using Opc.Ua;
using Opc.Ua.Server;
using Shouldly;
using Xunit;
using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
using HistorianRead = ZB.MOM.WW.OtOpcUa.Core.Abstractions.HistoryReadResult;
using SdkHistoryReadResult = Opc.Ua.HistoryReadResult;

namespace ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests;

/// <summary>
///     R08:1.1 (#736): the paged-vs-unpaged fuzz for the server raw cap. Every paging change in this repo has had boundary
///     bugs that the hand-written cases missed (silent truncation, an endless cursor, an empty page between every value), so
///     the cap is proved the only way that has worked: random series (bursts, gaps, sub-millisecond stamps, tie clusters),
///     random windows (empty, on a tie, one step past a tie, between samples, forward / reverse / end-only, with and without
///     bounds) and random caps — <c>NumValuesPerNode</c> 0 / 1 / 2 / small / huge, the server cap 2 … 1000 or off, the
///     tie-cluster budget 3 / 8 / 65536 — paged through a REAL SDK node manager against a wire-faithful fake of each backend
///     (SqlServer: tick precision, native reverse, <c>TOP</c>; QuestDB: µs truncation of bounds AND stamps, native reverse,
///     <c>LIMIT</c>; Gateway: forward only, <c>[start, end)</c> after its own boundary-row filtering, walked back by the
///     server) that enforces the backend half of the cap exactly as the real sources do (a read asking for 0 or more than
///     the cap reads cap + 1 rows and is refused when the window holds more).
///     <para>
///         The oracle is the same node manager's one-shot answer with the cap OFF (<c>NumValuesPerNode = 0</c>, no limit),
///         which for a plain read is also checked against the series itself. The concatenation of the pages must equal it
///         exactly; the read must terminate within the number of values plus two pages; no page may exceed the cap; the fake
///         must never see a request for 0 or more than the cap (the "bounded by the same cap" invariant); and a
///         <c>BadTooManyOperations</c> is accepted only when a tie cluster (or, on the Gateway shape, one millisecond) is
///         larger than the budget that the cap and <c>MaxTieClusterOverfetch</c> leave — after an exact prefix. The run is
///         non-vacuous by construction: it asserts that empty windows, mid-tie windows, reverse reads, caps of 1 and 2 and
///         every shape occurred, and that most iterations were exact.
///     </para>
///     <para>
///         Iterations: <c>OTOPCUA_PAGING_FUZZ_ITERATIONS</c> (default 10 000, the lane gate). Seeds are stable, so a failure
///         names a reproducible seed.
///     </para>
/// </summary>
public sealed class NodeManagerHistoryReadRawCapFuzzTests(BoundsServerFixture fx) : IClassFixture<BoundsServerFixture>
{
    private static readonly DateTime T0 = new(2026, 10, 22, 12, 0, 0, DateTimeKind.Utc);

    /// <summary>The backend the fake is shaped like.</summary>
    public enum Shape { SqlServer, QuestDb, Gateway }

    private enum Direction { Forward, Reverse, EndOnly }

    private static int Iterations =>
        int.TryParse(Environment.GetEnvironmentVariable("OTOPCUA_PAGING_FUZZ_ITERATIONS"), out var n) && n > 0 ? n : 10_000;

    [Fact]
    public async Task Paged_equals_unpaged_on_random_series_windows_and_caps_and_always_terminates()
    {
        var iterations = Iterations;
        var counts = new Dictionary<string, int>();
        void Count(string key) => counts[key] = counts.GetValueOrDefault(key) + 1;

        await fx.Gate.WaitAsync(TestContext.Current.CancellationToken);
        try
        {
            for (var seed = 0; seed < iterations; seed++)
            {
                var rng = new Random(seed);
                var shape = (Shape)(seed % 3);
                var resolution = shape == Shape.QuestDb ? 10L : 1L;
                var series = RandomSeries(rng, resolution);
                var cap = rng.Next(10) switch { 0 => 0, 1 or 2 => 3, 3 => 4, 4 => 5, 5 => 7, 6 => 10, 7 => 16, 8 => 33, _ => 1000 };
                var n = rng.Next(9) switch { 0 or 1 => 0u, 2 or 3 => 1u, 4 or 5 => 2u, 6 => (uint)rng.Next(3, 20), 7 => (uint)rng.Next(20, 200), _ => 100_000u };
                var tieBudget = rng.Next(4) switch { 0 => 3, 1 => 8, _ => 65536 };
                var direction = rng.Next(5) switch { 0 => Direction.Reverse, 1 => Direction.EndOnly, _ => Direction.Forward };
                if (direction == Direction.EndOnly && n == 0) direction = Direction.Forward; // Part 11: one bound only with a cap
                var bounds = direction != Direction.EndOnly && rng.Next(4) == 0;

                var a = PickBoundary(rng, series, resolution);
                var b = PickBoundary(rng, series, resolution);
                var (lo, hi) = a <= b ? (a, b) : (b, a);
                var (start, end) = direction switch
                {
                    Direction.Forward => (lo, hi),
                    Direction.Reverse => (hi, lo),
                    _ => (DateTime.MinValue, hi),
                };
                var tag = $"seed {seed} {shape} cap {cap} N {n} tie {tieBudget} {direction} bounds {bounds} [{start:O} .. {end:O}]";

                // The walk-back (Gateway shape only: reverse / end-only reads, and the bound lookups of any bounded read) refuses
                // a millisecond holding more samples than its budget — computed up front, over the whole series, because the
                // walk and the lookups read around the window.
                var walks = shape == Shape.Gateway && (direction != Direction.Forward || bounds);
                var msMax = walks ? MillisecondMax(series) : 0;

                // The oracle: the one-shot answer with the cap off (and, for a plain read, the series itself).
                var truthSource = new CappedFake(series, shape, cap: 0);
                fx.Nm.HistoricalSourceForTest(truthSource, tieBudget);
                // An end-only read is paged backward from EndTime (the node manager flips it to a reverse read down to the
                // beginning of time); its oracle is the explicit reverse read down to the Unix epoch — the walk-back's floor,
                // and Part 11 (the SDK checks it) needs both bounds set when NumValuesPerNode = 0.
                var (truth, truthError, truthCp) = direction == Direction.EndOnly
                    ? Read(hi, DateTime.UnixEpoch, 0, bounds: false, cp: null)
                    : Read(start, end, 0, bounds, cp: null);
                if (truthError.StatusCode.Code == StatusCodes.BadTooManyOperations)
                {
                    // Only the walk-back can refuse an uncapped one-shot read, and only past its budget (max(1, M) with no cap).
                    (walks && msMax > Math.Max(1, tieBudget)).ShouldBeTrue($"the oracle read was refused: {tag} (ms max {msMax})");
                    Count("oracle refused");
                    continue;
                }
                truthError.StatusCode.Code.ShouldBe(StatusCodes.Good, $"oracle: {tag}");
                truthCp.ShouldBeNull($"oracle (no cap, N = 0) is one page: {tag}");
                if (!bounds)
                    Render(truth).ShouldBe(Render(Oracle(series, start, end, direction, resolution)), $"oracle vs series: {tag}");
                truthSource.OverCapRequests.ShouldBe(0, $"the uncapped oracle source must never be asked for more than it allows: {tag}");

                var windowTies = LargestTieCluster(truth);
                if (truth.Count == 0) Count("empty");
                if (windowTies >= 2) Count("ties in window");
                if (EndsOnTie(series, start, end, direction, resolution)) Count("window ends on a tie");
                if (direction == Direction.Reverse) Count("reverse");
                if (direction == Direction.EndOnly) Count("end-only");
                if (n == 1) Count("N=1");
                if (n == 2) Count("N=2");
                if (n == 0) Count("N=0");
                if (cap == 3) Count("cap=3");
                if (bounds) Count("bounds");
                Count(shape.ToString());

                // A boundary tie cluster as large as the cap leaves a resumed page no room: the server steps past it with one
                // empty page carrying a continuation point (the documented #694 ceiling behaviour) — the one empty page allowed.
                var oversizedClusters = cap > 0
                    ? truth.Where(v => !(v.Value is null && v.StatusCode.Code == StatusCodes.BadBoundNotFound))
                        .GroupBy(v => v.SourceTimestamp.Ticks).Count(g => g.Count() >= Math.Max(2, cap - 1))
                    : 0;

                // The read under test.
                var source = new CappedFake(series, shape, cap);
                fx.Nm.HistoricalSourceForTest(source, tieBudget);
                var pages = new List<IReadOnlyList<DataValue>>();
                var collected = new List<DataValue>();
                byte[]? cp = null;
                StatusCode status;
                do
                {
                    var (values, error, next) = Read(start, end, n, bounds, cp);
                    status = error.StatusCode;
                    if (!StatusCode.IsGood(status)) break;
                    pages.Add(values);
                    collected.AddRange(values);
                    cp = next;
                    pages.Count.ShouldBeLessThanOrEqualTo(truth.Count + 2 + oversizedClusters, $"paging must terminate: {tag} pages {Sizes(pages)}");
                }
                while (cp is not null);

                source.OverCapRequests.ShouldBe(0, $"the server asked the backend for 0 or more than the cap: {tag}");
                source.BadRequests.ShouldBe(0, $"a reverse window reached a forward-only backend: {tag}");

                if (status.Code == StatusCodes.BadTooManyOperations)
                {
                    // Legitimate only past the budget the cap leaves: a tie cluster larger than min(M, cap − 1), or on the
                    // Gateway shape a millisecond holding more than min(M, cap − 2) samples (the walk-back's budget).
                    var overfetchBound = cap > 0 ? Math.Min(Math.Max(1, tieBudget), Math.Max(1, cap - 1)) : Math.Max(1, tieBudget);
                    var walkBudget = cap > 0 ? Math.Max(1, Math.Min(tieBudget, cap - 2)) : Math.Max(1, tieBudget);
                    var legit = windowTies > overfetchBound || (walks && msMax > walkBudget);
                    legit.ShouldBeTrue($"refused with no tie cluster over {overfetchBound} (max {windowTies}; ms max {msMax} vs {walkBudget}): {tag}");
                    Render(collected).ShouldBe(Render(truth.Take(collected.Count)), $"the prefix before the refusal: {tag}");
                    Count("refused");
                    continue;
                }

                status.Code.ShouldBe(StatusCodes.Good, $"{tag}");
                Render(collected).ShouldBe(Render(truth), $"paged ≠ unpaged: {tag} pages {Sizes(pages)}");
                if (cap > 0)
                {
                    // A bounded read whose data fits one page carries its start bound, the data and its end bound: the node
                    // manager fits the end bound against the client's N but not against the server cap (follow-up F2), so that
                    // one page may hold cap + 1 values; every other page is at most the cap.
                    var bothBoundsOnOnePage = bounds && pages.Count == 1;
                    pages.ShouldAllBe(p => p.Count <= cap + (bothBoundsOnOnePage ? 1 : 0), $"a page over the cap: {tag} pages {Sizes(pages)}");
                    if (bothBoundsOnOnePage && pages[0].Count == cap + 1) Count("bounded single page at cap+1 (F2)");
                }
                if (n > 0) pages.ShouldAllBe(p => p.Count <= n, $"a page over N: {tag} pages {Sizes(pages)}");
                for (var i = 0; i < pages.Count - 1; i++)
                {
                    if (pages[i].Count > 0) continue;
                    // Allowed only as the step past a boundary tie cluster the cap cannot page within.
                    var previousLast = i > 0 && pages[i - 1].Count > 0 ? pages[i - 1][^1].SourceTimestamp.Ticks : (long?)null;
                    var clusterAtBoundary = previousLast is { } t ? truth.Count(v => v.SourceTimestamp.Ticks == t) : 0;
                    (cap > 0 && clusterAtBoundary >= Math.Max(2, cap - 1)).ShouldBeTrue(
                        $"an empty page with a continuation point (page {i}, boundary cluster {clusterAtBoundary}): {tag} pages {Sizes(pages)}");
                    Count("empty step over an oversized tie cluster");
                }
                if (pages.Count > 1) Count("paged");
                if (pages.Count > 1 && n == 0) Count("paged at N=0");
                // A cost guard against a read explosion, not a budget. A native shape reads once per page plus the bound
                // lookups. The Gateway shape's walk-back has its own cost profile — ~30 reads a page on a dense tag
                // (#691/#693), far more on a sparse series under a tiny tie budget (seed 320: 82 reads with NO cap, seed 6428:
                // 120+) — so it is judged RELATIVE to the same request with the cap off (the pre-#736 behaviour), per page:
                // the cap must not make the walk explode compared to before. The baseline asks for the page size the cap
                // imposes (N = cap − 1): an uncapped N = 0 reverse read used to be ONE int.MaxValue forward read — the very
                // memory vector the cap removes — so "what N = 0 cost before" is not the comparison.
                var allowedReads = 8 * pages.Count + 8;
                if (shape == Shape.Gateway)
                {
                    var baseline = new CappedFake(series, shape, cap: 0);
                    fx.Nm.HistoricalSourceForTest(baseline, tieBudget);
                    var baseN = cap > 0 && (n == 0 || n > (uint)cap) ? (uint)(cap - 1) : n;
                    var basePages = 0;
                    byte[]? baseCp = null;
                    do
                    {
                        var (_, baseError, next) = Read(start, end, baseN, bounds, baseCp);
                        if (!StatusCode.IsGood(baseError.StatusCode)) break;
                        basePages++;
                        baseCp = next;
                    }
                    while (baseCp is not null && basePages <= truth.Count + 2 + oversizedClusters);
                    allowedReads = 2 * baseline.Reads * Math.Max(1, pages.Count) / Math.Max(1, basePages) + 32 * pages.Count + 16;
                }
                source.Reads.ShouldBeLessThanOrEqualTo(allowedReads,
                    $"backend reads exploded: {source.Reads} for {pages.Count} pages (allowed {allowedReads}): {tag}");
                Count("exact");
            }
        }
        finally
        {
            fx.Nm.HistoricalSourceForTest(NullHistorianDataSource.Instance, 65536);
            fx.Gate.Release();
        }

        // Non-vacuous: every category the brief names occurred, and most runs were exact.
        var summary = string.Join(", ", counts.OrderBy(kv => kv.Key).Select(kv => $"{kv.Key}={kv.Value}"));
        foreach (var required in new[] { "empty", "ties in window", "window ends on a tie", "reverse", "end-only", "N=1", "N=2", "N=0", "cap=3", "bounds", "paged", "paged at N=0", "SqlServer", "QuestDb", "Gateway" })
            counts.GetValueOrDefault(required).ShouldBeGreaterThan(iterations / 200, $"{required} under-represented: {summary}");
        counts.GetValueOrDefault("exact").ShouldBeGreaterThan(iterations * 6 / 10, summary);
        TestContext.Current.TestOutputHelper?.WriteLine($"raw-cap fuzz: {iterations} iterations — {summary}");
    }

    // --- generators -----------------------------------------------------------------------------

    private static DateTime Ts(DataValueSnapshot s) => s.SourceTimestampUtc ?? DateTime.MinValue;

    private static DateTime Trunc(DateTime t, long resolution) => new(t.Ticks - t.Ticks % resolution, DateTimeKind.Utc);

    /// <summary>Random ascending series: bursts, gaps, sub-millisecond stamps and tie clusters; stamps stored at the resolution.</summary>
    private static List<DataValueSnapshot> RandomSeries(Random rng, long resolution)
    {
        var list = new List<DataValueSnapshot>();
        var t = T0.Ticks - rng.Next(0, 3) * TimeSpan.TicksPerHour;
        var count = rng.Next(1, 120);
        var v = 0;
        while (list.Count < count)
        {
            t += rng.Next(6) switch
            {
                0 => rng.Next(1, 10),                                  // sub-millisecond (sub-µs on QuestDB: collapses onto a tie)
                1 => rng.Next(1, 5) * TimeSpan.TicksPerMillisecond,    // dense
                2 => rng.Next(1, 3) * TimeSpan.TicksPerSecond,
                3 => rng.Next(1, 30) * TimeSpan.TicksPerMinute,        // a gap
                _ => rng.Next(1, 1000) * TimeSpan.TicksPerMillisecond,
            };
            var ties = rng.Next(7) == 0 ? rng.Next(2, 9) : 1;
            for (var k = 0; k < ties; k++)
            {
                var ts = Trunc(new DateTime(t, DateTimeKind.Utc), resolution);
                var status = rng.Next(12) == 0 ? StatusCodes.Bad : StatusCodes.Good;
                list.Add(new DataValueSnapshot(status == StatusCodes.Bad ? null : (double)v, status, ts, ts));
                v++;
            }
        }

        return list;
    }

    /// <summary>A window bound: before / after the data, exactly on a sample (a tie, maybe), one step past one, a tick before one, or between two.</summary>
    private static DateTime PickBoundary(Random rng, List<DataValueSnapshot> series, long resolution)
    {
        var first = Ts(series[0]);
        var last = Ts(series[^1]);
        var i = rng.Next(series.Count);
        var t = Ts(series[i]);
        switch (rng.Next(9))
        {
            case 0: return first.AddSeconds(-rng.Next(1, 100));
            case 1: return last.AddSeconds(rng.Next(1, 100));
            case 2:
            case 3: return t;
            case 4: return t.AddTicks(resolution);
            case 5: return t.AddTicks(-1); // sub-resolution: a microsecond backend truncates it back ONTO the sample
            case 6: return t.AddMilliseconds(1);
            default:
                var next = i + 1 < series.Count ? Ts(series[i + 1]) : t.AddSeconds(10);
                return new DateTime(t.Ticks + (long)((next.Ticks - t.Ticks) * rng.NextDouble()), DateTimeKind.Utc);
        }
    }

    /// <summary>The series' own answer for a plain (no bounds) read: forward <c>[start, end)</c>, point, reverse / end-only <c>(end, start]</c> newest first.</summary>
    private static List<DataValue> Oracle(List<DataValueSnapshot> series, DateTime start, DateTime end, Direction direction, long resolution)
    {
        var s = Trunc(start, resolution);
        var e = Trunc(end, resolution);
        IEnumerable<DataValueSnapshot> window = direction switch
        {
            // start == end is Part 11's point read whichever way it was generated: the node manager reads it forward.
            Direction.Forward or Direction.Reverse when s == e => series.Where(x => Ts(x) == s),
            Direction.Forward => series.Where(x => Ts(x) >= s && Ts(x) < e),
            Direction.Reverse => series.Where(x => Ts(x) > e && Ts(x) <= s).Reverse(),
            _ => series.Where(x => Ts(x) <= e).Reverse(),
        };
        return window.Select(x => new DataValue(new Variant(x.Value), new StatusCode(x.StatusCode), Ts(x), Ts(x))).ToList();
    }

    /// <summary>The largest tie cluster among the returned stored values (a bound placeholder is not a stored value).</summary>
    private static int LargestTieCluster(IReadOnlyList<DataValue> truth) =>
        truth.Where(v => !(v.Value is null && v.StatusCode.Code == StatusCodes.BadBoundNotFound))
            .GroupBy(v => v.SourceTimestamp.Ticks).Select(g => g.Count()).DefaultIfEmpty(0).Max();

    /// <summary>The most samples within one millisecond anywhere in the series (the walk-back's refusal rule).</summary>
    private static int MillisecondMax(List<DataValueSnapshot> series)
    {
        var msMax = 0;
        for (int i = 0, j = 0; i < series.Count; i++)
        {
            while (Ts(series[i]).Ticks - Ts(series[j]).Ticks >= TimeSpan.TicksPerMillisecond) j++;
            msMax = Math.Max(msMax, i - j + 1);
        }

        return msMax;
    }

    private static bool EndsOnTie(List<DataValueSnapshot> series, DateTime start, DateTime end, Direction direction, long resolution)
    {
        var edge = Trunc(direction == Direction.Forward ? end : direction == Direction.Reverse ? start : end, resolution);
        return series.Count(x => Ts(x) == edge) >= 2;
    }

    // --- helpers ----------------------------------------------------------------------------------

    private static string Sizes(IEnumerable<IReadOnlyList<DataValue>> pages) => "[" + string.Join(",", pages.Select(p => p.Count)) + "]";

    private static string Render(IEnumerable<DataValue> values) =>
        string.Join(" | ", values.Select(v => $"{v.SourceTimestamp.Ticks}={v.Value ?? "null"}/{v.StatusCode.Code:X8}"));

    private (IReadOnlyList<DataValue> Values, ServiceResult Error, byte[]? Cp) Read(DateTime start, DateTime end, uint max, bool bounds, byte[]? cp)
    {
        var context = new OperationContext(new RequestHeader(), secureChannelContext: null, RequestType.HistoryRead, identity: null);
        var details = new ReadRawModifiedDetails
        {
            StartTime = start, EndTime = end, NumValuesPerNode = max, ReturnBounds = bounds, IsReadModified = false,
        };
        var nodesToRead = new List<HistoryReadValueId> { new() { NodeId = fx.NodeId, ContinuationPoint = cp } };
        var results = new List<SdkHistoryReadResult> { null! };
        var errors = new List<ServiceResult> { null! };
        fx.Nm.HistoryRead(context, details, TimestampsToReturn.Both, releaseContinuationPoints: false, nodesToRead, results, errors);

        var values = new List<DataValue>();
        if (results[0]?.HistoryData is { } ho && !ExtensionObject.IsNull(ho))
            values.AddRange(((HistoryData)ExtensionObject.ToEncodeable(ho)).DataValues);
        return (values, errors[0], results[0]?.ContinuationPoint);
    }

    /// <summary>
    ///     A series-backed historian shaped like one backend, enforcing the backend half of the server raw cap exactly as
    ///     the real data sources do: it reports the cap as <see cref="MaxRawValuesPerRead"/>; a read asking for 0 or more than
    ///     the cap reads cap + 1 rows and throws <see cref="HistoryReadLimitExceededException"/> when the window holds more
    ///     (never a truncated answer). It also records every such request (<see cref="OverCapRequests"/>) — the server must
    ///     never issue one — and every reverse window sent to the forward-only shape (<see cref="BadRequests"/>).
    /// </summary>
    private sealed class CappedFake(IReadOnlyList<DataValueSnapshot> series, Shape shape, int cap) : IHistorianDataSource
    {
        private readonly Lock _gate = new();

        public int Reads { get; private set; }
        public int OverCapRequests { get; private set; }
        public int BadRequests { get; private set; }

        public bool SupportsReverseRaw => shape != Shape.Gateway;

        public long TimestampResolutionTicks => shape == Shape.QuestDb ? 10 : 1;

        public int MaxRawValuesPerRead => cap;

        private DateTime Trunc(DateTime t) => NodeManagerHistoryReadRawCapFuzzTests.Trunc(t, TimestampResolutionTicks);

        public Task<HistorianRead> ReadRawAsync(
            string fullReference, DateTime startUtc, DateTime endUtc, uint maxValuesPerNode, CancellationToken cancellationToken)
        {
            var probe = cap > 0 && (maxValuesPerNode == 0 || maxValuesPerNode > (uint)cap);
            lock (_gate)
            {
                Reads++;
                if (probe) OverCapRequests++;
                if (shape == Shape.Gateway && startUtc > endUtc) BadRequests++;
            }

            if (shape == Shape.Gateway && startUtc > endUtc)
                throw new InvalidOperationException("start_time must be less than or equal to end_time. (InvalidArgument)");

            startUtc = Trunc(startUtc);
            endUtc = Trunc(endUtc);
            List<DataValueSnapshot> window;
            if (startUtc > endUtc) window = series.Where(s => Ts(s) > endUtc && Ts(s) <= startUtc).Reverse().ToList();
            else if (startUtc == endUtc) window = series.Where(s => Ts(s) == startUtc).ToList();
            else window = series.Where(s => Ts(s) >= startUtc && Ts(s) < endUtc).ToList();

            var top = probe ? cap + 1 : maxValuesPerNode == 0 ? int.MaxValue : (int)Math.Min(maxValuesPerNode, int.MaxValue);
            var page = window.Take(top).ToList();
            if (probe && page.Count > cap)
                throw new HistoryReadLimitExceededException(cap, $"more than MaxRawValuesPerRead ({cap}) samples");
            return Task.FromResult(new HistorianRead(page, null));
        }

        public Task<HistorianRead> ReadProcessedAsync(
            string fullReference, DateTime startUtc, DateTime endUtc, TimeSpan interval,
            HistoryAggregateType aggregate, CancellationToken cancellationToken) =>
            Task.FromResult(new HistorianRead(Array.Empty<DataValueSnapshot>(), null));

        public Task<HistorianRead> ReadAtTimeAsync(
            string fullReference, IReadOnlyList<DateTime> timestampsUtc, CancellationToken cancellationToken) =>
            Task.FromResult(new HistorianRead(Array.Empty<DataValueSnapshot>(), null));

        public Task<HistoricalEventsResult> ReadEventsAsync(
            string? sourceName, DateTime startUtc, DateTime endUtc, int maxEvents, CancellationToken cancellationToken) =>
            Task.FromResult(new HistoricalEventsResult(Array.Empty<HistoricalEvent>(), null));

        public HistorianHealthSnapshot GetHealthSnapshot() => NullHistorianDataSource.Instance.GetHealthSnapshot();

        public void Dispose() { }
    }
}

/// <summary>Test-only wiring shared by the paging fuzz: swap the source and the tie budget, with a fresh continuation store.</summary>
internal static class NodeManagerFuzzExtensions
{
    public static void HistoricalSourceForTest(this OtOpcUaNodeManager nm, IHistorianDataSource source, int tieBudget)
    {
        nm.HistoryContinuationStore = new InMemoryHistoryContinuationStore();
        nm.MaxTieClusterOverfetch = tieBudget;
        nm.HistorianDataSource = source;
    }
}