using System;
using Svrnty.CQRS.Events.Abstractions.Storage;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;

namespace Svrnty.CQRS.Events.Abstractions.Storage;

/// <summary>
/// Store for tracking read receipts (consumer acknowledgments of processed events).
/// </summary>
/// <remarks>
/// <para>
/// <strong>Purpose:</strong>
/// Read receipts provide visibility into consumer progress through event streams.
/// Unlike idempotency (which prevents duplicates), read receipts track progress.
/// </para>
/// <para>
/// <strong>Use Cases:</strong>
/// - Dashboard showing consumer lag/progress
/// - Resuming from last processed position
/// - Monitoring consumer health
/// - Detecting stuck consumers
/// </para>
/// </remarks>
public interface IReadReceiptStore
{
    /// <summary>
    /// Records that a consumer has successfully processed an event.
    /// </summary>
    /// <param name="consumerId">The consumer identifier.</param>
    /// <param name="streamName">The name of the event stream.</param>
    /// <param name="eventId">The unique event identifier.</param>
    /// <param name="offset">The event's offset/position in the stream.</param>
    /// <param name="acknowledgedAt">When the event was acknowledged.</param>
    /// <param name="cancellationToken">Cancellation token.</param>
    Task AcknowledgeEventAsync(
        string consumerId,
        string streamName,
        string eventId,
        long offset,
        DateTimeOffset acknowledgedAt,
        CancellationToken cancellationToken = default);

    /// <summary>
    /// Gets the last acknowledged offset for a consumer on a specific stream.
    /// </summary>
    /// <param name="consumerId">The consumer identifier.</param>
    /// <param name="streamName">The name of the event stream.</param>
    /// <param name="cancellationToken">Cancellation token.</param>
    /// <returns>The last acknowledged offset, or null if no receipts exist.</returns>
    Task<long?> GetLastAcknowledgedOffsetAsync(
        string consumerId,
        string streamName,
        CancellationToken cancellationToken = default);

    /// <summary>
    /// Gets statistics about a consumer's progress on a stream.
    /// </summary>
    /// <param name="consumerId">The consumer identifier.</param>
    /// <param name="streamName">The name of the event stream.</param>
    /// <param name="cancellationToken">Cancellation token.</param>
    /// <returns>Consumer progress statistics.</returns>
    Task<ConsumerProgress?> GetConsumerProgressAsync(
        string consumerId,
        string streamName,
        CancellationToken cancellationToken = default);

    /// <summary>
    /// Gets all consumers that are tracking a specific stream.
    /// </summary>
    /// <param name="streamName">The name of the event stream.</param>
    /// <param name="cancellationToken">Cancellation token.</param>
    /// <returns>List of consumer IDs tracking this stream.</returns>
    Task<IReadOnlyList<string>> GetConsumersForStreamAsync(
        string streamName,
        CancellationToken cancellationToken = default);

    /// <summary>
    /// Cleans up old read receipts.
    /// </summary>
    /// <param name="olderThan">Delete receipts older than this timestamp.</param>
    /// <param name="cancellationToken">Cancellation token.</param>
    /// <returns>Number of receipts deleted.</returns>
    Task<int> CleanupAsync(
        DateTimeOffset olderThan,
        CancellationToken cancellationToken = default);
}

/// <summary>
/// Represents a consumer's progress on a specific stream.
/// </summary>
public sealed class ConsumerProgress
{
    /// <summary>
    /// The consumer identifier.
    /// </summary>
    public required string ConsumerId { get; init; }

    /// <summary>
    /// The stream name.
    /// </summary>
    public required string StreamName { get; init; }

    /// <summary>
    /// The last acknowledged offset.
    /// </summary>
    public required long LastOffset { get; init; }

    /// <summary>
    /// When the last event was acknowledged.
    /// </summary>
    public required DateTimeOffset LastAcknowledgedAt { get; init; }

    /// <summary>
    /// Total number of events acknowledged.
    /// </summary>
    public required long TotalAcknowledged { get; init; }

    /// <summary>
    /// When the consumer first started tracking this stream.
    /// </summary>
    public DateTimeOffset? FirstAcknowledgedAt { get; init; }
}