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 * OPC Foundation MIT License 1.00
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using System;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;

namespace Opc.Ua.Client.Subscriptions.Fakes
{
    /// <summary>
    /// Hand-rolled fake for <see cref="IMessageAckQueue"/>. Records every
    /// invocation and lets tests override return behaviour via callback
    /// fields. Replaces <c>Mock&lt;IMessageAckQueue&gt;</c>.
    /// </summary>
    internal sealed class FakeMessageAckQueue : IMessageAckQueue
    {
        /// <summary>
        /// An owned snapshot of queued acknowledgements, captured under the queue's mutation lock.
        /// </summary>
        public List<SubscriptionAcknowledgement> QueuedAcks
        {
            get
            {
                lock (m_lock)
                {
                    return [.. m_queuedAcks];
                }
            }
        }

        public List<uint> CompletedSubscriptions { get; } = [];

        /// <summary>
        /// Instances passed to <see cref="CompleteAsync"/>, so tests can
        /// assert the subscription is retired by identity and not only by
        /// the server assigned id.
        /// </summary>
        public List<IMessageProcessor> CompletedProcessors { get; } = [];
        public int UpdateCalls { get; private set; }
        public int PublishingQuiescenceCalls { get; private set; }

        /// <summary>
        /// Optional override for <see cref="QueueAsync"/>. If null, returns
        /// completed.
        /// </summary>
        public Func<SubscriptionAcknowledgement, CancellationToken, ValueTask>? OnQueueAsync { get; set; }

        /// <summary>
        /// Optional override for <see cref="CompleteAsync"/>. If null,
        /// returns completed.
        /// </summary>
        public Func<uint, CancellationToken, ValueTask>? OnCompleteAsync { get; set; }

        /// <summary>
        /// Records an acknowledgement as if a processor had queued it.
        /// </summary>
        public ValueTask QueueAsync(SubscriptionAcknowledgement ack,
            CancellationToken ct = default)
        {
            return QueueAsync(null, 0, ack, ct);
        }

        /// <inheritdoc/>
        public ValueTask QueueAsync(IMessageProcessor? owner, long generation,
            SubscriptionAcknowledgement ack, CancellationToken ct = default)
        {
            lock (m_lock)
            {
                m_queuedAcks.Add(ack);
                m_queuedGenerations.Add(generation);
            }
            return OnQueueAsync?.Invoke(ack, ct) ?? default;
        }

        /// <summary>
        /// The sequence numbers whose acknowledgement was tracked before delivery.
        /// </summary>
        public List<uint> TrackedSequenceNumbers
        {
            get
            {
                lock (m_lock)
                {
                    return [.. m_tracked];
                }
            }
        }

        /// <inheritdoc/>
        public void TrackAcknowledgement(IMessageProcessor owner, long generation, uint sequenceNumber)
        {
            lock (m_lock)
            {
                m_tracked.Add(sequenceNumber);
            }
        }

        /// <summary>
        /// The owner generation each entry of <see cref="QueuedAcks"/> was stamped with.
        /// </summary>
        public List<long> QueuedAckGenerations
        {
            get
            {
                lock (m_lock)
                {
                    return [.. m_queuedGenerations];
                }
            }
        }

        /// <summary>
        /// Removes every queued acknowledgement for <paramref name="subscriptionId"/>.
        /// </summary>
        /// <returns>The number of removed acknowledgements.</returns>
        public int ClearQueuedAcks(uint subscriptionId)
        {
            lock (m_lock)
            {
                int removed = 0;
                for (int i = m_queuedAcks.Count - 1; i >= 0; i--)
                {
                    if (m_queuedAcks[i].SubscriptionId == subscriptionId)
                    {
                        m_queuedAcks.RemoveAt(i);
                        m_queuedGenerations.RemoveAt(i);
                        removed++;
                    }
                }
                return removed;
            }
        }

        /// <summary>
        /// Waits until at least <paramref name="count"/> acknowledgements have
        /// been queued. The processor under test dispatches a notification
        /// before it queues the acknowledgement for it, so a test that only
        /// waits for the dispatch races the enqueue that follows it.
        /// </summary>
        /// <param name="count">
        /// The number of queued acknowledgements to wait for.
        /// </param>
        /// <param name="timeoutMs">How long to wait before giving up.</param>
        /// <exception cref="TimeoutException">
        /// The acknowledgements did not arrive in time. Failing here rather
        /// than returning quietly keeps the diagnosis at the right level: a
        /// caller that went on to assert the count would only report the
        /// mismatch, which cannot distinguish an acknowledgement that was
        /// never queued from one that merely arrived late - and if that
        /// assertion is ever loosened, a silent return would hide the race
        /// this helper exists to close.
        /// </exception>
        public async Task WaitForQueuedAckAsync(int count, int timeoutMs = 5000)
        {
            const int kPollIntervalMs = 10;
            var timeout = TimeSpan.FromMilliseconds(timeoutMs);
            long start = TimeProvider.System.GetTimestamp();
            while (true)
            {
                int queued;
                lock (m_lock)
                {
                    queued = m_queuedAcks.Count;
                }
                if (queued >= count)
                {
                    return;
                }
                // Measure against a monotonic clock rather than accumulating
                // the poll interval: Task.Delay routinely overshoots, so
                // counting the requested interval silently stretches the
                // effective timeout well past what the caller asked for.
                if (TimeProvider.System.GetElapsedTime(start) >= timeout)
                {
                    throw new TimeoutException(
                        $"Expected at least {count} queued acknowledgement(s) within " +
                        $"{timeout.TotalSeconds:0.##}s but only {queued} arrived.");
                }
                await Task.Delay(kPollIntervalMs).ConfigureAwait(false);
            }
        }

        public ValueTask CompleteAsync(IMessageProcessor subscription,
            uint subscriptionId,
            CancellationToken ct = default)
        {
            CompletedProcessors.Add(subscription);
            CompletedSubscriptions.Add(subscriptionId);
            return OnCompleteAsync?.Invoke(subscriptionId, ct) ?? default;
        }

        public ValueTask RunWithPublishingQuiescedAsync(
            Func<CancellationToken, ValueTask> operation,
            CancellationToken ct = default)
        {
            PublishingQuiescenceCalls++;
            return operation(ct);
        }

        /// <summary>
        /// Optional override for <see cref="RunWithSessionAvailableAsync"/>,
        /// e.g. to simulate a session that is not available. If null, runs
        /// the operation.
        /// </summary>
        public Func<Func<CancellationToken, ValueTask>, CancellationToken, ValueTask>?
            OnRunWithSessionAvailableAsync
        { get; set; }

        public ValueTask RunWithSessionAvailableAsync(
            Func<CancellationToken, ValueTask> operation,
            CancellationToken ct = default)
        {
            return OnRunWithSessionAvailableAsync?.Invoke(operation, ct) ?? operation(ct);
        }

        /// <summary>
        /// Records the subscription ids dropped via
        /// <see cref="DropPendingForSubscription"/> so tests can assert
        /// stale-ack pruning happened during recovery.
        /// </summary>
        public List<uint> DroppedSubscriptions { get; } = [];

        public int DropPendingForSubscription(IMessageProcessor owner, uint subscriptionId)
        {
            // Like the manager, queued entries are not scanned: they fail send-time
            // validation because their owner's generation advanced.
            DroppedSubscriptions.Add(subscriptionId);
            return 0;
        }

        /// <summary>
        /// Ids reported through <see cref="OnSubscriptionCreated"/>.
        /// </summary>
        public List<uint> CreatedSubscriptions { get; } = [];

        /// <summary>
        /// Creation times reported through <see cref="OnSubscriptionCreated"/>.
        /// </summary>
        public List<DateTimeUtc> CreatedAt { get; } = [];

        /// <inheritdoc/>
        public void OnSubscriptionCreated(IMessageProcessor owner, uint subscriptionId,
            DateTimeUtc createdAt = default)
        {
            CreatedSubscriptions.Add(subscriptionId);
            CreatedAt.Add(createdAt);
        }

        /// <summary>
        /// Test-controlled answer for
        /// <see cref="IMessageAckQueue.OwnsSubscriptionId"/>. Defaults to
        /// <c>true</c>, i.e. the caller still owns the id.
        /// </summary>
        public Func<IMessageProcessor, uint, bool>? OnOwnsSubscriptionId { get; set; }

        /// <summary>
        /// Ids <see cref="OwnsSubscriptionId"/> was asked about.
        /// </summary>
        public List<uint> OwnershipChecks { get; } = [];

        public bool OwnsSubscriptionId(IMessageProcessor subscription, uint subscriptionId)
        {
            OwnershipChecks.Add(subscriptionId);
            return OnOwnsSubscriptionId?.Invoke(subscription, subscriptionId) ?? true;
        }

        public void Update()
        {
            UpdateCalls++;
        }

        /// <summary>
        /// Test-controlled value for the
        /// <see cref="IMessageAckQueue.PoolNotifications"/> setter the
        /// subscription dispatcher reads to decide whether to perform the
        /// pooled-notification reuse walk.
        /// </summary>
        public bool PoolNotifications { get; set; }

        /// <inheritdoc/>
        public CancellationToken ShutdownToken { get; set; }

        private readonly Lock m_lock = new();

        /// <summary>The queue's mutable storage, accessed only under <see cref="m_lock"/>.</summary>
        private readonly List<SubscriptionAcknowledgement> m_queuedAcks = [];
        private readonly List<long> m_queuedGenerations = [];
        private readonly List<uint> m_tracked = [];
    }
}