using System.Collections.Generic;
using System.Threading;

namespace ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient;

/// <summary>
///     Per-driver counters surfaced via <see cref="Core.Abstractions.DriverHealth.Diagnostics"/>
///     for the <c>driver-diagnostics</c> RPC (task #276). Hot-path increments use
///     <see cref="Interlocked"/> so they're lock-free; the read path snapshots into a
///     <see cref="IReadOnlyDictionary{TKey, TValue}"/> keyed by stable counter names.
/// </summary>
/// <remarks>
///     The counters are operational metrics, not config — they reset to zero when the
///     driver instance is recreated (Reinitialize tear-down + rebuild) and there is no
///     persistence across process restarts. NotificationsPerSecond is a simple decay-EWMA
///     so a quiet subscription doesn't latch the value at the last burst rate.
/// </remarks>
internal sealed class OpcUaClientDiagnostics
{
    // ---- Hot-path counters (Interlocked) ----

    private long _publishRequestCount;
    private long _notificationCount;
    private long _missingPublishRequestCount;
    private long _droppedNotificationCount;
    private long _sessionResetCount;

    // ---- EWMA state for NotificationsPerSecond ----
    //
    // Use ticks (long) for the timestamp so we can swap atomically. The rate is a double
    // updated under a tight lock — the EWMA arithmetic (load, blend, store) isn't naturally
    // atomic on doubles, and the spinlock is held only for arithmetic so contention is
    // bounded. A subscription firing at 10 kHz with one driver instance is dominated by
    // the SDK's notification path, not this lock.
    private readonly object _ewmaLock = new();
    private double _notificationsPerSecond;
    private long _lastNotificationTicks;

    /// <summary>Half-life ~5 seconds — recent activity dominates but a paused subscription decays toward zero.</summary>
    private static readonly TimeSpan EwmaHalfLife = TimeSpan.FromSeconds(5);

    // ---- Reconnect state (lock-free, single-writer in OnReconnectComplete) ----
    private long _lastReconnectUtcTicks;

    public long PublishRequestCount => Interlocked.Read(ref _publishRequestCount);
    public long NotificationCount => Interlocked.Read(ref _notificationCount);
    public long MissingPublishRequestCount => Interlocked.Read(ref _missingPublishRequestCount);
    public long DroppedNotificationCount => Interlocked.Read(ref _droppedNotificationCount);
    public long SessionResetCount => Interlocked.Read(ref _sessionResetCount);

    public DateTime? LastReconnectUtc
    {
        get
        {
            var ticks = Interlocked.Read(ref _lastReconnectUtcTicks);
            return ticks == 0 ? null : new DateTime(ticks, DateTimeKind.Utc);
        }
    }

    public double NotificationsPerSecond
    {
        get { lock (_ewmaLock) return _notificationsPerSecond; }
    }

    public void IncrementPublishRequest() => Interlocked.Increment(ref _publishRequestCount);

    public void IncrementMissingPublishRequest() => Interlocked.Increment(ref _missingPublishRequestCount);

    public void IncrementDroppedNotification() => Interlocked.Increment(ref _droppedNotificationCount);

    /// <summary>Records one delivered notification (any monitored item) + folds the inter-arrival into the EWMA rate.</summary>
    public void RecordNotification() => RecordNotification(DateTime.UtcNow);

    internal void RecordNotification(DateTime nowUtc)
    {
        Interlocked.Increment(ref _notificationCount);

        // EWMA over instantaneous rate. instRate = 1 / dt (events per second since last sample).
        // Decay factor a = 2^(-dt/halfLife) puts a five-second window on the smoothing — recent
        // bursts win, idle periods bleed back to zero.
        var nowTicks = nowUtc.Ticks;
        lock (_ewmaLock)
        {
            if (_lastNotificationTicks == 0)
            {
                _lastNotificationTicks = nowTicks;
                // First sample: seed at 0 — we don't know the prior rate. The next sample
                // produces a real instRate.
                return;
            }
            var dtTicks = nowTicks - _lastNotificationTicks;
            if (dtTicks <= 0)
            {
                // Same-tick collisions on bursts: treat as no time elapsed for rate purposes
                // (count was already incremented above) so we don't divide by zero or feed
                // an absurd instRate spike.
                return;
            }
            var dtSeconds = (double)dtTicks / TimeSpan.TicksPerSecond;
            var instRate = 1.0 / dtSeconds;
            var alpha = System.Math.Pow(0.5, dtSeconds / EwmaHalfLife.TotalSeconds);
            _notificationsPerSecond = (alpha * _notificationsPerSecond) + ((1.0 - alpha) * instRate);
            _lastNotificationTicks = nowTicks;
        }
    }

    public void RecordSessionReset(DateTime nowUtc)
    {
        Interlocked.Increment(ref _sessionResetCount);
        Interlocked.Exchange(ref _lastReconnectUtcTicks, nowUtc.Ticks);
    }

    /// <summary>
    ///     Snapshot the counters into the dictionary shape <see cref="Core.Abstractions.DriverHealth.Diagnostics"/>
    ///     surfaces. Numeric-only (so the RPC can render generically); LastReconnectUtc is
    ///     emitted as ticks to keep the value type uniform.
    /// </summary>
    public IReadOnlyDictionary<string, double> Snapshot()
    {
        var dict = new Dictionary<string, double>(7, System.StringComparer.Ordinal)
        {
            ["PublishRequestCount"] = PublishRequestCount,
            ["NotificationCount"] = NotificationCount,
            ["NotificationsPerSecond"] = NotificationsPerSecond,
            ["MissingPublishRequestCount"] = MissingPublishRequestCount,
            ["DroppedNotificationCount"] = DroppedNotificationCount,
            ["SessionResetCount"] = SessionResetCount,
        };
        var last = LastReconnectUtc;
        if (last is not null)
            dict["LastReconnectUtcTicks"] = last.Value.Ticks;
        return dict;
    }
}