using System;
using Svrnty.CQRS.Events.Abstractions.Streaming;
using System.Collections.Generic;
using System.Diagnostics.Metrics;
using Svrnty.CQRS.Events.Abstractions;

namespace Svrnty.CQRS.Events.Metrics;

/// <summary>
/// Implementation of event stream metrics using System.Diagnostics.Metrics API.
/// Compatible with OpenTelemetry and Prometheus exporters.
/// </summary>
/// <remarks>
/// <para>
/// <strong>Metric Naming Convention:</strong>
/// All metrics use the prefix "svrnty.cqrs.events." for consistency.
/// Tags/labels are used for stream name, subscription ID, and event type dimensions.
/// </para>
/// <para>
/// <strong>OpenTelemetry Integration:</strong>
/// This implementation uses .NET's built-in Meter API which is automatically
/// discovered by OpenTelemetry's .NET instrumentation.
/// </para>
/// <para>
/// <strong>Prometheus Integration:</strong>
/// Use OpenTelemetry's Prometheus exporter to expose these metrics at /metrics endpoint.
/// </para>
/// </remarks>
public sealed class EventStreamMetrics : IEventStreamMetrics, IDisposable
{
    private const string MeterName = "Svrnty.CQRS.Events";
    private const string MeterVersion = "1.0.0";

    private readonly Meter _meter;

    // Counters (cumulative values that only increase)
    private readonly Counter<long> _eventsPublishedCounter;
    private readonly Counter<long> _eventsConsumedCounter;
    private readonly Counter<long> _errorsCounter;
    private readonly Counter<long> _retriesCounter;

    // Histograms (distribution of values)
    private readonly Histogram<double> _processingLatencyHistogram;

    // Observable Gauges (current point-in-time values)
    private readonly Dictionary<string, long> _consumerLagCache = new();
    private readonly Dictionary<string, long> _streamLengthCache = new();
    private readonly Dictionary<string, int> _activeConsumersCache = new();
    private readonly object _cacheLock = new();

    /// <summary>
    /// Initializes a new instance of the <see cref="EventStreamMetrics"/> class.
    /// </summary>
    public EventStreamMetrics()
    {
        _meter = new Meter(MeterName, MeterVersion);

        // Counter: Total events published
        _eventsPublishedCounter = _meter.CreateCounter<long>(
            name: "svrnty.cqrs.events.published",
            unit: "events",
            description: "Total number of events published to streams");

        // Counter: Total events consumed
        _eventsConsumedCounter = _meter.CreateCounter<long>(
            name: "svrnty.cqrs.events.consumed",
            unit: "events",
            description: "Total number of events consumed from subscriptions");

        // Counter: Total errors
        _errorsCounter = _meter.CreateCounter<long>(
            name: "svrnty.cqrs.events.errors",
            unit: "errors",
            description: "Total number of errors during event processing");

        // Counter: Total retries
        _retriesCounter = _meter.CreateCounter<long>(
            name: "svrnty.cqrs.events.retries",
            unit: "retries",
            description: "Total number of retry attempts for failed events");

        // Histogram: Processing latency distribution
        _processingLatencyHistogram = _meter.CreateHistogram<double>(
            name: "svrnty.cqrs.events.processing_latency",
            unit: "ms",
            description: "Event processing latency from publish to acknowledgment");

        // Observable Gauge: Consumer lag
        _meter.CreateObservableGauge(
            name: "svrnty.cqrs.events.consumer_lag",
            observeValues: () =>
            {
                lock (_cacheLock)
                {
                    var measurements = new List<Measurement<long>>(_consumerLagCache.Count);
                    foreach (var kvp in _consumerLagCache)
                    {
                        var parts = kvp.Key.Split(':', 2);
                        if (parts.Length == 2)
                        {
                            var tags = new KeyValuePair<string, object?>[]
                            {
                                new("stream", parts[0]),
                                new("subscription", parts[1])
                            };
                            measurements.Add(new Measurement<long>(kvp.Value, tags));
                        }
                    }
                    return measurements;
                }
            },
            unit: "events",
            description: "Number of events the consumer is behind the stream head");

        // Observable Gauge: Stream length
        _meter.CreateObservableGauge(
            name: "svrnty.cqrs.events.stream_length",
            observeValues: () =>
            {
                lock (_cacheLock)
                {
                    var measurements = new List<Measurement<long>>(_streamLengthCache.Count);
                    foreach (var kvp in _streamLengthCache)
                    {
                        var tags = new KeyValuePair<string, object?>[]
                        {
                            new("stream", kvp.Key)
                        };
                        measurements.Add(new Measurement<long>(kvp.Value, tags));
                    }
                    return measurements;
                }
            },
            unit: "events",
            description: "Current length of the event stream (total events)");

        // Observable Gauge: Active consumers
        _meter.CreateObservableGauge(
            name: "svrnty.cqrs.events.active_consumers",
            observeValues: () =>
            {
                lock (_cacheLock)
                {
                    var measurements = new List<Measurement<int>>(_activeConsumersCache.Count);
                    foreach (var kvp in _activeConsumersCache)
                    {
                        var parts = kvp.Key.Split(':', 2);
                        if (parts.Length == 2)
                        {
                            var tags = new KeyValuePair<string, object?>[]
                            {
                                new("stream", parts[0]),
                                new("subscription", parts[1])
                            };
                            measurements.Add(new Measurement<int>(kvp.Value, tags));
                        }
                    }
                    return measurements;
                }
            },
            unit: "consumers",
            description: "Number of active consumers for a subscription");
    }

    /// <inheritdoc />
    public void RecordEventPublished(string streamName, string eventType)
    {
        var tags = new KeyValuePair<string, object?>[]
        {
            new("stream", streamName),
            new("event_type", eventType)
        };
        _eventsPublishedCounter.Add(1, tags);
    }

    /// <inheritdoc />
    public void RecordEventConsumed(string streamName, string subscriptionId, string eventType)
    {
        var tags = new KeyValuePair<string, object?>[]
        {
            new("stream", streamName),
            new("subscription", subscriptionId),
            new("event_type", eventType)
        };
        _eventsConsumedCounter.Add(1, tags);
    }

    /// <inheritdoc />
    public void RecordProcessingLatency(string streamName, string subscriptionId, TimeSpan latency)
    {
        var tags = new KeyValuePair<string, object?>[]
        {
            new("stream", streamName),
            new("subscription", subscriptionId)
        };
        _processingLatencyHistogram.Record(latency.TotalMilliseconds, tags);
    }

    /// <inheritdoc />
    public void RecordConsumerLag(string streamName, string subscriptionId, long lag)
    {
        var key = $"{streamName}:{subscriptionId}";
        lock (_cacheLock)
        {
            _consumerLagCache[key] = lag;
        }
    }

    /// <inheritdoc />
    public void RecordError(string streamName, string? subscriptionId, string errorType)
    {
        var tags = subscriptionId != null
            ? new KeyValuePair<string, object?>[]
            {
                new("stream", streamName),
                new("subscription", subscriptionId),
                new("error_type", errorType)
            }
            : new KeyValuePair<string, object?>[]
            {
                new("stream", streamName),
                new("error_type", errorType)
            };
        _errorsCounter.Add(1, tags);
    }

    /// <inheritdoc />
    public void RecordRetry(string streamName, string subscriptionId, int attemptNumber)
    {
        var tags = new KeyValuePair<string, object?>[]
        {
            new("stream", streamName),
            new("subscription", subscriptionId),
            new("attempt", attemptNumber)
        };
        _retriesCounter.Add(1, tags);
    }

    /// <inheritdoc />
    public void RecordStreamLength(string streamName, long length)
    {
        lock (_cacheLock)
        {
            _streamLengthCache[streamName] = length;
        }
    }

    /// <inheritdoc />
    public void RecordActiveConsumers(string streamName, string subscriptionId, int consumerCount)
    {
        var key = $"{streamName}:{subscriptionId}";
        lock (_cacheLock)
        {
            _activeConsumersCache[key] = consumerCount;
        }
    }

    /// <summary>
    /// Disposes the meter and releases resources.
    /// </summary>
    public void Dispose()
    {
        _meter?.Dispose();
    }
}