namespace QuestionsApi.Services;

public sealed class AnswerStatisticsMaintenanceService : BackgroundService
{
    private readonly AnswerStatisticsStore _statistics;
    private readonly QuestionAttemptStore _attempts;
    private readonly ServiceMetrics _metrics;
    private readonly ILogger<AnswerStatisticsMaintenanceService> _logger;
    private readonly TimeSpan _interval;
    private readonly string _owner = $"{Environment.MachineName}-{Guid.NewGuid():N}";

    public AnswerStatisticsMaintenanceService(
        AnswerStatisticsStore statistics,
        QuestionAttemptStore attempts,
        ServiceMetrics metrics,
        IConfiguration configuration,
        ILogger<AnswerStatisticsMaintenanceService> logger)
    {
        _statistics = statistics;
        _attempts = attempts;
        _metrics = metrics;
        _logger = logger;
        _interval = TimeSpan.FromSeconds(Math.Max(30, configuration.GetValue("AnswerStatistics:ReconcileSeconds", 300)));
    }

    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        try
        {
            if (await _statistics.TryAcquireBackfillLeaseAsync(_owner, TimeSpan.FromMinutes(30)))
            {
                _logger.LogInformation("Backfilling materialized answer statistics");
                await _statistics.RebuildAllAsync();
                await _statistics.MarkBackfillCompleteAsync(_owner);
                _metrics.AnswerStatisticsBackfillCompleted();
                _metrics.Reconciled();
            }
            else if (await _statistics.IsBackfillCompleteAsync())
            {
                _metrics.AnswerStatisticsBackfillCompleted();
            }
        }
        catch (Exception exception)
        {
            _logger.LogError(exception, "Initial answer-statistics backfill failed");
        }

        using var timer = new PeriodicTimer(_interval);
        while (await timer.WaitForNextTickAsync(stoppingToken))
        {
            try
            {
                if (await _statistics.IsBackfillCompleteAsync())
                    _metrics.AnswerStatisticsBackfillCompleted();
                var activeIds = await _attempts.GetActiveQuestionIdsAsync(DateTime.UtcNow.Subtract(_interval).Subtract(_interval));
                await _statistics.RebuildQuestionsAsync(activeIds);
                _metrics.Reconciled();
            }
            catch (Exception exception)
            {
                _logger.LogError(exception, "Answer-statistics reconciliation failed");
            }
        }
    }
}