using System.Collections.Concurrent;
using System.Collections.Immutable;

namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Runtime;

/// <summary>
///     Bookkeeping for live subscriptions. Maps each driver-issued <c>SubscriptionId</c> to the
///     set of (full-reference, gw item-handle) pairs the gateway returned, and maintains the
///     reverse map (item-handle → set of driver subscriptions) so the
///     <see cref="EventPump"/> can fan out a single OnDataChange event to every driver
///     subscription that includes the changed tag.
/// </summary>
/// <remarks>
///     A tag may legitimately appear in multiple driver subscriptions (separate clients or
///     OPC UA monitored items observing the same Galaxy attribute). Using a single shared
///     gw subscription per session and fanning out on the driver side keeps the gateway's
///     work bounded; the reverse map is the fan-out index.
/// </remarks>
internal sealed class SubscriptionRegistry
{
    private readonly ConcurrentDictionary<long, SubscriptionEntry> _bySubscriptionId = new();
    // Driver.Galaxy-012: use ImmutableHashSet<long> for the reverse map so removal is
    // O(log n) instead of "rebuild the entire ConcurrentBag from a LINQ filter on every
    // unsubscribe"; reads are lock-free because the immutable snapshot is published
    // atomically via ConcurrentDictionary AddOrUpdate.
    private readonly ConcurrentDictionary<int, ImmutableHashSet<long>> _subscribersByItemHandle = new();
    // Forward index for the Galaxy writer: fullRef (case-insensitive) → live item handle.
    // Maintained in lock-step with _subscribersByItemHandle; entries are cleaned up when
    // the last subscriber for a handle is removed, and TryResolveItemHandle guards against
    // stale entries by cross-checking _subscribersByItemHandle at read time.
    private readonly ConcurrentDictionary<string, int> _itemHandleByFullRef =
        new(StringComparer.OrdinalIgnoreCase);
    private long _nextSubscriptionId;

    /// <summary>Gets the number of tracked subscriptions.</summary>
    public int TrackedSubscriptionCount => _bySubscriptionId.Count;
    /// <summary>Gets the number of tracked item handles.</summary>
    public int TrackedItemHandleCount => _subscribersByItemHandle.Count;

    /// <summary>Allocate a fresh subscription id. Monotonic; unique per registry lifetime.</summary>
    public long NextSubscriptionId() => Interlocked.Increment(ref _nextSubscriptionId);

    /// <summary>
    ///     Register a subscription and the per-tag item handles the gateway returned for it.
    ///     Failed tags (item handle = 0 or negative) are stored anyway so unsubscribe can
    ///     emit per-tag UnsubscribeBulk for the ones that did succeed.
    /// </summary>
    /// <param name="subscriptionId">The subscription identifier.</param>
    /// <param name="bindings">The tag bindings for the subscription.</param>
    public void Register(long subscriptionId, IReadOnlyList<TagBinding> bindings)
    {
        var entry = new SubscriptionEntry(subscriptionId, bindings);
        _bySubscriptionId[subscriptionId] = entry;
        foreach (var binding in bindings)
        {
            if (binding.ItemHandle <= 0) continue; // failed gw subscribe — no events expected
            _subscribersByItemHandle.AddOrUpdate(
                binding.ItemHandle,
                _ => [subscriptionId],
                (_, set) => set.Add(subscriptionId));
            _itemHandleByFullRef[binding.FullReference] = binding.ItemHandle;
        }
    }

    /// <summary>
    ///     Remove a subscription. Returns the bindings the caller should pass to
    ///     <c>UnsubscribeBulkAsync</c>; null when the id was never registered.
    /// </summary>
    /// <param name="subscriptionId">The subscription identifier.</param>
    /// <returns>The bindings for the subscription, or null if not found.</returns>
    public IReadOnlyList<TagBinding>? Remove(long subscriptionId)
    {
        if (!_bySubscriptionId.TryRemove(subscriptionId, out var entry)) return null;

        foreach (var binding in entry.Bindings)
        {
            if (binding.ItemHandle <= 0) continue;
            // Driver.Galaxy-012: ImmutableHashSet.Remove is O(log n) and the result is
            // published atomically — no need to rebuild from a LINQ filter.
            if (!_subscribersByItemHandle.TryGetValue(binding.ItemHandle, out var set)) continue;
            var remaining = set.Remove(subscriptionId);
            if (remaining.IsEmpty)
            {
                _subscribersByItemHandle.TryRemove(binding.ItemHandle, out _);
                // Clean up the forward index only when this handle is still the current
                // mapping — a concurrent re-add for the same ref must not be clobbered.
                if (_itemHandleByFullRef.TryGetValue(binding.FullReference, out var fwd) && fwd == binding.ItemHandle)
                    _itemHandleByFullRef.TryRemove(binding.FullReference, out _);
            }
            else _subscribersByItemHandle[binding.ItemHandle] = remaining;
        }

        return entry.Bindings;
    }

    /// <summary>
    ///     Look up the (subscription id, full reference) pairs that should receive an
    ///     OnDataChange for the given gw item handle. Returns empty when nobody subscribes.
    /// </summary>
    /// <remarks>
    ///     Driver.Galaxy-012: O(1) per subscriber via the per-entry
    ///     <c>FullRefByItemHandle</c> index, rather than a <c>FirstOrDefault</c> linear
    ///     scan of the binding list. At 50k tags / 1Hz this turns each dispatch from a
    ///     50k-element scan into a single dictionary lookup.
    /// </remarks>
    /// <param name="itemHandle">The gateway item handle.</param>
    /// <returns>A list of subscription and reference pairs for the item handle.</returns>
    public IReadOnlyList<(long SubscriptionId, string FullReference)> ResolveSubscribers(int itemHandle)
    {
        if (!_subscribersByItemHandle.TryGetValue(itemHandle, out var bag)) return [];

        // Each subscription may include the tag once. Walk every active subscription that
        // claims this handle and pull the full ref from its index in O(1).
        var result = new List<(long, string)>();
        foreach (var subId in bag.Distinct())
        {
            if (!_bySubscriptionId.TryGetValue(subId, out var entry)) continue;
            if (entry.FullRefByItemHandle.TryGetValue(itemHandle, out var fullRef))
                result.Add((subId, fullRef));
        }
        return result;
    }

    /// <summary>
    ///     Resolve the live MXAccess item handle a current subscription holds for <paramref name="fullRef"/>,
    ///     or null when no live subscription covers it. The Galaxy writer borrows this handle to skip a
    ///     redundant AddItem round-trip on the first write to an already-subscribed tag. Guarded by the
    ///     authoritative live-handle set (<c>_subscribersByItemHandle</c>) so a stale forward-map entry
    ///     can never hand out a dead handle.
    /// </summary>
    /// <param name="fullRef">The dotted tag full reference (e.g. <c>TestMachine_002.TestFloat</c>).</param>
    /// <returns>The live item handle, or null when none is currently subscribed.</returns>
    public int? TryResolveItemHandle(string fullRef)
    {
        if (fullRef is null) return null;
        if (_itemHandleByFullRef.TryGetValue(fullRef, out var handle)
            && _subscribersByItemHandle.ContainsKey(handle))
            return handle;
        return null;
    }

    /// <summary>Snapshot every active binding for diagnostic output.</summary>
    public IReadOnlyList<TagBinding> SnapshotAllBindings() =>
        [.. _bySubscriptionId.Values.SelectMany(entry => entry.Bindings)];

    /// <summary>
    ///     Snapshot every active subscription with its bindings, grouped by subscription id.
    ///     Used by the reconnect replay path so it can re-issue SubscribeBulk per subscription
    ///     and then <see cref="Rebind"/> each one with the post-reconnect item handles.
    /// </summary>
    public IReadOnlyList<(long SubscriptionId, IReadOnlyList<TagBinding> Bindings)> SnapshotEntries() =>
        [.. _bySubscriptionId.Values.Select(entry => (entry.SubscriptionId, entry.Bindings))];

    /// <summary>
    ///     Replace an existing subscription's bindings with the item handles a post-reconnect
    ///     SubscribeBulk returned, rebuilding the reverse fan-out map so events on the new
    ///     handles dispatch and the now-dead pre-reconnect handles are dropped. No-op when the
    ///     subscription id is unknown (it was unsubscribed during the reconnect window).
    /// </summary>
    /// <param name="subscriptionId">The subscription identifier.</param>
    /// <param name="newBindings">The new tag bindings after reconnection.</param>
    public void Rebind(long subscriptionId, IReadOnlyList<TagBinding> newBindings)
    {
        if (!_bySubscriptionId.TryGetValue(subscriptionId, out var oldEntry)) return;

        // Drop this subscription from every reverse-map set it currently appears in. The
        // pre-reconnect item handles are stale once the gw re-issues fresh ones.
        // Driver.Galaxy-012: ImmutableHashSet.Remove is O(log n) — no LINQ rebuild.
        foreach (var binding in oldEntry.Bindings)
        {
            if (binding.ItemHandle <= 0) continue;
            if (!_subscribersByItemHandle.TryGetValue(binding.ItemHandle, out var set)) continue;
            var remaining = set.Remove(subscriptionId);
            if (remaining.IsEmpty)
            {
                _subscribersByItemHandle.TryRemove(binding.ItemHandle, out _);
                // Clean up the forward index only when this handle is still the current
                // mapping — the add loop below will overwrite it with the fresh handle.
                if (_itemHandleByFullRef.TryGetValue(binding.FullReference, out var fwd) && fwd == binding.ItemHandle)
                    _itemHandleByFullRef.TryRemove(binding.FullReference, out _);
            }
            else _subscribersByItemHandle[binding.ItemHandle] = remaining;
        }

        _bySubscriptionId[subscriptionId] = new SubscriptionEntry(subscriptionId, newBindings);
        foreach (var binding in newBindings)
        {
            if (binding.ItemHandle <= 0) continue; // failed gw subscribe — no events expected
            _subscribersByItemHandle.AddOrUpdate(
                binding.ItemHandle,
                _ => [subscriptionId],
                (_, set) => set.Add(subscriptionId));
            _itemHandleByFullRef[binding.FullReference] = binding.ItemHandle;
        }
    }

    /// <summary>
    ///     Per-subscription bookkeeping. <see cref="FullRefByItemHandle"/> is an index
    ///     over <see cref="Bindings"/> keyed by item handle so <c>ResolveSubscribers</c>
    ///     is O(1) per subscriber instead of a linear scan of every binding
    ///     (Driver.Galaxy-012). Failed bindings (item handle ≤ 0) are excluded from the
    ///     index because the EventPump only dispatches for positive handles.
    /// </summary>
    /// <summary>Per-subscription bookkeeping entry.</summary>
    private sealed class SubscriptionEntry
    {
        /// <summary>Gets the subscription identifier.</summary>
        public long SubscriptionId { get; }
        /// <summary>Gets the tag bindings for the subscription.</summary>
        public IReadOnlyList<TagBinding> Bindings { get; }
        /// <summary>Gets the index of full references by item handle.</summary>
        public IReadOnlyDictionary<int, string> FullRefByItemHandle { get; }

        /// <summary>Initializes a new subscription entry.</summary>
        /// <param name="subscriptionId">The subscription identifier.</param>
        /// <param name="bindings">The tag bindings for the subscription.</param>
        public SubscriptionEntry(long subscriptionId, IReadOnlyList<TagBinding> bindings)
        {
            SubscriptionId = subscriptionId;
            Bindings = bindings;
            var index = new Dictionary<int, string>(bindings.Count);
            foreach (var binding in bindings)
            {
                if (binding.ItemHandle <= 0) continue; // failed gw subscribe — no events expected
                // Last-write-wins on duplicates; the driver doesn't double-register a handle
                // within a single subscription, but be defensive.
                index[binding.ItemHandle] = binding.FullReference;
            }
            FullRefByItemHandle = index;
        }
    }
}

/// <summary>
///     One (full reference, gw item handle) pair returned by SubscribeBulk. Item handle is
///     zero or negative when the gateway rejected this individual tag (bad name, duplicate);
///     the registry keeps the binding so the caller can surface a per-tag failure status.
/// </summary>
internal sealed record TagBinding(string FullReference, int ItemHandle);