
        _users = db.GetCollection<UserRating>("UserRatings");
        _questions = db.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<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
            };
        }

        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));

        userRating.OverallElo = Math.Max(MinElo, oldOverall + overallDelta);
        userRating.CategoryElos[category] = Math.Max(MinElo, oldCategory + categoryDelta);
        userRating.RatedQuestionCount++;
        userRating.UpdatedUtc = DateTime.UtcNow;

        questionRating.Elo = Math.Max(MinElo, oldQuestion + questionDelta);
        questionRating.AttemptCount++;
        questionRating.UpdatedUtc = DateTime.UtcNow;

        await _users.ReplaceOneAsync(r => r.UserId == userRating.UserId, userRating, new ReplaceOptions { IsUpsert = true });
        await _questions.ReplaceOneAsync(r => r.QuestionExternalId == questionRating.QuestionExternalId, questionRating, new ReplaceOptions { IsUpsert = true });

        return new RatingChangeResponse
        {
            Rated = true,
            Category = category,
            UserOverallElo = userRating.OverallElo,
            UserOverallDelta = userRating.OverallElo - oldOverall,
            UserCategoryElo = userRating.CategoryElos[category],
            UserCategoryDelta = userRating.CategoryElos[category] - oldCategory,
            QuestionElo = questionRating.Elo,
            QuestionEloDelta = questionRating.Elo - oldQuestion
        };
    }

    private async Task<UserRating> GetOrCreateUserRatingAsync(string userId)
    {
        var rating = await _users.Find(r => r.UserId == userId).FirstOrDefaultAsync();
        if (rating is null)
        {
            rating = new UserRating { UserId = userId, CategoryElos = DefaultCategoryElos() };
            await _users.InsertOneAsync(rating);
            return rating;
        }

        var changed = false;
        foreach (var category in RatingCategory.Categories)
        {
            if (!rating.CategoryElos.ContainsKey(category))
            {
                rating.CategoryElos[category] = BaseElo;
                changed = true;
            }
        }
        if (changed)
            await _users.ReplaceOneAsync(r => r.UserId == userId, rating);
        return rating;
    }

    private async Task<QuestionRating> GetOrCreateQuestionRatingAsync(string questionExternalId)
    {
        var rating = await _questions.Find(r => r.QuestionExternalId == questionExternalId).FirstOrDefaultAsync();
        if (rating is not null)
            return rating;

        rating = new QuestionRating { QuestionExternalId = questionExternalId };
        await _questions.InsertOneAsync(rating);
        return rating;
    }

    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));
}