using S7.Net;
using Shouldly;
using Xunit;
using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
using S7NetDataType = global::S7.Net.DataType;

namespace ZB.MOM.WW.OtOpcUa.Driver.S7.Tests;

/// <summary>
///     Falsification gate for span-grouped S7 reads. Every tag must decode to the SAME value, the
///     same CLR box type and the same status code whether it was served by a wire request of its
///     own (the pre-existing per-tag path, reachable by setting
///     <c>SpanReadMaxGapBytes = -1</c>) or by a block read shared with its neighbours.
/// </summary>
/// <remarks>
///     <para>
///         The two drivers read the same in-memory PLC image through the same fake, so any
///         difference between the two result sets is a defect in the grouping path — there is no
///         other variable. Grouping is proven to have actually engaged by
///         <see cref="Grouping_issues_far_fewer_wire_requests_than_there_are_tags"/>: without that
///         assertion this whole file would pass vacuously if the planner ever produced only
///         leftovers, because "leftover" IS the per-tag path.
///     </para>
///     <para>
///         The fake serves <c>ReadBytesAsync</c> (block reads — the span path, plus the wide /
///         array / Timer-Counter tags that always read blocks) and <c>ReadAsync(string)</c> (the
///         narrow per-tag path) from ONE byte image per <c>(area, DB)</c>, decoding the latter
///         exactly as S7.Net does: <c>DBX</c> boxes a <c>bool</c>, <c>DBB</c> a <c>byte</c>,
///         <c>DBW</c> a <c>ushort</c>, <c>DBD</c> a <c>uint</c>.
///     </para>
/// </remarks>
[Trait("Category", "Unit")]
public sealed class S7SpanReadParityTests
{
    /// <summary>OPC UA <c>BadDeviceFailure</c> — what a device-layer read fault stamps.</summary>
    private const uint StatusBadDeviceFailure = 0x808B0000u;
    /// <summary>OPC UA <c>BadNodeIdUnknown</c> — what an unresolvable reference stamps.</summary>
    private const uint StatusBadNodeIdUnknown = 0x80340000u;
    /// <summary>
    ///     OPC UA <c>BadCommunicationError</c> — what the driver's catch-all failure mapping stamps,
    ///     which is where a malformed multi-var answer (an <c>InvalidDataException</c>) lands.
    /// </summary>
    private const uint StatusBadCommunicationError = 0x80050000u;

    // ── The mixed tag set ────────────────────────────────────────────────────────────────

    /// <summary>
    ///     A deliberately awkward batch: two Bools sharing one byte (bit 0 and bit 7), a Byte, a
    ///     word pair, three dwords behind a small hole, a wide Int64, an S7 STRING, a UInt16 array,
    ///     a far-away word past the gap budget, a second AREA, and a Counter (whose offset is a
    ///     device number, so it can never join a byte span).
    /// </summary>
    private static IReadOnlyList<RawTagEntry> MixedTags() => S7RawTags.Entries(
        new S7TagDefinition("Bit0", "DB1.DBX0.0", S7DataType.Bool, Writable: false),
        new S7TagDefinition("Bit7", "DB1.DBX0.7", S7DataType.Bool, Writable: false),
        new S7TagDefinition("Byte1", "DB1.DBB1", S7DataType.Byte, Writable: false),
        new S7TagDefinition("U16", "DB1.DBW2", S7DataType.UInt16, Writable: false),
        new S7TagDefinition("I16", "DB1.DBW4", S7DataType.Int16, Writable: false),
        new S7TagDefinition("U32", "DB1.DBD8", S7DataType.UInt32, Writable: false),
        new S7TagDefinition("I32", "DB1.DBD12", S7DataType.Int32, Writable: false),
        new S7TagDefinition("F32", "DB1.DBD16", S7DataType.Float32, Writable: false),
        new S7TagDefinition("I64", "DB1.DBB24", S7DataType.Int64, Writable: false),
        new S7TagDefinition("Str", "DB1.DBB36", S7DataType.String, Writable: false, StringLength: 10),
        new S7TagDefinition("Arr", "DB1.DBW60", S7DataType.UInt16, Writable: false, ArrayCount: 4),
        new S7TagDefinition("Far", "DB1.DBW200", S7DataType.UInt16, Writable: false),
        new S7TagDefinition("Mem", "MW0", S7DataType.UInt16, Writable: false),
        new S7TagDefinition("Cnt", "C1", S7DataType.Int32, Writable: false));

    /// <summary>Read references, in the order the batch presents them.</summary>
    private static readonly string[] MixedRefs =
    [
        "Bit0", "Bit7", "Byte1", "U16", "I16", "U32", "I32", "F32",
        "Str", "I64", "Arr", "Far", "Mem", "Cnt",
    ];

    /// <summary>Only the narrow, byte-addressed DB1 tags — every one of them lands in one span.</summary>
    private static IReadOnlyList<RawTagEntry> NarrowTags() => S7RawTags.Entries(
        new S7TagDefinition("A", "DB1.DBW0", S7DataType.UInt16, Writable: false),
        new S7TagDefinition("B", "DB1.DBW2", S7DataType.UInt16, Writable: false),
        new S7TagDefinition("C", "DB1.DBW4", S7DataType.UInt16, Writable: false));

    /// <summary>
    ///     <paramref name="count"/> narrow words spaced 8 bytes apart. With a gap budget below 6
    ///     none of them can merge, so every one becomes a single-member span the driver demotes to
    ///     the leftover path — which is exactly the population the multi-var batch serves.
    /// </summary>
    /// <param name="count">How many scattered words to author.</param>
    /// <returns>The raw-tag entries.</returns>
    private static IReadOnlyList<RawTagEntry> ScatteredWords(int count) => S7RawTags.Entries(
        [.. Enumerable.Range(0, count).Select(i =>
            new S7TagDefinition($"S{i}", $"DB1.DBW{i * 8}", S7DataType.UInt16, Writable: false))]);

    /// <summary>References for <see cref="ScatteredWords"/>, in authoring order.</summary>
    /// <param name="count">How many scattered words the batch reads.</param>
    /// <returns>The reference names.</returns>
    private static string[] ScatteredRefs(int count) => [.. Enumerable.Range(0, count).Select(i => $"S{i}")];

    /// <summary>
    ///     One of every narrow access width, spaced far enough apart that each is its own
    ///     single-member span — so the whole set reaches the multi-var pass and every box type
    ///     (<c>bool</c> / <c>byte</c> / <c>ushort</c> / <c>short</c> / <c>uint</c> / <c>int</c> /
    ///     <c>float</c>) is exercised through it.
    /// </summary>
    private static IReadOnlyList<RawTagEntry> ScatteredWidths() => S7RawTags.Entries(
        new S7TagDefinition("WBit", "DB1.DBX0.3", S7DataType.Bool, Writable: false),
        new S7TagDefinition("WByte", "DB1.DBB8", S7DataType.Byte, Writable: false),
        new S7TagDefinition("WU16", "DB1.DBW16", S7DataType.UInt16, Writable: false),
        new S7TagDefinition("WI16", "DB1.DBW24", S7DataType.Int16, Writable: false),
        new S7TagDefinition("WU32", "DB1.DBD32", S7DataType.UInt32, Writable: false),
        new S7TagDefinition("WI32", "DB1.DBD40", S7DataType.Int32, Writable: false),
        new S7TagDefinition("WF32", "DB1.DBD48", S7DataType.Float32, Writable: false),
        // A second AREA in the same batch — a multi-var request carries each item's own area/DB,
        // which is precisely what a block read cannot do.
        new S7TagDefinition("WMem", "MW0", S7DataType.UInt16, Writable: false));

    /// <summary>References for <see cref="ScatteredWidths"/>, in authoring order.</summary>
    private static readonly string[] ScatteredWidthRefs =
        ["WBit", "WByte", "WU16", "WI16", "WU32", "WI32", "WF32", "WMem"];

    // ── Parity ───────────────────────────────────────────────────────────────────────────

    /// <summary>
    ///     THE gate: span-grouped reads and per-tag reads produce identical snapshots — same
    ///     value, same box type, same status — for every tag in the mixed batch.
    /// </summary>
    [Fact]
    public async Task Span_grouped_reads_match_the_per_tag_path_snapshot_for_snapshot()
    {
        var images = Images();
        var ct = TestContext.Current.CancellationToken;

        var grouping = new ImageS7PlcFactory(images);
        var perTag = new ImageS7PlcFactory(images);

        using var groupedDriver = new S7Driver(Options(MixedTags(), maxGap: 16), "s7-grouped", grouping);
        using var perTagDriver = new S7Driver(Options(MixedTags(), maxGap: -1), "s7-per-tag", perTag);
        await groupedDriver.InitializeAsync("{}", ct);
        await perTagDriver.InitializeAsync("{}", ct);

        var grouped = await groupedDriver.ReadAsync(MixedRefs, ct);
        var individual = await perTagDriver.ReadAsync(MixedRefs, ct);

        grouped.Count.ShouldBe(MixedRefs.Length);
        for (var i = 0; i < MixedRefs.Length; i++)
        {
            var name = MixedRefs[i];
            grouped[i].StatusCode.ShouldBe(individual[i].StatusCode, $"status differs for '{name}'");
            grouped[i].StatusCode.ShouldBe(0u, $"'{name}' should have read Good");
            grouped[i].Value.ShouldNotBeNull($"'{name}' returned no value");
            grouped[i].Value!.GetType().ShouldBe(individual[i].Value!.GetType(), $"box type differs for '{name}'");
            grouped[i].Value.ShouldBe(individual[i].Value, $"value differs for '{name}'");
        }

        // The per-tag control really did take the per-tag path (no grouping happened there): its
        // only block reads are the four tags that ALWAYS read blocks (Int64, String, array, Counter).
        perTag.ReadBytesCalls.ShouldBe(4);
        perTag.StringReadCalls.ShouldBe(10);
    }

    /// <summary>
    ///     Proves grouping actually engaged. Without this the parity test above would pass
    ///     vacuously if the planner degenerated to all-leftovers, since a leftover IS a per-tag
    ///     read.
    /// </summary>
    /// <remarks>
    ///     14 tags collapse to 3 wire requests: ONE block read covering the eleven DB1 tags from
    ///     byte 0 to byte 68, the Counter's own unavoidable block read, and ONE multi-var request
    ///     carrying the two tags a span can't help — the far DB1 word (past the gap budget) and the
    ///     lone Memory word, which is in a different AREA entirely. Neither is worth a block read of
    ///     its own, and batching them together is something only a multi-var request can do.
    /// </remarks>
    [Fact]
    public async Task Grouping_issues_far_fewer_wire_requests_than_there_are_tags()
    {
        var ct = TestContext.Current.CancellationToken;
        var factory = new ImageS7PlcFactory(Images());
        using var driver = new S7Driver(Options(MixedTags(), maxGap: 16), "s7-wire-count", factory);
        await driver.InitializeAsync("{}", ct);

        _ = await driver.ReadAsync(MixedRefs, ct);

        (factory.ReadBytesCalls + factory.StringReadCalls + factory.MultiVarCalls)
            .ShouldBeLessThan(MixedRefs.Length);
        factory.ReadBytesCalls.ShouldBeLessThan(MixedRefs.Length);
        factory.ReadBytesCalls.ShouldBe(2);
        // NO per-address reads at all: the nine narrow tags inside the big span are served by its
        // block read, and the two that ended up alone in a span of their own are batched together
        // into a single multi-var request.
        factory.StringReadCalls.ShouldBe(0);
        factory.MultiVarCalls.ShouldBe(1);
    }

    // ── Failure parity ───────────────────────────────────────────────────────────────────

    /// <summary>
    ///     A failed span stamps EVERY one of its members with exactly the status the per-tag path
    ///     would have produced for that exception, and degrades health identically. A span read is
    ///     one wire request serving many tags, so a fault that used to affect one tag now affects
    ///     all of them — they must all be told the same thing.
    /// </summary>
    [Fact]
    public async Task A_span_read_failure_degrades_every_member_like_the_per_tag_path()
    {
        var ct = TestContext.Current.CancellationToken;
        // ErrorCode.ReadData with a message that is NOT access-denied ("not allowed") — a genuine
        // device-layer fault, which the driver maps to BadDeviceFailure + Degraded.
        var fault = new PlcException(ErrorCode.ReadData, "S7 CPU reported a device fault");

        var groupingFactory = new ImageS7PlcFactory(Images()) { ReadFault = fault };
        var perTagFactory = new ImageS7PlcFactory(Images()) { ReadFault = fault };

        using var groupedDriver = new S7Driver(Options(NarrowTags(), maxGap: 16), "s7-grouped-fail", groupingFactory);
        using var perTagDriver = new S7Driver(Options(NarrowTags(), maxGap: -1), "s7-per-tag-fail", perTagFactory);
        await groupedDriver.InitializeAsync("{}", ct);
        await perTagDriver.InitializeAsync("{}", ct);

        string[] refs = ["A", "B", "C"];
        var grouped = await groupedDriver.ReadAsync(refs, ct);
        var individual = await perTagDriver.ReadAsync(refs, ct);

        // Every member of the failed span — not just the first — is stamped Bad.
        grouped.Count.ShouldBe(3);
        grouped.ShouldAllBe(s => s.StatusCode == StatusBadDeviceFailure);
        grouped.ShouldAllBe(s => s.Value == null);
        grouped.Select(s => s.StatusCode).ShouldBe(individual.Select(s => s.StatusCode));

        groupedDriver.GetHealth().State.ShouldBe(DriverState.Degraded);
        groupedDriver.GetHealth().State.ShouldBe(perTagDriver.GetHealth().State);

        // One failed block read, not three — the span path fails once for the whole group.
        groupingFactory.ReadBytesCalls.ShouldBe(1);
    }

    /// <summary>
    ///     Unresolved references still stamp <c>BadNodeIdUnknown</c> and never enter a span, and
    ///     the resolvable tags around them still read Good.
    /// </summary>
    [Fact]
    public async Task An_unresolved_reference_is_still_BadNodeIdUnknown_under_grouping()
    {
        var ct = TestContext.Current.CancellationToken;
        var factory = new ImageS7PlcFactory(Images());
        using var driver = new S7Driver(Options(NarrowTags(), maxGap: 16), "s7-unknown", factory);
        await driver.InitializeAsync("{}", ct);

        var results = await driver.ReadAsync(["A", "NoSuchTag", "C"], ct);

        results[0].StatusCode.ShouldBe(0u);
        results[1].StatusCode.ShouldBe(StatusBadNodeIdUnknown);
        results[1].Value.ShouldBeNull();
        results[2].StatusCode.ShouldBe(0u);
    }

    // ── Multi-var leftovers ──────────────────────────────────────────────────────────────

    /// <summary>
    ///     The batchable leftovers ride ONE multi-var request per chunk of
    ///     <c>MultiVarReadMaxItems</c> and issue no per-address reads at all. 13 scattered words
    ///     that the span planner cannot merge used to cost 13 wire requests; they now cost 2.
    /// </summary>
    [Fact]
    public async Task Batchable_leftovers_ride_multi_var_requests_instead_of_one_read_each()
    {
        var ct = TestContext.Current.CancellationToken;
        var factory = new ImageS7PlcFactory(Images());
        using var driver = new S7Driver(Options(ScatteredWords(13), maxGap: 2), "s7-multivar", factory);
        await driver.InitializeAsync("{}", ct);

        var results = await driver.ReadAsync(ScatteredRefs(13), ct);

        results.ShouldAllBe(s => s.StatusCode == 0u);
        results.ShouldAllBe(s => s.Value != null);
        // ⌈13 / 12⌉ = 2 requests, and NOT ONE per-address read — the whole point of the batch.
        factory.MultiVarCalls.ShouldBe(2);
        factory.StringReadCalls.ShouldBe(0);
        factory.ReadBytesCalls.ShouldBe(0);
        // The chunking is a real split, not one oversized request plus an empty one.
        factory.MultiVarItems.Select(c => c.Count).ShouldBe([12, 1]);
    }

    /// <summary>
    ///     THE multi-var gate: a tag served by a multi-var request decodes to the same value, the
    ///     same CLR box type and the same status as the per-tag path reading the same image.
    /// </summary>
    [Fact]
    public async Task Multi_var_leftovers_match_the_per_tag_path_snapshot_for_snapshot()
    {
        var images = Images();
        var ct = TestContext.Current.CancellationToken;

        var batching = new ImageS7PlcFactory(images);
        var perTag = new ImageS7PlcFactory(images);

        using var batchingDriver = new S7Driver(Options(ScatteredWidths(), maxGap: 2), "s7-mv-parity", batching);
        using var perTagDriver = new S7Driver(Options(ScatteredWidths(), maxGap: -1), "s7-mv-control", perTag);
        await batchingDriver.InitializeAsync("{}", ct);
        await perTagDriver.InitializeAsync("{}", ct);

        var batched = await batchingDriver.ReadAsync(ScatteredWidthRefs, ct);
        var individual = await perTagDriver.ReadAsync(ScatteredWidthRefs, ct);

        for (var i = 0; i < ScatteredWidthRefs.Length; i++)
        {
            var name = ScatteredWidthRefs[i];
            batched[i].StatusCode.ShouldBe(0u, $"'{name}' should have read Good");
            batched[i].StatusCode.ShouldBe(individual[i].StatusCode, $"status differs for '{name}'");
            batched[i].Value.ShouldNotBeNull($"'{name}' returned no value");
            batched[i].Value!.GetType().ShouldBe(individual[i].Value!.GetType(), $"box type differs for '{name}'");
            batched[i].Value.ShouldBe(individual[i].Value, $"value differs for '{name}'");
        }

        // Proves the batching side really took the multi-var path (otherwise this file's parity
        // claim would be vacuous), and the control side really did not.
        batching.MultiVarCalls.ShouldBe(1);
        batching.StringReadCalls.ShouldBe(0);
        perTag.MultiVarCalls.ShouldBe(0);
        perTag.StringReadCalls.ShouldBe(ScatteredWidthRefs.Length);
    }

    /// <summary>
    ///     A multi-var request that faults falls back to the per-tag path for that chunk's tags and
    ///     still serves them Good. Batching is an optimisation: it must never be able to turn a tag
    ///     Bad that reading it one at a time would have served.
    /// </summary>
    [Fact]
    public async Task A_failing_multi_var_chunk_falls_back_to_per_tag_reads_and_still_serves_good()
    {
        var ct = TestContext.Current.CancellationToken;
        var factory = new ImageS7PlcFactory(Images())
        {
            // A device-layer fault — the shape that WOULD stamp BadDeviceFailure had it come from
            // the per-tag path. Through the multi-var pass it must cost nothing but a retry.
            MultiVarFault = new PlcException(ErrorCode.ReadData, "S7 CPU rejected the multi-var request"),
        };
        using var driver = new S7Driver(Options(ScatteredWords(5), maxGap: 2), "s7-mv-fallback", factory);
        await driver.InitializeAsync("{}", ct);

        var results = await driver.ReadAsync(ScatteredRefs(5), ct);

        results.ShouldAllBe(s => s.StatusCode == 0u);
        results.ShouldAllBe(s => s.Value != null);
        factory.MultiVarCalls.ShouldBe(1);
        // Every tag of the failed chunk was re-read individually.
        factory.StringReadCalls.ShouldBe(5);
        // And the failed batch attempt did NOT degrade the driver — nothing actually failed to read.
        driver.GetHealth().State.ShouldBe(DriverState.Healthy);
    }

    /// <summary>
    ///     Tags the per-tag path owns — Timer and Counter, whose offsets are device numbers rather
    ///     than byte addresses — are never handed to a multi-var request, even though they share the
    ///     leftover list with the narrow scalars that are.
    /// </summary>
    [Fact]
    public async Task Timer_and_counter_leftovers_never_enter_a_multi_var_request()
    {
        var ct = TestContext.Current.CancellationToken;
        var tags = S7RawTags.Entries(
            new S7TagDefinition("Tmr", "T2", S7DataType.Float64, Writable: false),
            new S7TagDefinition("Cnt", "C1", S7DataType.Int32, Writable: false),
            new S7TagDefinition("N0", "DB1.DBW0", S7DataType.UInt16, Writable: false),
            new S7TagDefinition("N1", "DB1.DBW100", S7DataType.UInt16, Writable: false));
        var factory = new ImageS7PlcFactory(Images());
        using var driver = new S7Driver(Options(tags, maxGap: 2), "s7-mv-partition", factory);
        await driver.InitializeAsync("{}", ct);

        var results = await driver.ReadAsync(["Tmr", "Cnt", "N0", "N1"], ct);

        results.ShouldAllBe(s => s.StatusCode == 0u);
        results[0].Value.ShouldBeOfType<double>();
        results[1].Value.ShouldBeOfType<int>();

        // One multi-var request carrying ONLY the two narrow words; the Timer and the Counter kept
        // their own block reads.
        factory.MultiVarCalls.ShouldBe(1);
        factory.MultiVarItems[0].Count.ShouldBe(2);
        factory.MultiVarItems[0].ShouldAllBe(d => d.DataType == S7NetDataType.DataBlock);
        factory.ReadBytesCalls.ShouldBe(2);
    }

    // ── Malformed multi-var answers ──────────────────────────────────────────────────────

    /// <summary>
    ///     ONE undecodable item in an otherwise-good multi-var chunk spoils only its own tag: its
    ///     chunk-mates keep the values the batch already carried, and NOBODY is re-read individually.
    ///     A per-ITEM decode fault is not a chunk fault — the whole-chunk fallback exists for a
    ///     failed REQUEST, and firing it here would re-read tags that already have good values.
    /// </summary>
    /// <remarks>
    ///     Health ends <c>Degraded</c>, NOT <c>Healthy</c>. This pin used to assert the opposite:
    ///     the decode loop stamped health per item in chunk order, so the good item after the bad
    ///     one re-stamped it and a pass with failures reported <c>Healthy</c> — last-writer-wins.
    ///     That was deliberately replaced (perf follow-ups round 3) by <b>worst-state-wins</b>: the
    ///     whole read pass accumulates its outcomes and writes health ONCE at the end, so a mixed
    ///     pass can no longer report Healthy. Only the driver-level health claim changed — the
    ///     per-tag statuses below are exactly as they were, because a per-ITEM decode fault still
    ///     spoils only its own tag.
    /// </remarks>
    [Fact]
    public async Task One_undecodable_multi_var_item_stamps_only_its_own_tag_Bad()
    {
        var ct = TestContext.Current.CancellationToken;
        var images = Images();
        var factory = new ImageS7PlcFactory(images) { NullItemIndex = 1 };
        using var driver = new S7Driver(Options(ScatteredWords(3), maxGap: 2), "s7-mv-item-fault", factory);
        await driver.InitializeAsync("{}", ct);

        var results = await driver.ReadAsync(ScatteredRefs(3), ct);

        // The one item the CPU answered with nothing.
        results[1].StatusCode.ShouldBe(StatusBadCommunicationError);
        results[1].Value.ShouldBeNull();

        // Its chunk-mates are untouched, values and all (S{i} lives at DB1.DBW{i * 8}).
        var db1 = images[(S7NetDataType.DataBlock, 1)];
        static ushort Word(byte[] image, int at) => (ushort)((image[at] << 8) | image[at + 1]);
        results[0].StatusCode.ShouldBe(0u);
        results[0].Value.ShouldBe(Word(db1, 0));
        results[2].StatusCode.ShouldBe(0u);
        results[2].Value.ShouldBe(Word(db1, 16));

        // One batch, and NO per-tag fallback at all — not even for the tag that failed to decode,
        // which would have failed identically on the per-tag path.
        factory.MultiVarCalls.ShouldBe(1);
        factory.StringReadCalls.ShouldBe(0);
        factory.ReadBytesCalls.ShouldBe(0);
        driver.GetHealth().State.ShouldBe(DriverState.Degraded);
    }

    // ── Worst-state-wins read-pass health ────────────────────────────────────────────────

    /// <summary>
    ///     A pass with BOTH failures and successes reports <c>Degraded</c> — the failure is not
    ///     erased by the later success — AND still advances <c>LastSuccessfulRead</c>, because
    ///     something genuinely was read.
    /// </summary>
    /// <remarks>
    ///     Advancing the timestamp on a Degraded pass is load-bearing, not cosmetic: the
    ///     comms-loss watchdog (CLAUDE.md §"Comms-loss detection") trips on non-Healthy state
    ///     <b>AND</b> a frozen <c>LastSuccessfulRead</c>. Several drivers latch Degraded off one
    ///     unreadable address, so freezing the timestamp on any non-Healthy pass would reconnect a
    ///     perfectly reachable PLC every 90 s forever.
    /// </remarks>
    [Fact]
    public async Task A_mixed_read_pass_reports_Degraded_and_still_advances_LastSuccessfulRead()
    {
        var ct = TestContext.Current.CancellationToken;
        var factory = new ImageS7PlcFactory(Images()) { NullItemIndex = 1 };
        using var driver = new S7Driver(Options(ScatteredWords(3), maxGap: 2), "s7-mixed-pass", factory);
        await driver.InitializeAsync("{}", ct);

        // The connect stamped a LastSuccessfulRead; the pass must move it PAST that value, so a
        // frozen timestamp is distinguishable from a merely non-null one.
        var before = driver.GetHealth().LastSuccessfulRead;
        before.ShouldNotBeNull();

        var results = await driver.ReadAsync(ScatteredRefs(3), ct);

        results[0].StatusCode.ShouldBe(0u);
        results[1].StatusCode.ShouldBe(StatusBadCommunicationError);
        results[2].StatusCode.ShouldBe(0u);

        var health = driver.GetHealth();
        health.State.ShouldBe(DriverState.Degraded, "a pass that had a failure must not report Healthy");
        health.LastSuccessfulRead.ShouldNotBeNull();
        health.LastSuccessfulRead!.Value.ShouldBeGreaterThan(
            before!.Value, "two of the three tags read fine — the watchdog's evidence must advance");
    }

    /// <summary>
    ///     The SPAN path obeys the same rule: one member that fails to decode leaves the pass
    ///     <c>Degraded</c> even though a later member of the same block decoded fine.
    /// </summary>
    /// <remarks>
    ///     The block read itself succeeds — this is the per-member decode guard, the span-path
    ///     sibling of the multi-var per-item guard above. The poisoned member is an S7 STRING laid
    ///     over the image's filler pattern, whose two-byte header declares a current length larger
    ///     than its capacity; <c>S7String.FromByteArray</c> rejects that. It is deliberately the
    ///     FIRST member (lower offset) so the good member decodes after it — the exact ordering
    ///     last-writer-wins used to erase.
    /// </remarks>
    [Fact]
    public async Task One_undecodable_span_member_leaves_the_pass_Degraded()
    {
        var ct = TestContext.Current.CancellationToken;
        // The STRING occupies DB1 bytes 100..111 (StringLength + 2), so the word at 112 is
        // adjacent and lands in the SAME span.
        var tags = S7RawTags.Entries(
            new S7TagDefinition("BadStr", "DB1.DBB100", S7DataType.String, Writable: false, StringLength: 10),
            new S7TagDefinition("GoodWord", "DB1.DBW112", S7DataType.UInt16, Writable: false));
        var factory = new ImageS7PlcFactory(Images());

        // The poison is the FILLER, not a fake knob: Images() writes (i * 37 + 11) & 0xFF, which
        // steps +37 per byte and so runs ASCENDING except in the ~15% of positions where it wraps
        // past 256. Byte 100 is not one of those, so the STRING header at 100/101 reads
        // maxLen 127 / curLen 164 — a current length larger than the capacity, which
        // S7String.FromByteArray rejects. Asserted rather than assumed, so a change to the filler
        // pattern fails HERE, loudly, instead of silently turning this into a passing no-op test.
        var db1Image = factory.Images[(S7NetDataType.DataBlock, 1)];
        db1Image[101].ShouldBeGreaterThan(
            db1Image[100], "the S7 STRING at DB1.DBB100 must declare curLen > maxLen to be undecodable");
        using var driver = new S7Driver(Options(tags, maxGap: 16), "s7-span-decode-fault", factory);
        await driver.InitializeAsync("{}", ct);

        var before = driver.GetHealth().LastSuccessfulRead;
        var results = await driver.ReadAsync(["BadStr", "GoodWord"], ct);

        // ONE block read served both members — this really is the span path, not two per-tag reads.
        factory.ReadBytesCalls.ShouldBe(1);
        factory.StringReadCalls.ShouldBe(0);

        // Only the poisoned member is Bad; its block-mate keeps its value.
        results[0].StatusCode.ShouldNotBe(0u);
        results[0].Value.ShouldBeNull();
        results[1].StatusCode.ShouldBe(0u);
        results[1].Value.ShouldNotBeNull();

        var health = driver.GetHealth();
        health.State.ShouldBe(DriverState.Degraded, "a span member that failed to decode must not be erased");
        health.LastSuccessfulRead!.Value.ShouldBeGreaterThan(before!.Value);
    }

    /// <summary>
    ///     Faulted outranks BOTH Degraded and Healthy: an access-denied read (the permanent PUT/GET
    ///     configuration fault) on one tag survives a later device-fault tag AND a later tag that
    ///     read successfully, all in the same pass.
    /// </summary>
    /// <remarks>
    ///     The ordering is Healthy &lt; Degraded &lt; Faulted, so the config alert — the one state
    ///     an operator must act on — is precisely the one last-writer-wins was most likely to
    ///     erase, since the denied tag is usually a minority of the batch. Both later outcomes are
    ///     present deliberately: the success falsifies "any success clears the failure", the
    ///     Degraded falsifies "the last failure wins". <c>LastSuccessfulRead</c> still advances,
    ///     because one tag really was read.
    /// </remarks>
    [Fact]
    public async Task An_access_denied_tag_keeps_the_pass_Faulted_even_after_a_later_tag_succeeds()
    {
        var ct = TestContext.Current.CancellationToken;
        var factory = new ImageS7PlcFactory(Images());
        // A: "not allowed" is what IsAccessDenied matches — PUT/GET disabled on an S7-1200/1500,
        // which is the Faulted config alert. B: an ordinary device-layer fault, merely Degraded,
        // observed AFTER it. C: no fault at all.
        factory.PerTagFaults["DB1.DBW0"] = new PlcException(ErrorCode.ReadData, "Accessing object not allowed");
        factory.PerTagFaults["DB1.DBW2"] = new PlcException(ErrorCode.ReadData, "S7 CPU reported a device fault");
        // maxGap -1 turns grouping off entirely, so every tag takes the per-tag read path and the
        // faults can be aimed at individual addresses.
        using var driver = new S7Driver(Options(NarrowTags(), maxGap: -1), "s7-access-denied", factory);
        await driver.InitializeAsync("{}", ct);

        var before = driver.GetHealth().LastSuccessfulRead;
        var results = await driver.ReadAsync(["A", "B", "C"], ct);

        results[0].StatusCode.ShouldNotBe(0u);
        results[1].StatusCode.ShouldBe(StatusBadDeviceFailure);
        results[2].StatusCode.ShouldBe(0u);

        var health = driver.GetHealth();
        health.State.ShouldBe(
            DriverState.Faulted, "a config alert must outrank a later device fault AND a later success");
        health.LastError.ShouldNotBeNull();
        // The FIRST failure is the causal one — the later device fault does not overwrite it.
        health.LastError!.ShouldContain("PUT/GET");
        health.LastSuccessfulRead!.Value.ShouldBeGreaterThan(
            before!.Value, "one tag read fine, so the watchdog's evidence must still advance");
    }

    /// <summary>
    ///     The pass's message must explain the state it reports. When a merely-Degraded fault is
    ///     observed BEFORE the access-denied one, escalating to <c>Faulted</c> has to re-point
    ///     <c>LastError</c> at the access-denied cause — reporting the config alert under the
    ///     earlier device-fault message tells the operator to chase the wrong thing.
    /// </summary>
    /// <remarks>
    ///     This is the reversed ordering of
    ///     <see cref="An_access_denied_tag_keeps_the_pass_Faulted_even_after_a_later_tag_succeeds"/>,
    ///     and the pair is the point: keeping the pass's chronologically-first message satisfies
    ///     that test and fails this one, so only a message that moves WITH the severity passes both.
    /// </remarks>
    [Fact]
    public async Task Escalating_to_Faulted_re_points_the_error_message_at_the_access_denied_cause()
    {
        var ct = TestContext.Current.CancellationToken;
        var factory = new ImageS7PlcFactory(Images());
        // A: an ordinary device-layer fault (Degraded) FIRST. B: the access-denied config alert
        // (Faulted) after it. C: no fault at all.
        factory.PerTagFaults["DB1.DBW0"] = new PlcException(ErrorCode.ReadData, "S7 CPU reported a device fault");
        factory.PerTagFaults["DB1.DBW2"] = new PlcException(ErrorCode.ReadData, "Accessing object not allowed");
        using var driver = new S7Driver(Options(NarrowTags(), maxGap: -1), "s7-escalation-message", factory);
        await driver.InitializeAsync("{}", ct);

        var before = driver.GetHealth().LastSuccessfulRead;
        var results = await driver.ReadAsync(["A", "B", "C"], ct);

        results[0].StatusCode.ShouldBe(StatusBadDeviceFailure);
        results[1].StatusCode.ShouldNotBe(0u);
        results[2].StatusCode.ShouldBe(0u);

        var health = driver.GetHealth();
        health.State.ShouldBe(DriverState.Faulted, "the access-denied fault must still outrank the earlier one");
        health.LastError.ShouldNotBeNull();
        health.LastError!.ShouldContain(
            "PUT/GET",
            customMessage: "the message must explain the FAULTED state, not the earlier Degraded one");
        health.LastSuccessfulRead!.Value.ShouldBeGreaterThan(before!.Value);
    }

    /// <summary>
    ///     Cancellation mid-pass abandons the batch but must NOT discard the evidence the pass had
    ///     already gathered: a tag that read successfully before the cancellation still advances
    ///     <c>LastSuccessfulRead</c>.
    /// </summary>
    /// <remarks>
    ///     Deciding health once at the end of the pass makes this a real risk, which per-item
    ///     writes did not have. On a partially-working device a repeatedly-cancelled poll would
    ///     freeze <c>LastSuccessfulRead</c> while tags were demonstrably still being read, and the
    ///     comms-loss watchdog — which trips on non-Healthy state AND a frozen timestamp — would
    ///     reconnect a CPU that is answering. Flushing from a <c>finally</c> is what prevents it.
    /// </remarks>
    [Fact]
    public async Task A_cancelled_pass_still_flushes_the_evidence_it_had_already_gathered()
    {
        var ct = TestContext.Current.CancellationToken;
        // Narrow first (a per-address read, which the hang knob does NOT intercept), then a wide
        // Int64 (a BLOCK read, which it does) — so the pass banks one success and is then cancelled
        // mid-flight on the very next tag.
        var tags = S7RawTags.Entries(
            new S7TagDefinition("Narrow", "DB1.DBW0", S7DataType.UInt16, Writable: false),
            new S7TagDefinition("Wide", "DB1.DBB8", S7DataType.Int64, Writable: false));
        var factory = new ImageS7PlcFactory(Images()) { HangReadBytes = true };
        // maxGap -1 keeps both tags on the per-tag path, in reference order.
        using var driver = new S7Driver(Options(tags, maxGap: -1), "s7-cancel-flush", factory);
        await driver.InitializeAsync("{}", ct);

        var before = driver.GetHealth().LastSuccessfulRead;

        using var cts = new CancellationTokenSource();
        var reading = driver.ReadAsync(["Narrow", "Wide"], cts.Token);
        await factory.ReadBytesEntered.Task.WaitAsync(TimeSpan.FromSeconds(10), ct);
        await cts.CancelAsync();

        await Should.ThrowAsync<OperationCanceledException>(async () => await reading);

        // The first tag really was served before the cancellation — one per-address read happened.
        factory.StringReadCalls.ShouldBe(1);

        var health = driver.GetHealth();
        health.LastSuccessfulRead!.Value.ShouldBeGreaterThan(
            before!.Value, "the banked success must survive the abandoned batch");
    }

    /// <summary>
    ///     A pass that observes NOTHING — every reference unresolvable, so not one tag was read and
    ///     not one failed — reports nothing: it must leave the driver's prior <c>Faulted</c> config
    ///     alert (and its <c>LastSuccessfulRead</c>) exactly as it found them.
    /// </summary>
    /// <remarks>
    ///     Deciding health once per pass means the pass has to answer "what did I observe?", and a
    ///     degenerate pass observed nothing at all — writing <c>Healthy</c> for it would invent
    ///     evidence and silently clear a PUT/GET config alert an operator still has to act on. The
    ///     per-item health writes this replaced had that property for free (no item, no write); the
    ///     end-of-pass guard is what preserves it.
    /// </remarks>
    [Fact]
    public async Task A_pass_that_observes_nothing_leaves_a_prior_Faulted_untouched()
    {
        var ct = TestContext.Current.CancellationToken;
        var factory = new ImageS7PlcFactory(Images());
        factory.PerTagFaults["DB1.DBW0"] = new PlcException(ErrorCode.ReadData, "Accessing object not allowed");
        using var driver = new S7Driver(Options(NarrowTags(), maxGap: -1), "s7-observes-nothing", factory);
        await driver.InitializeAsync("{}", ct);

        // Drive the driver into the access-denied config alert first.
        _ = await driver.ReadAsync(["A"], ct);
        var faulted = driver.GetHealth();
        faulted.State.ShouldBe(DriverState.Faulted);

        // Now a pass whose every reference is unresolvable: they are stamped BadNodeIdUnknown
        // before the planner ever sees them, so no span, no multi-var item and no per-tag read
        // happens — the pass observes neither a success nor a failure.
        var results = await driver.ReadAsync(["NoSuchTag", "AlsoMissing"], ct);
        results.ShouldAllBe(s => s.StatusCode == StatusBadNodeIdUnknown);

        var after = driver.GetHealth();
        after.State.ShouldBe(DriverState.Faulted, "an observe-nothing pass must not clear the config alert");
        after.LastError.ShouldBe(faulted.LastError);
        after.LastSuccessfulRead.ShouldBe(faulted.LastSuccessfulRead, "nothing was read, so nothing advances");
    }

    /// <summary>
    ///     An answer of the wrong SHAPE — one item short — cannot be mapped back onto the request's
    ///     indexes, so the driver refuses to guess which tag got which value and falls the whole
    ///     chunk back to the per-tag path, which still serves every tag Good.
    /// </summary>
    [Fact]
    public async Task A_short_multi_var_answer_falls_back_to_per_tag_reads_and_still_serves_good()
    {
        var ct = TestContext.Current.CancellationToken;
        var factory = new ImageS7PlcFactory(Images()) { ShortAnswer = true };
        using var driver = new S7Driver(Options(ScatteredWords(5), maxGap: 2), "s7-mv-shape", factory);
        await driver.InitializeAsync("{}", ct);

        var results = await driver.ReadAsync(ScatteredRefs(5), ct);

        results.ShouldAllBe(s => s.StatusCode == 0u);
        results.ShouldAllBe(s => s.Value != null);
        factory.MultiVarCalls.ShouldBe(1);
        // Every tag of the rejected chunk was re-read individually — including the four the short
        // answer DID carry, because a mis-shaped answer says nothing trustworthy about any of them.
        factory.StringReadCalls.ShouldBe(5);
        driver.GetHealth().State.ShouldBe(DriverState.Healthy);
    }

    // ── Cancellation mid-read (STAB-15a) ─────────────────────────────────────────────────

    /// <summary>
    ///     Cancellation observed while a SPAN's block read is in flight abandons a half-read
    ///     ISO-on-TCP response on the single gated stream, so the handle can't be reused: the OCE
    ///     must propagate (the caller cancelled — nobody consumes the batch) AND the next read must
    ///     open a fresh connection. The per-tag path's sibling of this is pinned by
    ///     <c>S7DriverReconnectTests.Cancelled_read_marks_handle_dead_and_next_read_reopens</c>.
    /// </summary>
    [Fact]
    public async Task A_cancelled_span_block_read_marks_the_handle_dead_and_the_next_read_reopens()
    {
        var ct = TestContext.Current.CancellationToken;
        var factory = new ImageS7PlcFactory(Images()) { HangReadBytes = true };
        using var driver = new S7Driver(Options(NarrowTags(), maxGap: 16), "s7-span-cancel", factory);
        await driver.InitializeAsync("{}", ct);
        factory.CreatedPlcCount.ShouldBe(1);

        string[] refs = ["A", "B", "C"];
        using var cts = new CancellationTokenSource();
        var reading = driver.ReadAsync(refs, cts.Token);
        await factory.ReadBytesEntered.Task.WaitAsync(TimeSpan.FromSeconds(10), ct);
        await cts.CancelAsync();

        await Should.ThrowAsync<OperationCanceledException>(async () => await reading);

        // The reopen is the observable half of the contract: the desynced handle was DISPOSED — not
        // merely leaked behind a second connection — and the next read builds a new one.
        factory.HangReadBytes = false;
        var recovered = await driver.ReadAsync(refs, ct);
        recovered.ShouldAllBe(s => s.StatusCode == 0u);
        factory.CreatedPlcCount.ShouldBe(2);
        factory.Plcs[0].Disposed.ShouldBeTrue();
    }

    /// <summary>
    ///     The same contract on the MULTI-VAR leftover path: a cancelled batch request propagates
    ///     the OCE and marks the handle dead, so the next read reopens. Distinct from the per-chunk
    ///     fallback — cancellation is the one request fault that must NOT be retried per tag.
    /// </summary>
    [Fact]
    public async Task A_cancelled_multi_var_read_marks_the_handle_dead_and_the_next_read_reopens()
    {
        var ct = TestContext.Current.CancellationToken;
        var factory = new ImageS7PlcFactory(Images()) { HangMultiVar = true };
        using var driver = new S7Driver(Options(ScatteredWords(5), maxGap: 2), "s7-mv-cancel", factory);
        await driver.InitializeAsync("{}", ct);
        factory.CreatedPlcCount.ShouldBe(1);

        var refs = ScatteredRefs(5);
        using var cts = new CancellationTokenSource();
        var reading = driver.ReadAsync(refs, cts.Token);
        await factory.MultiVarEntered.Task.WaitAsync(TimeSpan.FromSeconds(10), ct);
        await cts.CancelAsync();

        await Should.ThrowAsync<OperationCanceledException>(async () => await reading);

        factory.HangMultiVar = false;
        var recovered = await driver.ReadAsync(refs, ct);
        recovered.ShouldAllBe(s => s.StatusCode == 0u);
        factory.CreatedPlcCount.ShouldBe(2);
        factory.Plcs[0].Disposed.ShouldBeTrue();
    }

    // ── Configurability ──────────────────────────────────────────────────────────────────

    /// <summary>
    ///     The grouping knobs must be reachable from a deployed driver config. <c>ParseOptions</c>
    ///     maps an explicit DTO rather than deserialising <see cref="S7DriverOptions"/>, so a knob
    ///     that is not mapped there is silently pinned to its default forever — which would leave
    ///     the documented "-1 disables grouping" escape hatch unreachable in production.
    /// </summary>
    [Fact]
    public void The_span_read_knobs_bind_from_driver_config_json()
    {
        var configured = S7DriverFactoryExtensions.ParseOptions(
            "s7-cfg",
            """{"host":"10.0.0.1","spanReadMaxGapBytes":-1,"spanReadMaxSpanBytes":64}""");

        configured.SpanReadMaxGapBytes.ShouldBe(-1);
        configured.SpanReadMaxSpanBytes.ShouldBe(64);

        // Omitted keys keep the shipped defaults rather than collapsing to zero.
        var defaulted = S7DriverFactoryExtensions.ParseOptions("s7-cfg", """{"host":"10.0.0.1"}""");
        defaulted.SpanReadMaxGapBytes.ShouldBe(S7DriverOptions.DefaultSpanReadMaxGapBytes);
        defaulted.SpanReadMaxSpanBytes.ShouldBe(S7DriverOptions.DefaultSpanReadMaxSpanBytes);
    }

    // ── Fixture ──────────────────────────────────────────────────────────────────────────

    /// <summary>Driver options over the given tags with the span-grouping knobs under test.</summary>
    private static S7DriverOptions Options(IReadOnlyList<RawTagEntry> tags, int maxGap) => new()
    {
        Host = "192.0.2.1",
        Timeout = TimeSpan.FromMilliseconds(250),
        Probe = new S7ProbeOptions { Enabled = false },
        SpanReadMaxGapBytes = maxGap,
        SpanReadMaxSpanBytes = 220,
        RawTags = tags,
    };

    /// <summary>
    ///     The in-memory PLC images the fake serves, one byte array per <c>(area, DB)</c>. Filled
    ///     with a deterministic non-repeating pattern so no tag reads a vacuous all-zero value, then
    ///     seeded with a well-formed S7 STRING field at DB1 byte 36 (<c>[maxLen][curLen][chars…]</c>,
    ///     which <c>S7String.FromByteArray</c> validates).
    /// </summary>
    private static Dictionary<(S7NetDataType Area, int Db), byte[]> Images()
    {
        var db1 = new byte[256];
        for (var i = 0; i < db1.Length; i++) db1[i] = (byte)((i * 37 + 11) & 0xFF);

        db1[36] = 10;              // declared max length
        db1[37] = 5;               // current length
        var chars = "HELLO"u8.ToArray();
        chars.CopyTo(db1, 38);
        for (var i = 38 + chars.Length; i < 36 + 12; i++) db1[i] = 0x20; // reserved padding

        var mem = new byte[64];
        for (var i = 0; i < mem.Length; i++) mem[i] = (byte)((i * 53 + 7) & 0xFF);

        var counters = new byte[64];
        for (var i = 0; i < counters.Length; i++) counters[i] = (byte)((i * 29 + 3) & 0xFF);

        var timers = new byte[64];
        for (var i = 0; i < timers.Length; i++) timers[i] = (byte)((i * 17 + 5) & 0xFF);

        return new Dictionary<(S7NetDataType, int), byte[]>
        {
            [(S7NetDataType.DataBlock, 1)] = db1,
            [(S7NetDataType.Memory, 0)] = mem,
            [(S7NetDataType.Counter, 0)] = counters,
            [(S7NetDataType.Timer, 0)] = timers,
        };
    }

    /// <summary>
    ///     Fake <see cref="IS7PlcFactory"/> serving both read shapes from one shared byte image per
    ///     <c>(area, DB)</c>, and counting each shape so a test can prove which path ran. Counters
    ///     live on the FACTORY (not the connection) so a reconnect cannot reset them.
    /// </summary>
    private sealed class ImageS7PlcFactory(Dictionary<(S7NetDataType Area, int Db), byte[]> images) : IS7PlcFactory
    {
        /// <summary>The shared per-area byte images both read shapes decode from.</summary>
        public Dictionary<(S7NetDataType Area, int Db), byte[]> Images { get; } = images;

        /// <summary>Number of block reads issued — one per planned span, plus the always-block tags.</summary>
        public int ReadBytesCalls;

        /// <summary>Number of per-address string reads issued — the un-grouped narrow path.</summary>
        public int StringReadCalls;

        /// <summary>Number of multi-var requests issued — one per leftover chunk.</summary>
        public int MultiVarCalls;

        /// <summary>The items of every multi-var request, in issue order, so a test can assert what was batched.</summary>
        public List<IReadOnlyList<global::S7.Net.Types.DataItem>> MultiVarItems { get; } = new();

        /// <summary>When set, every read throws this instead of serving the image.</summary>
        public Exception? ReadFault { get; init; }

        /// <summary>When set, only the MULTI-VAR read throws this — the per-tag fallback still serves the image.</summary>
        public Exception? MultiVarFault { get; init; }

        /// <summary>
        ///     Per-address read faults, keyed by the address string (e.g. <c>DB1.DBW0</c>): only
        ///     those addresses throw, every other one still serves the image. Lets a test put
        ///     faults of DIFFERENT severities and successes in ONE pass, which
        ///     <see cref="ReadFault"/> (all-or-nothing) cannot express.
        /// </summary>
        public Dictionary<string, Exception> PerTagFaults { get; } = new(StringComparer.OrdinalIgnoreCase);

        /// <summary>
        ///     When true, <c>ReadBytesAsync</c> hangs until its token is cancelled, so cancellation is
        ///     the only way out of a block read (STAB-15a). Settable so a test can turn it back off
        ///     after observing the cancellation and let the reopened handle read normally.
        /// </summary>
        public bool HangReadBytes { get; set; }

        /// <summary>When true, <c>ReadMultipleVarsAsync</c> hangs until its token is cancelled (STAB-15a).</summary>
        public bool HangMultiVar { get; set; }

        /// <summary>Completed the moment a hanging <c>ReadBytesAsync</c> is entered, so a test cancels a read that is genuinely in flight.</summary>
        public TaskCompletionSource ReadBytesEntered { get; } = new(TaskCreationOptions.RunContinuationsAsynchronously);

        /// <summary>Completed the moment a hanging <c>ReadMultipleVarsAsync</c> is entered.</summary>
        public TaskCompletionSource MultiVarEntered { get; } = new(TaskCreationOptions.RunContinuationsAsynchronously);

        /// <summary>
        ///     When set, the multi-var answer comes back the right shape but with THIS item's
        ///     <c>Value</c> left null — the per-item decode fault, which must spoil only that tag.
        /// </summary>
        public int? NullItemIndex { get; init; }

        /// <summary>
        ///     When true, the multi-var answer comes back ONE item short — a shape the driver cannot
        ///     map back onto the request's indexes, so it must reject the whole chunk.
        /// </summary>
        public bool ShortAnswer { get; init; }

        /// <summary>How many connections this factory has been asked to build — the reopen observable.</summary>
        public int CreatedPlcCount;

        /// <summary>
        ///     Every connection this factory built, in build order, so a test can assert what became
        ///     of the STALE one — a driver that leaked the old handle and merely opened a second
        ///     would satisfy <see cref="CreatedPlcCount"/> alone.
        /// </summary>
        public List<ImageS7Plc> Plcs { get; } = new();

        /// <inheritdoc />
        public IS7Plc Create(S7CpuType cpuType, string host, int port, short rack, short slot, TimeSpan timeout)
        {
            Interlocked.Increment(ref CreatedPlcCount);
            var plc = new ImageS7Plc(this);
            lock (Plcs) Plcs.Add(plc);
            return plc;
        }

        public sealed class ImageS7Plc(ImageS7PlcFactory owner) : IS7Plc
        {
            /// <inheritdoc />
            public bool IsConnected { get; private set; }

            /// <summary>True once the driver has disposed this handle — the discard half of a reopen.</summary>
            public bool Disposed { get; private set; }

            /// <inheritdoc />
            public Task OpenAsync(CancellationToken ct) { IsConnected = true; return Task.CompletedTask; }

            /// <inheritdoc />
            public void Close() => IsConnected = false;

            /// <inheritdoc />
            public async Task<byte[]> ReadBytesAsync(S7NetDataType area, int db, int startByteAdr, int count, CancellationToken ct)
            {
                // Counted BEFORE the injected fault so a test can assert how many block reads were
                // ATTEMPTED — a failure test that counted only successes would read zero.
                Interlocked.Increment(ref owner.ReadBytesCalls);
                if (owner.HangReadBytes)
                {
                    // Signal first, then hang: the test cancels only once the read is genuinely in
                    // flight, so it exercises cancel-mid-I/O rather than racing a timer.
                    owner.ReadBytesEntered.TrySetResult();
                    await Task.Delay(System.Threading.Timeout.Infinite, ct).ConfigureAwait(false);
                }
                if (owner.ReadFault is { } fault) throw fault;
                var image = owner.Images[(area, db)];
                if (startByteAdr + count > image.Length)
                {
                    throw new PlcException(ErrorCode.ReadData,
                        $"fake image overrun: {area} db{db} @{startByteAdr}+{count} exceeds {image.Length}");
                }
                return image[startByteAdr..(startByteAdr + count)];
            }

            /// <inheritdoc />
            /// <remarks>
            ///     Decodes each item from the same byte image the other two read shapes serve, boxing
            ///     exactly as S7.Net's <c>ParseBytes</c> does for <c>Count == 1</c>: <c>Bit</c> a
            ///     <c>bool</c> at <c>BitAdr</c>, <c>Byte</c> a <c>byte</c>, <c>Word</c> a
            ///     <c>ushort</c>, <c>DWord</c> a <c>uint</c>. Like the real library it populates each
            ///     item's <c>Value</c> in place and hands the same list back.
            /// </remarks>
            public async Task<IReadOnlyList<global::S7.Net.Types.DataItem>> ReadMultipleVarsAsync(
                List<global::S7.Net.Types.DataItem> items, CancellationToken ct)
            {
                // Counted (and captured) BEFORE the injected fault so a fallback test can assert the
                // batch was ATTEMPTED — counting only successes would read zero.
                Interlocked.Increment(ref owner.MultiVarCalls);
                lock (owner.MultiVarItems) owner.MultiVarItems.Add([.. items]);
                if (owner.HangMultiVar)
                {
                    owner.MultiVarEntered.TrySetResult();
                    await Task.Delay(System.Threading.Timeout.Infinite, ct).ConfigureAwait(false);
                }
                if (owner.MultiVarFault is { } mvFault) throw mvFault;
                if (owner.ReadFault is { } fault) throw fault;

                foreach (var item in items)
                {
                    var image = owner.Images[(item.DataType, item.DB)];
                    var at = item.StartByteAdr;
                    item.Value = item.VarType switch
                    {
                        VarType.Bit => (object)((image[at] & (1 << item.BitAdr)) != 0),
                        VarType.Byte => image[at],
                        VarType.Word => (ushort)((image[at] << 8) | image[at + 1]),
                        VarType.DWord => ((uint)image[at] << 24) | ((uint)image[at + 1] << 16)
                                         | ((uint)image[at + 2] << 8) | image[at + 3],
                        _ => throw new NotSupportedException($"fake cannot serve VarType {item.VarType}"),
                    };
                }

                // The two malformed-answer shapes, injected AFTER decoding so the answer is as good
                // as any other batch's apart from the one defect under test.
                if (owner.NullItemIndex is { } nullAt) items[nullAt].Value = null;
                if (owner.ShortAnswer) return items.GetRange(0, items.Count - 1);

                return items;
            }

            /// <inheritdoc />
            public Task<object?> ReadAsync(string address, CancellationToken ct)
            {
                Interlocked.Increment(ref owner.StringReadCalls);
                if (owner.PerTagFaults.TryGetValue(address, out var aimed)) throw aimed;
                if (owner.ReadFault is { } fault) throw fault;
                var addr = S7AddressParser.Parse(address);
                var image = owner.Images[(ToS7NetArea(addr.Area), addr.DbNumber)];
                var at = addr.ByteOffset;

                // Box exactly as S7.Net's string read does — the size suffix picks the type.
                return Task.FromResult<object?>(addr.Size switch
                {
                    S7Size.Bit => (object)((image[at] & (1 << addr.BitOffset)) != 0),
                    S7Size.Byte => image[at],
                    S7Size.Word => (ushort)((image[at] << 8) | image[at + 1]),
                    S7Size.DWord => ((uint)image[at] << 24) | ((uint)image[at + 1] << 16)
                                    | ((uint)image[at + 2] << 8) | image[at + 3],
                    _ => throw new NotSupportedException($"fake cannot serve size {addr.Size}"),
                });
            }

            /// <summary>Mirrors the driver's own area mapping so both read shapes hit the same image.</summary>
            private static S7NetDataType ToS7NetArea(S7Area area) => area switch
            {
                S7Area.DataBlock => S7NetDataType.DataBlock,
                S7Area.Memory => S7NetDataType.Memory,
                S7Area.Input => S7NetDataType.Input,
                S7Area.Output => S7NetDataType.Output,
                S7Area.Timer => S7NetDataType.Timer,
                S7Area.Counter => S7NetDataType.Counter,
                _ => throw new NotSupportedException($"fake cannot map area {area}"),
            };

            /// <inheritdoc />
            public Task WriteAsync(string address, object value, CancellationToken ct) => Task.CompletedTask;

            /// <inheritdoc />
            public Task WriteBytesAsync(S7NetDataType area, int db, int startByteAdr, byte[] value, CancellationToken ct)
                => Task.CompletedTask;

            /// <inheritdoc />
            public Task ReadStatusAsync(CancellationToken ct) => Task.CompletedTask;

            /// <inheritdoc />
            public void Dispose()
            {
                IsConnected = false;
                Disposed = true;
            }
        }
    }
}