step3/api/QuestionsApi/Services/CreditStore.cs

using MongoDB.Bson;
using MongoDB.Driver;
using QuestionsApi.Models;

namespace QuestionsApi.Services;

public sealed class CreditStore
{
    private readonly IMongoCollection<UserCreditAccount> _accounts;
    private readonly TimeProvider _clock;

    public CreditStore(MongoDatabaseProvider mongo, TimeProvider clock)
    {
        _accounts = mongo.Database.GetCollection<UserCreditAccount>("UserCreditAccounts");
        _clock = clock;
        _accounts.Indexes.CreateOne(new CreateIndexModel<UserCreditAccount>(
            Builders<UserCreditAccount>.IndexKeys.Ascending(value => value.UserId),
            new CreateIndexOptions { Unique = true, Name = "uq_credit_user" }));
    }

    public async Task<bool> AwardForFirstAnswerAsync(string userId, string attemptId)
    {
        if (string.IsNullOrWhiteSpace(attemptId)) return false;
        var now = _clock.GetUtcNow().UtcDateTime;
        await EnsureAndCleanAsync(userId, now);

        var filters = Builders<UserCreditAccount>.Filter;
        var belowCap = new BsonDocumentFilterDefinition<UserCreditAccount>(
            new BsonDocument("$expr", new BsonDocument("$lt", new BsonArray
            {
                new BsonDocument("$size", "$RecentEarnings"),
                CreditEconomy.RollingEarningLimit
            })));
        var filter = filters.Eq(value => value.UserId, userId) &
            filters.Not(filters.ElemMatch(value => value.RecentEarnings,
                earning => earning.OperationId == attemptId)) &
            belowCap;
        var update = Builders<UserCreditAccount>.Update
            .Inc(value => value.Credits, 1)
            .Push(value => value.RecentEarnings, new CreditEvent
            {
                OperationId = attemptId,
                CreatedUtc = now
            })
            .Set(value => value.UpdatedUtc, now);
        return (await _accounts.UpdateOneAsync(filter, update)).ModifiedCount == 1;
    }

    public async Task<CreditStatusResponse> GetStatusAsync(string userId)
    {
        var now = _clock.GetUtcNow().UtcDateTime;
        await EnsureAndCleanAsync(userId, now);
        var account = await _accounts.Find(value => value.UserId == userId).FirstAsync();
        return ToStatus(account);
    }

    public async Task<int> GetBankCycleAsync(string userId)
    {
        await EnsureAndCleanAsync(userId, _clock.GetUtcNow().UtcDateTime);
        return (await _accounts.Find(value => value.UserId == userId).FirstAsync()).BankCycle;
    }

    public async Task<CreditOperationResponse> PurchaseAsync(
        string userId, RzStoreProduct product, string operationId)
    {
        var now = _clock.GetUtcNow().UtcDateTime;
        await EnsureAndCleanAsync(userId, now);
        var existing = await _accounts.Find(value => value.UserId == userId).FirstAsync();
        var prior = existing.Purchases.FirstOrDefault(value => value.OperationId == operationId);
        if (prior is not null)
            return Response(prior.Sku == product.Sku
                ? CreditOperationResult.Succeeded
                : CreditOperationResult.OperationConflict, existing);

        var filter = Builders<UserCreditAccount>.Filter.Eq(value => value.UserId, userId) &
            Builders<UserCreditAccount>.Filter.Gte(value => value.Credits, product.CoinPrice) &
            Builders<UserCreditAccount>.Filter.Not(
                Builders<UserCreditAccount>.Filter.ElemMatch(
                    value => value.Purchases, purchase => purchase.OperationId == operationId));
        var update = Builders<UserCreditAccount>.Update
            .Inc(value => value.Credits, -product.CoinPrice)
            .Push(value => value.Purchases, new CreditPurchaseEvent
            {
                OperationId = operationId,
                Sku = product.Sku,
                CoinCost = product.CoinPrice,
                CreatedUtc = now
            })
            .Set(value => value.UpdatedUtc, now);
        update = product.Grant == RzStoreGrant.ExamToken
            ? update.Inc(value => value.ExamTokens, 1)
            : update.Inc(value => value.QBankResetTokens, 1);
        var account = await _accounts.FindOneAndUpdateAsync(
            filter, update, new FindOneAndUpdateOptions<UserCreditAccount>
            {
                ReturnDocument = ReturnDocument.After
            });
        if (account is not null) return Response(CreditOperationResult.Succeeded, account);

        account = await _accounts.Find(value => value.UserId == userId).FirstAsync();
        prior = account.Purchases.FirstOrDefault(value => value.OperationId == operationId);
        if (prior is not null)
            return Response(prior.Sku == product.Sku
                ? CreditOperationResult.Succeeded
                : CreditOperationResult.OperationConflict, account);
        return Response(CreditOperationResult.InsufficientCredits, account);
    }

    public async Task<CreditOperationResponse> ApplyQBankResetAsync(string userId, string operationId)
    {
        var now = _clock.GetUtcNow().UtcDateTime;
        await EnsureAndCleanAsync(userId, now);
        var filter = Builders<UserCreditAccount>.Filter.Eq(value => value.UserId, userId) &
            Builders<UserCreditAccount>.Filter.Gt(value => value.QBankResetTokens, 0) &
            Builders<UserCreditAccount>.Filter.AnyNin(value => value.AppliedOperations, [operationId]);
        var update = Builders<UserCreditAccount>.Update
            .Inc(value => value.QBankResetTokens, -1)
            .Inc(value => value.BankCycle, 1)
            .AddToSet(value => value.AppliedOperations, operationId)
            .Set(value => value.UpdatedUtc, now);
        var account = await _accounts.FindOneAndUpdateAsync(
            filter, update, new FindOneAndUpdateOptions<UserCreditAccount>
            {
                ReturnDocument = ReturnDocument.After
            });
        if (account is not null) return Response(CreditOperationResult.Succeeded, account);
        account = await _accounts.Find(value => value.UserId == userId).FirstAsync();
        if (account.AppliedOperations.Contains(operationId))
            return Response(CreditOperationResult.Succeeded, account);
        return Response(CreditOperationResult.InsufficientTokens, account);
    }

    public async Task<bool> TryConsumeExamTokenAsync(string userId, string operationId)
    {
        var now = _clock.GetUtcNow().UtcDateTime;
        await EnsureAndCleanAsync(userId, now);
        var filter = Builders<UserCreditAccount>.Filter.Eq(value => value.UserId, userId) &
            Builders<UserCreditAccount>.Filter.Gt(value => value.ExamTokens, 0) &
            Builders<UserCreditAccount>.Filter.AnyNin(value => value.AppliedOperations, [operationId]);
        var update = Builders<UserCreditAccount>.Update
            .Inc(value => value.ExamTokens, -1)
            .AddToSet(value => value.AppliedOperations, operationId)
            .Set(value => value.UpdatedUtc, now);
        return (await _accounts.UpdateOneAsync(filter, update)).ModifiedCount == 1;
    }

    public Task RefundExamTokenAsync(string userId, string operationId) =>
        _accounts.UpdateOneAsync(
            Builders<UserCreditAccount>.Filter.Eq(value => value.UserId, userId) &
            Builders<UserCreditAccount>.Filter.AnyEq(value => value.AppliedOperations, operationId),
            Builders<UserCreditAccount>.Update
                .Inc(value => value.ExamTokens, 1)
                .Pull(value => value.AppliedOperations, operationId)
                .Set(value => value.UpdatedUtc, _clock.GetUtcNow().UtcDateTime));

    private async Task EnsureAndCleanAsync(string userId, DateTime now)
    {
        await _accounts.UpdateOneAsync(
            value => value.UserId == userId,
            Builders<UserCreditAccount>.Update
                .SetOnInsert(value => value.UserId, userId)
                .SetOnInsert(value => value.Credits, 0)
                .SetOnInsert(value => value.ExamTokens, 1)
                .SetOnInsert(value => value.QBankResetTokens, 0)
                .SetOnInsert(value => value.BankCycle, 0)
                .SetOnInsert(value => value.InitialExamGrantApplied, true)
                .SetOnInsert(value => value.RecentEarnings, [])
                .SetOnInsert(value => value.Purchases, [])
                .SetOnInsert(value => value.AppliedOperations, [])
                .SetOnInsert(value => value.UpdatedUtc, now),
            new UpdateOptions { IsUpsert = true });
        await _accounts.UpdateOneAsync(
            Builders<UserCreditAccount>.Filter.Eq(value => value.UserId, userId) &
            Builders<UserCreditAccount>.Filter.Ne(value => value.InitialExamGrantApplied, true),
            Builders<UserCreditAccount>.Update
                .Inc(value => value.ExamTokens, 1)
                .Set(value => value.InitialExamGrantApplied, true));
        await _accounts.UpdateOneAsync(
            Builders<UserCreditAccount>.Filter.Eq(value => value.UserId, userId) &
            Builders<UserCreditAccount>.Filter.Exists(nameof(UserCreditAccount.RecentEarnings), false),
            Builders<UserCreditAccount>.Update.Set(value => value.RecentEarnings, []));
        await _accounts.UpdateOneAsync(
            Builders<UserCreditAccount>.Filter.Eq(value => value.UserId, userId) &
            Builders<UserCreditAccount>.Filter.Exists(nameof(UserCreditAccount.Purchases), false),
            Builders<UserCreditAccount>.Update.Set(value => value.Purchases, []));
        await _accounts.UpdateOneAsync(
            Builders<UserCreditAccount>.Filter.Eq(value => value.UserId, userId) &
            Builders<UserCreditAccount>.Filter.Exists(nameof(UserCreditAccount.AppliedOperations), false),
            Builders<UserCreditAccount>.Update.Set(value => value.AppliedOperations, []));
        var cutoff = now - CreditEconomy.EarningWindow;
        await _accounts.UpdateOneAsync(
            value => value.UserId == userId,
            Builders<UserCreditAccount>.Update.PullFilter(
                value => value.RecentEarnings,
                earning => earning.CreatedUtc <= cutoff));
    }

    private static CreditStatusResponse ToStatus(UserCreditAccount account) => new()
    {
        Credits = account.Credits,
        EarnedInRollingWindow = account.RecentEarnings.Count,
        RemainingEarningCapacity = Math.Max(
            0, CreditEconomy.RollingEarningLimit - account.RecentEarnings.Count),
        ExamTokens = account.ExamTokens,
        QBankResetTokens = account.QBankResetTokens,
        BankCycle = account.BankCycle
    };

    private static CreditOperationResponse Response(
        CreditOperationResult result, UserCreditAccount account) => new()
        {
            Result = result,
            Status = ToStatus(account)
        };
}