using System.Diagnostics.Metrics;
using Npgsql;
using Tiku.Infrastructure.Messaging;

namespace Tiku.Api.Observability;

public sealed class OutboxBacklogSnapshot
{
    private static readonly Meter Meter = new("Tiku.Messaging", "1.0.0");
    private long pending;
    private double oldestAgeSeconds;

    public OutboxBacklogSnapshot()
    {
        Meter.CreateObservableGauge("tiku.outbox.pending", () => Interlocked.Read(ref pending));
        Meter.CreateObservableGauge("tiku.outbox.oldest_age", () => Volatile.Read(ref oldestAgeSeconds), "s");
    }

    internal long Pending => Interlocked.Read(ref pending);
    internal double OldestAgeSeconds => Volatile.Read(ref oldestAgeSeconds);

    internal void Update(long count, DateTime? oldestSentTime)
    {
        Interlocked.Exchange(ref pending, count);
        Volatile.Write(ref oldestAgeSeconds, oldestSentTime is null
            ? 0
            : Math.Max(0, (DateTimeOffset.UtcNow - new DateTimeOffset(oldestSentTime.Value)).TotalSeconds));
    }
}

internal sealed class OutboxBacklogMonitor(
    NpgsqlDataSource dataSource,
    OutboxBacklogSnapshot snapshot,
    MessagingOptions messagingOptions,
    ILogger<OutboxBacklogMonitor> logger) : BackgroundService
{
    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        while (!stoppingToken.IsCancellationRequested)
        {
            try
            {
                if (messagingOptions.IsConfigured)
                {
                    await using var command = dataSource.CreateCommand(
                        "SELECT count(*), min(sent_time) FROM outbox_message;");
                    await using var reader = await command.ExecuteReaderAsync(stoppingToken);
                    if (await reader.ReadAsync(stoppingToken))
                    {
                        snapshot.Update(reader.GetInt64(0), reader.IsDBNull(1) ? null : reader.GetDateTime(1));
                    }
                }
            }
            catch (OperationCanceledException) when (stoppingToken.IsCancellationRequested)
            {
                break;
            }
            catch (Exception exception)
            {
                logger.LogWarning(exception, "Outbox backlog metric collection failed.");
            }

            await Task.Delay(TimeSpan.FromSeconds(30), stoppingToken);
        }
    }
}