using System;
using System.Threading;

namespace Svrnty.CQRS.Events.Logging;

/// <summary>
/// Manages correlation ID propagation across async operations for distributed tracing.
/// </summary>
/// <remarks>
/// <para>
/// <strong>Phase 6 Feature:</strong>
/// Uses AsyncLocal to maintain correlation ID context across async boundaries.
/// Enables full request tracing across event streams, subscriptions, and consumers.
/// </para>
/// <para>
/// <strong>Usage Pattern:</strong>
/// <code>
/// using (CorrelationContext.Begin(correlationId))
/// {
///     // All operations within this scope will have access to the correlation ID
///     await PublishEventAsync(myEvent);
///     _logger.LogEventPublished(eventId, eventType, streamName, CorrelationContext.Current);
/// }
/// </code>
/// </para>
/// </remarks>
public static class CorrelationContext
{
    private static readonly AsyncLocal<string?> _correlationId = new();

    /// <summary>
    /// Gets the current correlation ID for this async context.
    /// </summary>
    /// <returns>The current correlation ID, or null if not set.</returns>
    public static string? Current => _correlationId.Value;

    /// <summary>
    /// Begins a new correlation context with the specified ID.
    /// </summary>
    /// <param name="correlationId">The correlation ID to use for this context.</param>
    /// <returns>A disposable scope that restores the previous correlation ID when disposed.</returns>
    /// <remarks>
    /// If correlationId is null, a new GUID will be generated.
    /// Always use with a using statement to ensure proper cleanup.
    /// </remarks>
    public static IDisposable Begin(string? correlationId = null)
    {
        return new CorrelationScope(correlationId ?? Guid.NewGuid().ToString());
    }

    /// <summary>
    /// Sets the correlation ID for the current async context.
    /// </summary>
    /// <param name="correlationId">The correlation ID to set.</param>
    /// <remarks>
    /// Prefer using Begin() with a using statement for automatic cleanup.
    /// </remarks>
    internal static void Set(string? correlationId)
    {
        _correlationId.Value = correlationId;
    }

    private sealed class CorrelationScope : IDisposable
    {
        private readonly string? _previousCorrelationId;
        private bool _disposed;

        public CorrelationScope(string correlationId)
        {
            _previousCorrelationId = Current;
            Set(correlationId);
        }

        public void Dispose()
        {
            if (!_disposed)
            {
                Set(_previousCorrelationId);
                _disposed = true;
            }
        }
    }
}