using System;
using Svrnty.CQRS.Events.HealthCheck;
using Svrnty.CQRS.Events.Abstractions.Models;
using Svrnty.CQRS.Events.Abstractions.EventStore;
using Svrnty.CQRS.Events.Abstractions.Streaming;
using Svrnty.CQRS.Events.Subscriptions;
using Svrnty.CQRS.Events.Abstractions.Configuration;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Options;
using Svrnty.CQRS.Events.Abstractions;

namespace Svrnty.CQRS.Events.HealthCheck;

/// <summary>
/// Implementation of stream health checks.
/// </summary>
public sealed class StreamHealthCheck : IStreamHealthCheck
{
    private readonly IEnumerable<IStreamConfiguration> _streamConfigurations;
    private readonly IEnumerable<Subscription> _subscriptions;
    private readonly IEventStreamStore _streamStore;
    private readonly ILogger<StreamHealthCheck>? _logger;
    private readonly StreamHealthCheckOptions _options;

    public StreamHealthCheck(
        IEnumerable<IStreamConfiguration> streamConfigurations,
        IEnumerable<Subscription> subscriptions,
        IEventStreamStore streamStore,
        IOptions<StreamHealthCheckOptions>? options = null,
        ILogger<StreamHealthCheck>? logger = null)
    {
        _streamConfigurations = streamConfigurations ?? Enumerable.Empty<IStreamConfiguration>();
        _subscriptions = subscriptions ?? Enumerable.Empty<Subscription>();
        _streamStore = streamStore ?? throw new ArgumentNullException(nameof(streamStore));
        _logger = logger;
        _options = options?.Value ?? new StreamHealthCheckOptions();
    }

    public async Task<HealthCheckResult> CheckStreamHealthAsync(string streamName, CancellationToken cancellationToken = default)
    {
        var sw = Stopwatch.StartNew();
        try
        {
            using var cts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
            cts.CancelAfter(_options.HealthCheckTimeout);

            var streamConfig = _streamConfigurations.FirstOrDefault(s => s.StreamName == streamName);
            if (streamConfig == null)
            {
                return HealthCheckResult.Unhealthy(
                    $"Stream '{streamName}' does not exist",
                    duration: sw.Elapsed);
            }

            // Check if we can get the stream length (validates stream is readable)
            var streamLength = await _streamStore.GetStreamLengthAsync(streamName, cts.Token);

            var data = new Dictionary<string, object>
            {
                ["streamName"] = streamName,
                ["streamLength"] = streamLength,
                ["streamType"] = streamConfig.Type.ToString(),
                ["deliverySemantics"] = streamConfig.DeliverySemantics.ToString(),
                ["scope"] = streamConfig.Scope.ToString()
            };

            return HealthCheckResult.Healthy(
                $"Stream '{streamName}' is healthy (length: {streamLength})",
                data,
                sw.Elapsed);
        }
        catch (OperationCanceledException) when (cancellationToken.IsCancellationRequested)
        {
            throw;
        }
        catch (OperationCanceledException)
        {
            return HealthCheckResult.Unhealthy(
                $"Health check for stream '{streamName}' timed out after {_options.HealthCheckTimeout.TotalSeconds}s",
                duration: sw.Elapsed);
        }
        catch (Exception ex)
        {
            _logger?.LogError(ex, "Error checking health of stream '{StreamName}'", streamName);
            return HealthCheckResult.Unhealthy(
                $"Error checking stream '{streamName}': {ex.Message}",
                ex,
                duration: sw.Elapsed);
        }
    }

    public async Task<HealthCheckResult> CheckSubscriptionHealthAsync(string streamName, string subscriptionName, CancellationToken cancellationToken = default)
    {
        var sw = Stopwatch.StartNew();
        try
        {
            using var cts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
            cts.CancelAfter(_options.HealthCheckTimeout);

            var streamConfig = _streamConfigurations.FirstOrDefault(s => s.StreamName == streamName);
            if (streamConfig == null)
            {
                return HealthCheckResult.Unhealthy(
                    $"Stream '{streamName}' does not exist",
                    duration: sw.Elapsed);
            }

            var subscription = _subscriptions.FirstOrDefault(s => s.StreamName == streamName && s.SubscriptionId == subscriptionName);
            if (subscription == null)
            {
                return HealthCheckResult.Unhealthy(
                    $"Subscription '{subscriptionName}' does not exist on stream '{streamName}'",
                    duration: sw.Elapsed);
            }

            // Get stream length and consumer offset
            var streamLength = await _streamStore.GetStreamLengthAsync(streamName, cts.Token);
            var consumerOffset = await _streamStore.GetConsumerOffsetAsync(streamName, subscriptionName, cts.Token);

            // Calculate lag
            var lag = streamLength - consumerOffset;

            // Get last update time to detect stalled consumers
            var lastUpdateTime = await _streamStore.GetConsumerLastUpdateTimeAsync(streamName, subscriptionName, cts.Token);
            var timeSinceUpdate = DateTimeOffset.UtcNow - lastUpdateTime;

            var data = new Dictionary<string, object>
            {
                ["streamName"] = streamName,
                ["subscriptionName"] = subscriptionName,
                ["streamLength"] = streamLength,
                ["consumerOffset"] = consumerOffset,
                ["lag"] = lag,
                ["lastUpdateTime"] = lastUpdateTime,
                ["timeSinceUpdate"] = timeSinceUpdate.TotalSeconds,
                ["subscriptionMode"] = subscription.Mode.ToString()
            };

            // Check for unhealthy conditions
            if (lag >= _options.UnhealthyConsumerLagThreshold)
            {
                return HealthCheckResult.Unhealthy(
                    $"Subscription '{subscriptionName}' has excessive lag: {lag} events (threshold: {_options.UnhealthyConsumerLagThreshold})",
                    data: data,
                    duration: sw.Elapsed);
            }

            if (timeSinceUpdate >= _options.UnhealthyStalledThreshold && streamLength > consumerOffset)
            {
                return HealthCheckResult.Unhealthy(
                    $"Subscription '{subscriptionName}' appears stalled: no progress for {timeSinceUpdate.TotalMinutes:F1} minutes (threshold: {_options.UnhealthyStalledThreshold.TotalMinutes} minutes)",
                    data: data,
                    duration: sw.Elapsed);
            }

            // Check for degraded conditions
            if (lag >= _options.DegradedConsumerLagThreshold)
            {
                return HealthCheckResult.Degraded(
                    $"Subscription '{subscriptionName}' has elevated lag: {lag} events (threshold: {_options.DegradedConsumerLagThreshold})",
                    data: data,
                    duration: sw.Elapsed);
            }

            if (timeSinceUpdate >= _options.DegradedStalledThreshold && streamLength > consumerOffset)
            {
                return HealthCheckResult.Degraded(
                    $"Subscription '{subscriptionName}' has slow progress: no updates for {timeSinceUpdate.TotalMinutes:F1} minutes (threshold: {_options.DegradedStalledThreshold.TotalMinutes} minutes)",
                    data: data,
                    duration: sw.Elapsed);
            }

            return HealthCheckResult.Healthy(
                $"Subscription '{subscriptionName}' is healthy (lag: {lag} events, last update: {timeSinceUpdate.TotalSeconds:F1}s ago)",
                data,
                sw.Elapsed);
        }
        catch (OperationCanceledException) when (cancellationToken.IsCancellationRequested)
        {
            throw;
        }
        catch (OperationCanceledException)
        {
            return HealthCheckResult.Unhealthy(
                $"Health check for subscription '{subscriptionName}' timed out after {_options.HealthCheckTimeout.TotalSeconds}s",
                duration: sw.Elapsed);
        }
        catch (Exception ex)
        {
            _logger?.LogError(ex, "Error checking health of subscription '{SubscriptionName}' on stream '{StreamName}'", subscriptionName, streamName);
            return HealthCheckResult.Unhealthy(
                $"Error checking subscription '{subscriptionName}': {ex.Message}",
                ex,
                duration: sw.Elapsed);
        }
    }

    public async Task<IReadOnlyDictionary<string, HealthCheckResult>> CheckAllStreamsAsync(CancellationToken cancellationToken = default)
    {
        var results = new ConcurrentDictionary<string, HealthCheckResult>();

        var tasks = _streamConfigurations.Select(async stream =>
        {
            var result = await CheckStreamHealthAsync(stream.StreamName, cancellationToken);
            results[stream.StreamName] = result;
        });

        await Task.WhenAll(tasks);
        return results;
    }

    public async Task<IReadOnlyDictionary<string, HealthCheckResult>> CheckAllSubscriptionsAsync(CancellationToken cancellationToken = default)
    {
        var results = new ConcurrentDictionary<string, HealthCheckResult>();

        var tasks = _subscriptions.Select(async subscription =>
        {
            var result = await CheckSubscriptionHealthAsync(subscription.StreamName, subscription.SubscriptionId, cancellationToken);
            var key = $"{subscription.StreamName}:{subscription.SubscriptionId}";
            results[key] = result;
        });

        await Task.WhenAll(tasks);
        return results;
    }
}