using Microsoft.Extensions.Logging;
using ZB.MOM.WW.OtOpcUa.Core.Abstractions;

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

/// <summary>
///     The <see cref="IAlarmSource"/> half of <see cref="MTConnectDriver"/> (#626): CONDITION data items as
///     native OPC UA Part 9 alarms.
/// </summary>
/// <remarks>
///     <para>
///         <b>Which tags are alarms.</b> The server decides, not this driver: a raw tag whose <c>TagConfig</c>
///         carries the cross-driver <c>alarm</c> object is materialised as a Part 9 condition at its RawPath, and
///         its RawPath arrives here through <see cref="SubscribeAlarmsAsync"/>. Every event this driver raises
///         uses that RawPath as <see cref="AlarmEventArgs.ConditionId"/> — the key the host routes on. A CONDITION
///         tag WITHOUT an <c>alarm</c> object is unchanged: it stays the plain String value it always was.
///     </para>
///     <para>
///         <b>One condition per DataItem.</b> MTConnect can hold several simultaneous conditions on one
///         DataItem (distinct <c>nativeCode</c>s); the Part 9 condition shows the most severe active one
///         (<see cref="MTConnectConditionTracker"/>), and an escalation or a new code re-raises it
///         (<see cref="MTConnectConditionAlarmMapper"/>).
///     </para>
///     <para>
///         <b>Delivery is diffed against what the server was actually TOLD.</b> <see cref="_deliveredConditions"/>
///         records a state only when an <see cref="OnAlarmEvent"/> handler was attached to receive it. The host's
///         <c>DriverInstanceActor</c> detaches its handler while it reconnects and re-subscribes on every
///         Connected entry, so a transition that happened while nobody listened stays pending and is delivered on
///         the re-subscribe — and an unchanged condition is NOT re-raised there, which would un-acknowledge it.
///     </para>
///     <para>
///         <b>Raised under <see cref="_alarmLock"/>.</b> Two threads emit — the <c>/sample</c> pump and a
///         subscribing caller — and the diff + record + raise must be one step or a Raise and its Clear could
///         reach the server reversed. The only production handler is <c>DriverInstanceActor</c>'s, a
///         non-blocking <c>Tell</c>. <see cref="_alarmLock"/> is never taken while holding
///         <see cref="_subscriptionLock"/> or the lifecycle semaphore, and never takes either.
///     </para>
///     <para>
///         <b>Comms loss emits nothing here</b>, matching every other native source: a driver that cannot reach
///         its Agent has nothing new to say, and the host annotates every condition it owns
///         <c>BadNoCommunication</c> on the connectivity edge (#477), keeping each one's Active state. On recovery
///         the re-baselined <c>/current</c> is diffed like any other document.
///     </para>
/// </remarks>
public sealed partial class MTConnectDriver : IAlarmSource
{
    /// <summary>Serializes condition-alarm diffing and delivery. See the class remarks.</summary>
    private readonly Lock _alarmLock = new();

    /// <summary>
    ///     ConditionId (RawPath) → the state the server was last told. Guarded by <see cref="_alarmLock"/>.
    ///     Survives re-subscribes and lifecycle restarts on purpose — it describes the SERVER's condition node,
    ///     which outlives both.
    /// </summary>
    private readonly Dictionary<string, MTConnectConditionState> _deliveredConditions = new(StringComparer.Ordinal);

    /// <summary>The CONDITION tracker for the current session; replaced alongside the observation index.</summary>
    private MTConnectConditionTracker _conditions = new();

    /// <summary>
    ///     The one live alarm subscription, or <c>null</c>. Written under <see cref="_subscriptionLock"/> (it
    ///     keeps the shared <c>/sample</c> pump alive); read with <see cref="Volatile.Read{T}"/> elsewhere.
    /// </summary>
    private AlarmSubscriptionState? _alarmSubscription;

    private long _nextAlarmSubscriptionId;

    /// <inheritdoc/>
    public event EventHandler<AlarmEventArgs>? OnAlarmEvent;

    /// <summary>The condition tracker the pump folds CONDITION observations into. Exposed for tests.</summary>
    internal MTConnectConditionTracker ConditionTracker => Volatile.Read(ref _conditions);

    /// <inheritdoc/>
    /// <remarks>
    ///     Collapses to ONE live subscription (the latest), like the Galaxy and FOCAS sources: the owning
    ///     <c>DriverInstanceActor</c> is the only caller and re-subscribes on every Connected entry. Starts the
    ///     shared <c>/sample</c> pump when nothing else had (a driver whose only authored tags are alarms), and
    ///     immediately delivers the current state of every subscribed condition that differs from what the server
    ///     was last told — the re-establish step after a restart or a reconnect.
    /// </remarks>
    public Task<IAlarmSubscriptionHandle> SubscribeAlarmsAsync(
        IReadOnlyList<string> sourceNodeIds, CancellationToken cancellationToken)
    {
        ArgumentNullException.ThrowIfNull(sourceNodeIds);

        var references = new HashSet<string>(StringComparer.Ordinal);
        foreach (var reference in sourceNodeIds)
        {
            if (!string.IsNullOrWhiteSpace(reference)) references.Add(reference);
        }

        var handle = new MTConnectAlarmSubscriptionHandle(Interlocked.Increment(ref _nextAlarmSubscriptionId));

        lock (_subscriptionLock)
        {
            Volatile.Write(ref _alarmSubscription, new AlarmSubscriptionState(handle, references));
            StartSampleStreamCore(republishOnStart: false);
        }

        lock (_alarmLock)
        {
            // A condition that left the set has its node torn down by the server; if it comes back it is a NEW
            // node that has been told nothing, so forget what its predecessor was told.
            foreach (var stale in _deliveredConditions.Keys.Where(k => !references.Contains(k)).ToList())
            {
                _deliveredConditions.Remove(stale);
            }
        }

        EmitConditionTransitions(touchedDataItemIds: null, resync: true);

        _logger.LogDebug(
            "MTConnect driver {DriverInstanceId} opened alarm subscription {SubscriptionId} over {ConditionCount} condition(s).",
            _driverInstanceId, handle.DiagnosticId, references.Count);

        return Task.FromResult<IAlarmSubscriptionHandle>(handle);
    }

    /// <inheritdoc/>
    /// <remarks>An unknown or superseded handle is a no-op, matching <see cref="UnsubscribeAsync"/>.</remarks>
    public async Task UnsubscribeAlarmsAsync(IAlarmSubscriptionHandle handle, CancellationToken cancellationToken)
    {
        ArgumentNullException.ThrowIfNull(handle);

        bool stopStream;
        lock (_subscriptionLock)
        {
            if (Volatile.Read(ref _alarmSubscription) is not { } live || !ReferenceEquals(live.Handle, handle))
            {
                return;
            }

            Volatile.Write(ref _alarmSubscription, null);
            stopStream = !HasSubscribedReferencesCore();
        }

        if (stopStream)
        {
            // Outside the lock — see UnsubscribeAsync.
            await StopSampleStreamAsync(cancellationToken).ConfigureAwait(false);
        }
    }

    /// <inheritdoc/>
    /// <remarks>
    ///     MTConnect has no acknowledgement — an Agent's conditions clear when the machine says so. The Part 9
    ///     Acknowledge (and AddComment) is applied server-side; this is the no-op success the host routes it to,
    ///     so nothing is sent to the Agent and capability negotiation still succeeds. Same posture as FOCAS.
    /// </remarks>
    public Task AcknowledgeAsync(
        IReadOnlyList<AlarmAcknowledgeRequest> acknowledgements, CancellationToken cancellationToken)
    {
        ArgumentNullException.ThrowIfNull(acknowledgements);

        return Task.CompletedTask;
    }

    /// <summary>Whether the alarm subscription names any condition. <b>Caller must hold <see cref="_subscriptionLock"/>.</b></summary>
    private bool HasAlarmReferencesCore() => Volatile.Read(ref _alarmSubscription) is { References.Count: > 0 };

    /// <summary>
    ///     Diffs the subscribed conditions against what the server was last told and raises one event per
    ///     condition that moved. Never throws: a faulting handler is logged and the next condition still goes.
    /// </summary>
    /// <param name="touchedDataItemIds">
    ///     Only conditions bound to these DataItems are evaluated; <c>null</c> evaluates every subscribed one.
    /// </param>
    /// <param name="resync">Whether this is a (re)subscribe — see <see cref="MTConnectConditionAlarmMapper"/>.</param>
    private void EmitConditionTransitions(IReadOnlySet<string>? touchedDataItemIds, bool resync)
    {
        if (touchedDataItemIds is { Count: 0 }) return;

        var subscription = Volatile.Read(ref _alarmSubscription);
        if (subscription is not { References.Count: > 0 }) return;

        lock (_alarmLock)
        {
            // Nobody to tell (the host detached while it reconnects): record nothing, so the re-subscribe that
            // follows still sees the difference and delivers it.
            var handlers = OnAlarmEvent;
            if (handlers is null) return;

            var binding = Volatile.Read(ref _binding);
            var tracker = Volatile.Read(ref _conditions);

            foreach (var conditionId in subscription.References)
            {
                var dataItemId = binding.ToDataItemId(conditionId);
                if (touchedDataItemIds is not null && !touchedDataItemIds.Contains(dataItemId)) continue;

                var current = tracker.Get(dataItemId);
                if (current is null) continue;

                _deliveredConditions.TryGetValue(conditionId, out var delivered);
                var evt = MTConnectConditionAlarmMapper.Transition(
                    delivered, current, conditionId, dataItemId, subscription.Handle, resync);
                _deliveredConditions[conditionId] = current;

                if (evt is null) continue;

                foreach (var registration in handlers.GetInvocationList())
                {
                    try
                    {
                        ((EventHandler<AlarmEventArgs>)registration).Invoke(this, evt);
                    }
                    catch (Exception ex)
                    {
                        _logger.LogWarning(
                            ex,
                            "MTConnect driver {DriverInstanceId} caught a fault from an alarm subscriber for {ConditionId}; delivery continues.",
                            _driverInstanceId, conditionId);
                    }
                }
            }
        }
    }

    /// <summary>The live alarm subscription: its handle and the ConditionIds (RawPaths) it names.</summary>
    private sealed record AlarmSubscriptionState(MTConnectAlarmSubscriptionHandle Handle, HashSet<string> References);
}

/// <summary>Handle returned by <see cref="MTConnectDriver.SubscribeAlarmsAsync"/>.</summary>
/// <param name="Id">The driver-local subscription number.</param>
public sealed record MTConnectAlarmSubscriptionHandle(long Id) : IAlarmSubscriptionHandle
{
    /// <inheritdoc />
    public string DiagnosticId => $"mtconnect-alarm-sub-{Id}";
}