using System;
using Svrnty.CQRS.Events.Abstractions.Storage;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.Extensions.Logging;
using Svrnty.CQRS.Events.Abstractions;

namespace Svrnty.CQRS.Events.Storage;

/// <summary>
/// In-memory implementation of <see cref="IReadReceiptStore"/>.
/// </summary>
/// <remarks>
/// <para>
/// <strong>Scope:</strong>
/// - Single application instance only (not distributed)
/// - Lost on application restart
/// - Suitable for development and testing
/// </para>
/// <para>
/// <strong>Thread Safety:</strong>
/// Uses ConcurrentDictionary for thread-safe operations.
/// </para>
/// </remarks>
public sealed class InMemoryReadReceiptStore : IReadReceiptStore
{
    private readonly ILogger<InMemoryReadReceiptStore> _logger;

    // Key: "{consumerId}:{streamName}"
    private readonly ConcurrentDictionary<string, ConsumerStreamState> _consumerStates = new();

    public InMemoryReadReceiptStore(ILogger<InMemoryReadReceiptStore> logger)
    {
        _logger = logger ?? throw new ArgumentNullException(nameof(logger));
    }

    /// <inheritdoc />
    public Task AcknowledgeEventAsync(
        string consumerId,
        string streamName,
        string eventId,
        long offset,
        DateTimeOffset acknowledgedAt,
        CancellationToken cancellationToken = default)
    {
        if (string.IsNullOrWhiteSpace(consumerId))
            throw new ArgumentException("Consumer ID cannot be null or whitespace.", nameof(consumerId));
        if (string.IsNullOrWhiteSpace(streamName))
            throw new ArgumentException("Stream name cannot be null or whitespace.", nameof(streamName));
        if (string.IsNullOrWhiteSpace(eventId))
            throw new ArgumentException("Event ID cannot be null or whitespace.", nameof(eventId));

        var key = GetKey(consumerId, streamName);

        _consumerStates.AddOrUpdate(
            key,
            // Add new state
            _ => new ConsumerStreamState
            {
                ConsumerId = consumerId,
                StreamName = streamName,
                LastEventId = eventId,
                LastOffset = offset,
                LastAcknowledgedAt = acknowledgedAt,
                FirstAcknowledgedAt = acknowledgedAt,
                TotalAcknowledged = 1
            },
            // Update existing state
            (_, existing) =>
            {
                // Only update if this offset is newer
                if (offset > existing.LastOffset)
                {
                    existing.LastEventId = eventId;
                    existing.LastOffset = offset;
                    existing.LastAcknowledgedAt = acknowledgedAt;
                }
                existing.TotalAcknowledged++;
                return existing;
            });

        _logger.LogDebug(
            "Acknowledged event {EventId} at offset {Offset} for consumer {ConsumerId} on stream {StreamName}",
            eventId,
            offset,
            consumerId,
            streamName);

        return Task.CompletedTask;
    }

    /// <inheritdoc />
    public Task<long?> GetLastAcknowledgedOffsetAsync(
        string consumerId,
        string streamName,
        CancellationToken cancellationToken = default)
    {
        if (string.IsNullOrWhiteSpace(consumerId))
            throw new ArgumentException("Consumer ID cannot be null or whitespace.", nameof(consumerId));
        if (string.IsNullOrWhiteSpace(streamName))
            throw new ArgumentException("Stream name cannot be null or whitespace.", nameof(streamName));

        var key = GetKey(consumerId, streamName);

        if (_consumerStates.TryGetValue(key, out var state))
        {
            return Task.FromResult<long?>(state.LastOffset);
        }

        return Task.FromResult<long?>(null);
    }

    /// <inheritdoc />
    public Task<ConsumerProgress?> GetConsumerProgressAsync(
        string consumerId,
        string streamName,
        CancellationToken cancellationToken = default)
    {
        if (string.IsNullOrWhiteSpace(consumerId))
            throw new ArgumentException("Consumer ID cannot be null or whitespace.", nameof(consumerId));
        if (string.IsNullOrWhiteSpace(streamName))
            throw new ArgumentException("Stream name cannot be null or whitespace.", nameof(streamName));

        var key = GetKey(consumerId, streamName);

        if (_consumerStates.TryGetValue(key, out var state))
        {
            var progress = new ConsumerProgress
            {
                ConsumerId = state.ConsumerId,
                StreamName = state.StreamName,
                LastOffset = state.LastOffset,
                LastAcknowledgedAt = state.LastAcknowledgedAt,
                TotalAcknowledged = state.TotalAcknowledged,
                FirstAcknowledgedAt = state.FirstAcknowledgedAt
            };

            return Task.FromResult<ConsumerProgress?>(progress);
        }

        return Task.FromResult<ConsumerProgress?>(null);
    }

    /// <inheritdoc />
    public Task<IReadOnlyList<string>> GetConsumersForStreamAsync(
        string streamName,
        CancellationToken cancellationToken = default)
    {
        if (string.IsNullOrWhiteSpace(streamName))
            throw new ArgumentException("Stream name cannot be null or whitespace.", nameof(streamName));

        var consumers = _consumerStates.Values
            .Where(state => state.StreamName == streamName)
            .Select(state => state.ConsumerId)
            .Distinct()
            .ToList();

        return Task.FromResult<IReadOnlyList<string>>(consumers);
    }

    /// <inheritdoc />
    public Task<int> CleanupAsync(
        DateTimeOffset olderThan,
        CancellationToken cancellationToken = default)
    {
        var keysToRemove = _consumerStates
            .Where(kvp => kvp.Value.LastAcknowledgedAt < olderThan)
            .Select(kvp => kvp.Key)
            .ToList();

        var removedCount = 0;
        foreach (var key in keysToRemove)
        {
            if (_consumerStates.TryRemove(key, out _))
            {
                removedCount++;
            }
        }

        if (removedCount > 0)
        {
            _logger.LogInformation(
                "Cleaned up {RemovedCount} read receipt records older than {OlderThan}",
                removedCount,
                olderThan);
        }

        return Task.FromResult(removedCount);
    }

    private static string GetKey(string consumerId, string streamName)
    {
        return $"{consumerId}:{streamName}";
    }

    private sealed class ConsumerStreamState
    {
        public required string ConsumerId { get; init; }
        public required string StreamName { get; init; }
        public string LastEventId { get; set; } = string.Empty;
        public long LastOffset { get; set; }
        public DateTimeOffset LastAcknowledgedAt { get; set; }
        public DateTimeOffset FirstAcknowledgedAt { get; init; }
        public long TotalAcknowledged { get; set; }
    }
}