step3/api/QuestionsApi/Services/CcsRatingStore.cs

using MongoDB.Driver;
using QuestionsApi.Models;

namespace QuestionsApi.Services;

public sealed class CcsRatingStore
{
    private const int BaseElo = 1000;
    private const int MinimumElo = 100;
    private readonly IMongoCollection<CcsUserRating> _users;
    private readonly IMongoCollection<CcsCaseRating> _cases;
    private readonly IMongoCollection<CcsRatedCaseResult> _results;

    public CcsRatingStore(MongoDatabaseProvider mongo)
    {
        _users = mongo.Database.GetCollection<CcsUserRating>("CcsUserRatings");
        _cases = mongo.Database.GetCollection<CcsCaseRating>("CcsCaseRatings");
        _results = mongo.Database.GetCollection<CcsRatedCaseResult>("CcsRatedCaseResults");
        _users.Indexes.CreateOne(new CreateIndexModel<CcsUserRating>(
            Builders<CcsUserRating>.IndexKeys.Ascending(value => value.UserId), new CreateIndexOptions { Unique = true }));
        _cases.Indexes.CreateOne(new CreateIndexModel<CcsCaseRating>(
            Builders<CcsCaseRating>.IndexKeys.Ascending(value => value.CaseExternalId), new CreateIndexOptions { Unique = true }));
        _results.Indexes.CreateOne(new CreateIndexModel<CcsRatedCaseResult>(
            Builders<CcsRatedCaseResult>.IndexKeys.Ascending(value => value.UserId).Ascending(value => value.CaseExternalId),
            new CreateIndexOptions { Unique = true }));
    }

    public async Task<CcsRatingProfileResponse> GetProfileAsync(string userId)
    {
        var rating = await GetOrCreateUserAsync(userId);
        return new CcsRatingProfileResponse { Elo = rating.Elo, RatedCaseCount = rating.RatedCaseCount };
    }

    public async Task<CcsRatingChange> ApplyAsync(CcsAttempt attempt)
    {
        var actual = Math.Clamp((attempt.Score?.Percent ?? 0) / 100.0, 0, 1);
        if (!attempt.RatingEligible || attempt.Mode != CcsAttemptModes.Timed)
            return Unrated("not_eligible", actual);

        try
        {
            await _results.InsertOneAsync(new CcsRatedCaseResult
            {
                UserId = attempt.UserId,
                CaseExternalId = attempt.CaseExternalId,
                AttemptId = attempt.Id ?? ""
            });
        }
        catch (MongoWriteException exception) when (exception.WriteError?.Category == ServerErrorCategory.DuplicateKey)
        {
            return Unrated("repeat_case", actual);
        }

        var user = await GetOrCreateUserAsync(attempt.UserId);
        var caseRating = await GetOrCreateCaseAsync(attempt.CaseExternalId);
        var expected = ExpectedScore(user.Elo, caseRating.Elo);
        var userDelta = (int)Math.Round(UserK(user.RatedCaseCount) * (actual - expected), MidpointRounding.AwayFromZero);
        var caseDelta = (int)Math.Round(CaseK(caseRating.RatedAttemptCount) * (expected - actual), MidpointRounding.AwayFromZero);
        var updatedUser = await UpdateUserAsync(user, userDelta);
        var updatedCase = await UpdateCaseAsync(caseRating, caseDelta);
        return new CcsRatingChange
        {
            Rated = true,
            Reason = "first_timed_attempt",
            UserEloBefore = user.Elo,
            UserElo = updatedUser.Elo,
            UserEloDelta = updatedUser.Elo - user.Elo,
            CaseEloBefore = caseRating.Elo,
            CaseElo = updatedCase.Elo,
            CaseEloDelta = updatedCase.Elo - caseRating.Elo,
            ActualScore = actual,
            AppliedUtc = DateTime.UtcNow
        };
    }

    public static double ExpectedScore(int userElo, int caseElo) =>
        1.0 / (1.0 + Math.Pow(10.0, (caseElo - userElo) / 400.0));

    private static int UserK(int count) => count < 20 ? 40 : 24;
    private static int CaseK(int count) => count < 20 ? 32 : 24;

    private static CcsRatingChange Unrated(string reason, double actual) => new()
    {
        Rated = false, Reason = reason, ActualScore = actual
    };

    private async Task<CcsUserRating> GetOrCreateUserAsync(string userId)
    {
        try
        {
            await _users.UpdateOneAsync(value => value.UserId == userId,
                Builders<CcsUserRating>.Update
                    .SetOnInsert(value => value.UserId, userId)
                    .SetOnInsert(value => value.Elo, BaseElo)
                    .SetOnInsert(value => value.RatedCaseCount, 0)
                    .SetOnInsert(value => value.UpdatedUtc, DateTime.UtcNow),
                new UpdateOptions { IsUpsert = true });
        }
        catch (MongoWriteException exception) when (exception.WriteError?.Category == ServerErrorCategory.DuplicateKey)
        {
            // Another API replica created the initial rating first.
        }
        return await _users.Find(value => value.UserId == userId).FirstAsync();
    }

    private async Task<CcsCaseRating> GetOrCreateCaseAsync(string externalId)
    {
        try
        {
            await _cases.UpdateOneAsync(value => value.CaseExternalId == externalId,
                Builders<CcsCaseRating>.Update
                    .SetOnInsert(value => value.CaseExternalId, externalId)
                    .SetOnInsert(value => value.Elo, BaseElo)
                    .SetOnInsert(value => value.RatedAttemptCount, 0)
                    .SetOnInsert(value => value.UpdatedUtc, DateTime.UtcNow),
                new UpdateOptions { IsUpsert = true });
        }
        catch (MongoWriteException exception) when (exception.WriteError?.Category == ServerErrorCategory.DuplicateKey)
        {
            // Another API replica created the initial rating first.
        }
        return await _cases.Find(value => value.CaseExternalId == externalId).FirstAsync();
    }

    private async Task<CcsUserRating> UpdateUserAsync(CcsUserRating current, int delta)
    {
        while (true)
        {
            var nextElo = Math.Max(MinimumElo, current.Elo + delta);
            var updated = await _users.FindOneAndUpdateAsync(
                value => value.UserId == current.UserId && value.Elo == current.Elo && value.RatedCaseCount == current.RatedCaseCount,
                Builders<CcsUserRating>.Update.Set(value => value.Elo, nextElo)
                    .Inc(value => value.RatedCaseCount, 1).Set(value => value.UpdatedUtc, DateTime.UtcNow),
                new FindOneAndUpdateOptions<CcsUserRating> { ReturnDocument = ReturnDocument.After });
            if (updated is not null) return updated;
            current = await _users.Find(value => value.UserId == current.UserId).FirstAsync();
        }
    }

    private async Task<CcsCaseRating> UpdateCaseAsync(CcsCaseRating current, int delta)
    {
        while (true)
        {
            var nextElo = Math.Max(MinimumElo, current.Elo + delta);
            var updated = await _cases.FindOneAndUpdateAsync(
                value => value.CaseExternalId == current.CaseExternalId && value.Elo == current.Elo && value.RatedAttemptCount == current.RatedAttemptCount,
                Builders<CcsCaseRating>.Update.Set(value => value.Elo, nextElo)
                    .Inc(value => value.RatedAttemptCount, 1).Set(value => value.UpdatedUtc, DateTime.UtcNow),
                new FindOneAndUpdateOptions<CcsCaseRating> { ReturnDocument = ReturnDocument.After });
            if (updated is not null) return updated;
            current = await _cases.Find(value => value.CaseExternalId == current.CaseExternalId).FirstAsync();
        }
    }
}