// -----------------------------------------------------------------------------
// ConcelierCacheMetrics.cs
// Sprint: SPRINT_8200_0013_0001_GW_valkey_advisory_cache
// Task: VCACHE-8200-027, VCACHE-8200-028
// Description: OpenTelemetry metrics for cache operations
// -----------------------------------------------------------------------------

using System.Diagnostics;
using System.Diagnostics.Metrics;

namespace StellaOps.Concelier.Cache.Valkey;

/// <summary>
/// Metrics instrumentation for the Concelier advisory cache.
/// </summary>
public sealed class ConcelierCacheMetrics : IDisposable
{
    /// <summary>
    /// Activity source name for cache operations.
    /// </summary>
    public const string ActivitySourceName = "StellaOps.Concelier.Cache";

    /// <summary>
    /// Meter name for cache metrics.
    /// </summary>
    public const string MeterName = "StellaOps.Concelier.Cache";

    private readonly Meter _meter;
    private readonly Counter<long> _hitsCounter;
    private readonly Counter<long> _missesCounter;
    private readonly Counter<long> _evictionsCounter;
    private readonly Histogram<double> _latencyHistogram;
    private readonly ObservableGauge<long> _hotSetSizeGauge;

    private long _lastKnownHotSetSize;

    /// <summary>
    /// Activity source for tracing cache operations.
    /// </summary>
    public static ActivitySource ActivitySource { get; } = new(ActivitySourceName, "1.0.0");

    /// <summary>
    /// Initializes a new instance of <see cref="ConcelierCacheMetrics"/>.
    /// </summary>
    public ConcelierCacheMetrics()
    {
        _meter = new Meter(MeterName, "1.0.0");

        _hitsCounter = _meter.CreateCounter<long>(
            "concelier_cache_hits_total",
            unit: "{hits}",
            description: "Total number of cache hits");

        _missesCounter = _meter.CreateCounter<long>(
            "concelier_cache_misses_total",
            unit: "{misses}",
            description: "Total number of cache misses");

        _evictionsCounter = _meter.CreateCounter<long>(
            "concelier_cache_evictions_total",
            unit: "{evictions}",
            description: "Total number of cache evictions");

        _latencyHistogram = _meter.CreateHistogram<double>(
            "concelier_cache_latency_ms",
            unit: "ms",
            description: "Cache operation latency in milliseconds");

        _hotSetSizeGauge = _meter.CreateObservableGauge(
            "concelier_cache_hot_set_size",
            () => _lastKnownHotSetSize,
            unit: "{entries}",
            description: "Current number of entries in the hot advisory set");
    }

    /// <summary>
    /// Records a cache hit.
    /// </summary>
    public void RecordHit() => _hitsCounter.Add(1);

    /// <summary>
    /// Records a cache miss.
    /// </summary>
    public void RecordMiss() => _missesCounter.Add(1);

    /// <summary>
    /// Records a cache eviction.
    /// </summary>
    /// <param name="reason">The reason for eviction.</param>
    public void RecordEviction(string reason = "ttl")
    {
        _evictionsCounter.Add(1, new KeyValuePair<string, object?>("reason", reason));
    }

    /// <summary>
    /// Records operation latency.
    /// </summary>
    /// <param name="milliseconds">Latency in milliseconds.</param>
    /// <param name="operation">The operation type (get, set, invalidate).</param>
    public void RecordLatency(double milliseconds, string operation)
    {
        _latencyHistogram.Record(milliseconds, new KeyValuePair<string, object?>("operation", operation));
    }

    /// <summary>
    /// Updates the hot set size gauge.
    /// </summary>
    /// <param name="size">Current hot set size.</param>
    public void UpdateHotSetSize(long size)
    {
        _lastKnownHotSetSize = size;
    }

    /// <summary>
    /// Starts an activity for tracing a cache operation.
    /// </summary>
    /// <param name="operationName">Name of the operation.</param>
    /// <returns>The activity, or null if tracing is disabled.</returns>
    public static Activity? StartActivity(string operationName)
    {
        return ActivitySource.StartActivity(operationName, ActivityKind.Internal);
    }

    /// <summary>
    /// Starts an activity with tags.
    /// </summary>
    /// <param name="operationName">Name of the operation.</param>
    /// <param name="tags">Tags to add to the activity.</param>
    /// <returns>The activity, or null if tracing is disabled.</returns>
    public static Activity? StartActivity(string operationName, params (string Key, object? Value)[] tags)
    {
        var activity = ActivitySource.StartActivity(operationName, ActivityKind.Internal);
        if (activity is not null)
        {
            foreach (var (key, value) in tags)
            {
                activity.SetTag(key, value);
            }
        }
        return activity;
    }

    /// <inheritdoc />
    public void Dispose()
    {
        _meter.Dispose();
    }
}

/// <summary>
/// Extension methods for timing cache operations.
/// </summary>
public static class CacheMetricsExtensions
{
    /// <summary>
    /// Times an async operation and records the latency.
    /// </summary>
    public static async Task<T> TimeAsync<T>(
        this ConcelierCacheMetrics metrics,
        string operation,
        Func<Task<T>> action)
    {
        var sw = Stopwatch.StartNew();
        try
        {
            return await action().ConfigureAwait(false);
        }
        finally
        {
            sw.Stop();
            metrics.RecordLatency(sw.Elapsed.TotalMilliseconds, operation);
        }
    }

    /// <summary>
    /// Times an async operation and records the latency.
    /// </summary>
    public static async Task TimeAsync(
        this ConcelierCacheMetrics metrics,
        string operation,
        Func<Task> action)
    {
        var sw = Stopwatch.StartNew();
        try
        {
            await action().ConfigureAwait(false);
        }
        finally
        {
            sw.Stop();
            metrics.RecordLatency(sw.Elapsed.TotalMilliseconds, operation);
        }
    }
}