using System.Diagnostics;
using System.Net;
using System.Net.Http.Headers;
using System.Text;
using Shouldly;
using Xunit;

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

/// <summary>
///     Task 7 — the <c>/current</c> + <c>/sample</c> legs: <see cref="MTConnectStreamsParser"/>
///     turning an <c>MTConnectStreams</c> document into <see cref="MTConnectStreamsResult"/>, the
///     ring-buffer sequence-gap signal (<see cref="MTConnectAgentClient.IsSequenceGap"/>), and the
///     <c>multipart/x-mixed-replace</c> chunk framing + heartbeat watchdog on the long-poll leg.
///     Every test runs against canned fixtures or a stubbed <see cref="HttpMessageHandler"/> — no
///     socket is opened anywhere in this file.
/// </summary>
public sealed class MTConnectStreamsParseTests
{
    private const string CurrentFixture = "Fixtures/current.xml";
    private const string SampleFixture = "Fixtures/sample.xml";
    private const string GapFixture = "Fixtures/sample-gap.xml";
    private const string UnavailableFixture = "Fixtures/current-unavailable.xml";

    /// <summary>The fixtures deliberately share one instanceId, so a gap — not a restart — is under test.</summary>
    private const long FixtureInstanceId = 1655000000L;

    private static readonly string[] AllFixtureDataItemIds =
    [
        "dev1_avail",
        "dev1_pos",
        "dev1_vibration_ts",
        "dev1_partcount",
        "dev1_execution",
        "dev1_program",
        "dev1_system_cond"
    ];

    // ------------------------------------------------------------------ header sequence parsing

    [Fact]
    public void Current_parse_reads_header_sequences_and_observations()
    {
        var result = MTConnectStreamsParser.Parse(File.ReadAllText(CurrentFixture));

        result.NextSequence.ShouldBeGreaterThan(0);
        result.Observations.ShouldNotBeEmpty();
    }

    [Theory]
    [InlineData(CurrentFixture, 1L, 108L)]
    [InlineData(SampleFixture, 108L, 113L)]
    [InlineData(GapFixture, 5000L, 5005L)]
    [InlineData(UnavailableFixture, 200L, 207L)]
    public void Parse_reads_first_and_next_sequence_and_the_instance_id_from_every_fixture(
        string fixture, long firstSequence, long nextSequence)
    {
        var result = MTConnectStreamsParser.Parse(File.ReadAllText(fixture));

        result.FirstSequence.ShouldBe(firstSequence);
        result.NextSequence.ShouldBe(nextSequence);
        result.InstanceId.ShouldBe(FixtureInstanceId);
    }

    // ------------------------------------------------------------------------ observation values

    [Fact]
    public void Current_parse_keys_every_observation_by_its_dataitem_id()
    {
        var result = MTConnectStreamsParser.Parse(File.ReadAllText(CurrentFixture));

        result.Observations.Select(o => o.DataItemId).ShouldBe(AllFixtureDataItemIds, ignoreOrder: true);
    }

    [Fact]
    public void Current_parse_reads_the_good_value_the_agent_reported()
    {
        var observations = ParseCurrent();

        // The observation ELEMENT is <Position>, not <POSITION>; the value is its text content.
        observations["dev1_pos"].Value.ShouldBe("123.4567");
        observations["dev1_execution"].Value.ShouldBe("ACTIVE");
        observations["dev1_program"].Value.ShouldBe("O1234");
        observations["dev1_avail"].Value.ShouldBe("AVAILABLE");
    }

    [Fact]
    public void Current_parse_carries_the_unavailable_text_sentinel_through_verbatim()
    {
        // Task 8 maps UNAVAILABLE => BadNoCommunication; this layer must not pre-interpret it.
        ParseCurrent()["dev1_partcount"].Value.ShouldBe("UNAVAILABLE");
    }

    [Fact]
    public void Current_parse_keeps_a_time_series_vector_as_its_raw_string()
    {
        ParseCurrent()["dev1_vibration_ts"].Value.ShouldBe("1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0");
    }

    // ---------------------------------------------------------------------- CONDITION observations

    [Fact]
    public void A_condition_observations_value_is_its_element_name_not_its_empty_text()
    {
        // <Normal dataItemId="dev1_system_cond" .../> — the element name IS the state, and the
        // element is empty, so a text-content read yields "" and loses the observation entirely.
        ParseCurrent()["dev1_system_cond"].Value.ShouldBe("Normal");
    }

    [Theory]
    [InlineData("Normal", "Normal")]
    [InlineData("Warning", "Warning")]
    [InlineData("Fault", "Fault")]
    public void A_condition_state_element_name_becomes_the_value(string element, string expected)
    {
        var result = MTConnectStreamsParser.Parse(StreamsDocument(
            $"""<Condition><{element} dataItemId="c" timestamp="2026-07-24T12:00:00Z" sequence="1" type="SYSTEM"/></Condition>"""));

        result.Observations.ShouldHaveSingleItem().Value.ShouldBe(expected);
    }

    [Fact]
    public void A_condition_with_text_content_still_takes_its_value_from_the_element_name()
    {
        // Agents put the operator-facing condition message in the element text; the STATE is the name.
        var result = MTConnectStreamsParser.Parse(StreamsDocument(
            """<Condition><Fault dataItemId="c" timestamp="2026-07-24T12:00:00Z" nativeCode="E17">Spindle overtemperature</Fault></Condition>"""));

        result.Observations.ShouldHaveSingleItem().Value.ShouldBe("Fault");
    }

    [Fact]
    public void An_unavailable_condition_element_is_the_unavailable_sentinel()
    {
        // <Unavailable/> is genuinely no-comms and must land on the exact same sentinel Task 8
        // matches for a Sample/Event's literal UNAVAILABLE text.
        var observations = ParseFixture(UnavailableFixture);

        observations["dev1_system_cond"].Value.ShouldBe("UNAVAILABLE");
    }

    [Fact]
    public void The_all_unavailable_fixture_is_all_unavailable()
    {
        var result = MTConnectStreamsParser.Parse(File.ReadAllText(UnavailableFixture));

        result.Observations.Select(o => o.DataItemId).ShouldBe(AllFixtureDataItemIds, ignoreOrder: true);
        result.Observations.ShouldAllBe(o => o.Value == "UNAVAILABLE");
    }

    // ------------------------------------------------------------------------------- timestamps

    [Fact]
    public void Parse_normalizes_every_timestamp_to_utc()
    {
        // A wrong Kind silently shifts the OPC UA SourceTimestamp by the machine's UTC offset.
        var result = MTConnectStreamsParser.Parse(File.ReadAllText(CurrentFixture));

        result.Observations.ShouldAllBe(o => o.TimestampUtc.Kind == DateTimeKind.Utc);
        ParseCurrent()["dev1_pos"].TimestampUtc
            .ShouldBe(new DateTime(2026, 7, 24, 12, 0, 0, 200, DateTimeKind.Utc));
    }

    [Fact]
    public void An_offset_timestamp_is_converted_to_utc_rather_than_taken_at_face_value()
    {
        var result = MTConnectStreamsParser.Parse(StreamsDocument(
            """<Events><Program dataItemId="p" timestamp="2026-07-24T14:00:00+02:00">O1</Program></Events>"""));

        var observation = result.Observations.ShouldHaveSingleItem();
        observation.TimestampUtc.Kind.ShouldBe(DateTimeKind.Utc);
        observation.TimestampUtc.ShouldBe(new DateTime(2026, 7, 24, 12, 0, 0, DateTimeKind.Utc));
    }

    // -------------------------------------------------------------- sequence-gap detection (both ways)

    [Fact]
    public void Sequence_gap_is_detected_when_firstSequence_exceeds_requested_from()
    {
        var chunk = MTConnectStreamsParser.Parse(File.ReadAllText(GapFixture));

        MTConnectAgentClient.IsSequenceGap(requestedFrom: 1, chunk).ShouldBeTrue();
    }

    [Fact]
    public void No_sequence_gap_is_reported_for_a_contiguous_chunk()
    {
        // The falsifiability leg: a helper hard-wired to true would pass the gap test alone.
        var contiguous = MTConnectStreamsParser.Parse(File.ReadAllText(SampleFixture));

        MTConnectAgentClient.IsSequenceGap(requestedFrom: 108, contiguous).ShouldBeFalse();
    }

    [Theory]
    [InlineData(4999L, true)]   // requested older than the buffer holds — data was lost
    [InlineData(5000L, false)]  // exactly the oldest retained sequence — nothing was lost
    [InlineData(5001L, false)]  // the agent simply had nothing older to send
    public void Sequence_gap_is_strictly_firstSequence_greater_than_requested_from(long requestedFrom, bool expected)
    {
        var chunk = MTConnectStreamsParser.Parse(File.ReadAllText(GapFixture));

        MTConnectAgentClient.IsSequenceGap(requestedFrom, chunk).ShouldBe(expected);
    }

    // --------------------------------------------------------------- legal but degenerate documents

    [Fact]
    public void A_document_with_a_valid_header_and_no_observations_parses_rather_than_throwing()
    {
        // /sample chunks are deltas; an idle agent legitimately sends a document with nothing in it.
        const string Xml = """
                           <MTConnectStreams xmlns="urn:mtconnect.org:MTConnectStreams:1.3">
                             <Header instanceId="7" firstSequence="10" lastSequence="9" nextSequence="10"/>
                             <Streams/>
                           </MTConnectStreams>
                           """;

        var result = MTConnectStreamsParser.Parse(Xml);

        result.Observations.ShouldBeEmpty();
        result.InstanceId.ShouldBe(7);
        result.NextSequence.ShouldBe(10);
        result.FirstSequence.ShouldBe(10);
    }

    [Fact]
    public void A_component_stream_may_omit_any_of_samples_events_and_condition()
    {
        // sample.xml carries no <Condition> at all; absence is normal, not an error.
        var result = MTConnectStreamsParser.Parse(File.ReadAllText(SampleFixture));

        result.Observations.Select(o => o.DataItemId).ShouldBe(
            ["dev1_pos", "dev1_vibration_ts", "dev1_partcount", "dev1_execution", "dev1_program"], ignoreOrder: true);
    }

    [Fact]
    public void Parse_collects_observations_across_every_device_and_component_stream()
    {
        const string Xml = """
                           <MTConnectStreams xmlns="urn:mtconnect.org:MTConnectStreams:2.0">
                             <Header instanceId="1" firstSequence="1" lastSequence="3" nextSequence="4"/>
                             <Streams>
                               <DeviceStream name="A" uuid="a">
                                 <ComponentStream component="Device" componentId="a">
                                   <Events><Availability dataItemId="a_avail" timestamp="2026-07-24T12:00:00Z">AVAILABLE</Availability></Events>
                                 </ComponentStream>
                               </DeviceStream>
                               <DeviceStream name="B" uuid="b">
                                 <ComponentStream component="Path" componentId="b_path">
                                   <Samples><Position dataItemId="b_pos" timestamp="2026-07-24T12:00:01Z">1.0</Position></Samples>
                                   <Condition><Warning dataItemId="b_cond" timestamp="2026-07-24T12:00:02Z" type="SYSTEM"/></Condition>
                                 </ComponentStream>
                               </DeviceStream>
                             </Streams>
                           </MTConnectStreams>
                           """;

        var result = MTConnectStreamsParser.Parse(Xml);

        result.Observations.Select(o => o.DataItemId).ShouldBe(["a_avail", "b_pos", "b_cond"]);
        result.Observations[2].Value.ShouldBe("Warning");
    }

    [Theory]
    [InlineData("urn:mtconnect.org:MTConnectStreams:1.3")]
    [InlineData("urn:mtconnect.org:MTConnectStreams:2.0")]
    [InlineData("")]
    public void Parse_is_agnostic_to_the_document_namespace_version(string namespaceUri)
    {
        var xmlns = namespaceUri.Length == 0 ? string.Empty : $" xmlns=\"{namespaceUri}\"";
        var xml = $"""
                   <MTConnectStreams{xmlns}>
                     <Header instanceId="1" firstSequence="1" lastSequence="1" nextSequence="2"/>
                     <Streams><DeviceStream><ComponentStream>
                       <Events><Program dataItemId="p" timestamp="2026-07-24T12:00:00Z">O1</Program></Events>
                     </ComponentStream></DeviceStream></Streams>
                   </MTConnectStreams>
                   """;

        MTConnectStreamsParser.Parse(xml).Observations.ShouldHaveSingleItem().DataItemId.ShouldBe("p");
    }

    [Fact]
    public void Parse_from_a_stream_matches_the_string_overload()
    {
        using var stream = File.OpenRead(CurrentFixture);

        var fromStream = MTConnectStreamsParser.Parse(stream);

        var fromString = MTConnectStreamsParser.Parse(File.ReadAllText(CurrentFixture));
        fromStream.NextSequence.ShouldBe(fromString.NextSequence);
        fromStream.Observations.ShouldBe(fromString.Observations);
    }

    // ------------------------------------------------------------- fails loudly, never silently empty

    [Theory]
    [InlineData("")]
    [InlineData("   ")]
    [InlineData("not xml at all")]
    [InlineData("<MTConnectStreams><Streams>")]
    public void Parse_throws_on_malformed_xml(string xml)
    {
        Should.Throw<InvalidDataException>(() => MTConnectStreamsParser.Parse(xml));
    }

    [Fact]
    public void Parse_throws_on_a_devices_document_served_where_streams_were_expected()
    {
        var ex = Should.Throw<InvalidDataException>(
            () => MTConnectStreamsParser.Parse("<MTConnectDevices><Devices/></MTConnectDevices>"));

        ex.Message.ShouldContain("MTConnectStreams");
        ex.Message.ShouldContain("MTConnectDevices");
    }

    [Fact]
    public void Parse_throws_on_an_mtconnect_error_document_and_surfaces_the_agent_error()
    {
        // An agent answers an out-of-range `from` with an MTConnectError document under HTTP 200.
        const string Xml = """
                           <MTConnectError xmlns="urn:mtconnect.org:MTConnectError:1.3">
                             <Header instanceId="1"/>
                             <Errors>
                               <Error errorCode="OUT_OF_RANGE">'from' must be greater than 100</Error>
                             </Errors>
                           </MTConnectError>
                           """;

        var ex = Should.Throw<InvalidDataException>(() => MTConnectStreamsParser.Parse(Xml));

        ex.Message.ShouldContain("OUT_OF_RANGE");
        ex.Message.ShouldContain("'from' must be greater than 100");
    }

    [Theory]
    [InlineData("")]                                                                        // no <Header> at all
    [InlineData("""<Header instanceId="1" firstSequence="1"/>""")]                           // no nextSequence
    [InlineData("""<Header instanceId="1" nextSequence="2"/>""")]                            // no firstSequence
    [InlineData("""<Header firstSequence="1" nextSequence="2"/>""")]                         // no instanceId
    [InlineData("""<Header instanceId="1" firstSequence="one" nextSequence="2"/>""")]        // non-numeric
    [InlineData("""<Header instanceId="1" firstSequence="1" nextSequence="lots"/>""")]       // non-numeric
    public void Parse_throws_on_an_unusable_header(string headerXml)
    {
        var xml = $"<MTConnectStreams>{headerXml}<Streams/></MTConnectStreams>";

        Should.Throw<InvalidDataException>(() => MTConnectStreamsParser.Parse(xml));
    }

    [Theory]
    [InlineData("""<Position timestamp="2026-07-24T12:00:00Z">1</Position>""")]              // no dataItemId
    [InlineData("""<Position dataItemId="p">1</Position>""")]                                // no timestamp
    [InlineData("""<Position dataItemId="p" timestamp="whenever">1</Position>""")]           // unparseable timestamp
    public void Parse_throws_on_a_malformed_observation(string observationXml)
    {
        Should.Throw<InvalidDataException>(
            () => MTConnectStreamsParser.Parse(StreamsDocument($"<Samples>{observationXml}</Samples>")));
    }

    [Fact]
    public void Parse_reads_only_unqualified_attributes()
    {
        // A prefixed x:dataItemId is a vendor extension, not the observation's identity — reading it
        // would invent a data item that the probe never declared.
        var xml = StreamsDocument(
            """<Samples><Position xmlns:x="urn:vendor" x:dataItemId="p" timestamp="2026-07-24T12:00:00Z">1</Position></Samples>""");

        Should.Throw<InvalidDataException>(() => MTConnectStreamsParser.Parse(xml));
    }

    // ------------------------------------------------------------- MTConnectAgentClient.CurrentAsync

    [Fact]
    public async Task CurrentAsync_requests_the_agent_current_path_and_returns_the_parsed_snapshot()
    {
        var handler = StubHandler.RespondingWith(await File.ReadAllTextAsync(CurrentFixture, Ct));
        using var client = NewClient(handler, new MTConnectDriverOptions { AgentUri = "http://agent:5000" });

        var result = await client.CurrentAsync(Ct);

        handler.LastRequestUri!.AbsoluteUri.ShouldBe("http://agent:5000/current");
        result.NextSequence.ShouldBe(108);
        result.Observations.Count.ShouldBe(7);
    }

    [Fact]
    public async Task CurrentAsync_scopes_the_request_to_the_configured_device_name()
    {
        var handler = StubHandler.RespondingWith(await File.ReadAllTextAsync(CurrentFixture, Ct));
        using var client = NewClient(
            handler, new MTConnectDriverOptions { AgentUri = "http://agent:5000/", DeviceName = "VMC 3Axis" });

        await client.CurrentAsync(Ct);

        handler.LastRequestUri!.AbsoluteUri.ShouldBe("http://agent:5000/VMC%203Axis/current");
    }

    [Fact]
    public async Task CurrentAsync_bounds_a_hung_agent_by_the_configured_request_timeout()
    {
        using var client = NewClient(
            StubHandler.Hanging(),
            new MTConnectDriverOptions { AgentUri = "http://agent:5000", RequestTimeoutMs = 50 });

        var started = Stopwatch.StartNew();
        var ex = await Should.ThrowAsync<TimeoutException>(async () => await client.CurrentAsync(Ct));

        ex.ShouldNotBeAssignableTo<OperationCanceledException>();
        ex.Message.ShouldContain("http://agent:5000/current");
        started.Elapsed.ShouldBeLessThan(TimeSpan.FromSeconds(10));
    }

    [Fact]
    public async Task CurrentAsync_throws_on_an_error_document_served_under_http_200()
    {
        var handler = StubHandler.RespondingWith(
            """
            <MTConnectError><Errors><Error errorCode="NO_DEVICE">nope</Error></Errors></MTConnectError>
            """);
        using var client = NewClient(handler, new MTConnectDriverOptions { AgentUri = "http://agent:5000" });

        var ex = await Should.ThrowAsync<InvalidDataException>(async () => await client.CurrentAsync(Ct));

        ex.Message.ShouldContain("NO_DEVICE");
    }

    [Fact]
    public async Task CurrentAsync_throws_on_a_non_success_status()
    {
        var handler = StubHandler.RespondingWith("nope", HttpStatusCode.ServiceUnavailable);
        using var client = NewClient(handler, new MTConnectDriverOptions { AgentUri = "http://agent:5000" });

        await Should.ThrowAsync<HttpRequestException>(async () => await client.CurrentAsync(Ct));
    }

    // -------------------------------------------------------------- MTConnectAgentClient.SampleAsync

    [Fact]
    public async Task SampleAsync_composes_the_from_interval_count_and_heartbeat_query()
    {
        var handler = StubHandler.RespondingWithMultipart("bnd", await File.ReadAllTextAsync(SampleFixture, Ct));
        using var client = NewClient(handler, new MTConnectDriverOptions
        {
            AgentUri = "http://agent:5000",
            DeviceName = "VMC-3Axis",
            SampleIntervalMs = 250,
            SampleCount = 64,
            HeartbeatMs = 7000
        });

        await DrainAsync(client.SampleAsync(108, Ct));

        handler.LastRequestUri!.AbsoluteUri.ShouldBe(
            "http://agent:5000/VMC-3Axis/sample?from=108&interval=250&count=64&heartbeat=7000");
    }

    [Fact]
    public async Task SampleAsync_yields_one_result_per_multipart_chunk()
    {
        var handler = StubHandler.RespondingWithMultipart(
            "--------bnd",
            await File.ReadAllTextAsync(SampleFixture, Ct),
            await File.ReadAllTextAsync(GapFixture, Ct));
        using var client = NewClient(handler, new MTConnectDriverOptions { AgentUri = "http://agent:5000" });

        var chunks = await DrainAsync(client.SampleAsync(108, Ct));

        chunks.Count.ShouldBe(2);
        chunks[0].FirstSequence.ShouldBe(108);
        chunks[0].NextSequence.ShouldBe(113);
        chunks[1].FirstSequence.ShouldBe(5000);
        chunks[1].NextSequence.ShouldBe(5005);
    }

    [Fact]
    public async Task SampleAsync_frames_chunks_from_the_boundary_when_the_agent_omits_content_length()
    {
        var handler = StubHandler.RespondingWithMultipart(
            "bnd",
            withContentLength: false,
            await File.ReadAllTextAsync(SampleFixture, Ct),
            await File.ReadAllTextAsync(GapFixture, Ct));
        using var client = NewClient(handler, new MTConnectDriverOptions { AgentUri = "http://agent:5000" });

        var chunks = await DrainAsync(client.SampleAsync(108, Ct));

        chunks.Select(c => c.FirstSequence).ShouldBe([108L, 5000L]);
    }

    [Fact]
    public async Task SampleAsync_exposes_the_gap_signal_to_the_caller_on_the_chunk_that_carries_it()
    {
        // Task 11's pump: advance `from` to NextSequence while contiguous, re-baseline on a gap.
        var handler = StubHandler.RespondingWithMultipart(
            "bnd",
            await File.ReadAllTextAsync(SampleFixture, Ct),
            await File.ReadAllTextAsync(GapFixture, Ct));
        using var client = NewClient(handler, new MTConnectDriverOptions { AgentUri = "http://agent:5000" });

        var from = 108L;
        var gaps = new List<bool>();
        await foreach (var chunk in client.SampleAsync(from, Ct))
        {
            gaps.Add(MTConnectAgentClient.IsSequenceGap(from, chunk));
            from = chunk.NextSequence;
        }

        gaps.ShouldBe([false, true]);
        from.ShouldBe(5005);
    }

    [Fact]
    public async Task SampleAsync_ignores_an_empty_keepalive_part()
    {
        var handler = StubHandler.RespondingWithMultipart(
            "bnd",
            await File.ReadAllTextAsync(SampleFixture, Ct),
            "",
            await File.ReadAllTextAsync(GapFixture, Ct));
        using var client = NewClient(handler, new MTConnectDriverOptions { AgentUri = "http://agent:5000" });

        var chunks = await DrainAsync(client.SampleAsync(108, Ct));

        chunks.Count.ShouldBe(2);
    }

    [Fact]
    public async Task SampleAsync_yields_a_heartbeat_document_that_carries_no_observations()
    {
        // The agent's real keep-alive is a well-formed but observation-free MTConnectStreams
        // document — it advances the sequence, so it must reach the caller, not be swallowed.
        const string Heartbeat = """
                                 <MTConnectStreams>
                                   <Header instanceId="1655000000" firstSequence="113" lastSequence="112" nextSequence="113"/>
                                   <Streams/>
                                 </MTConnectStreams>
                                 """;
        var handler = StubHandler.RespondingWithMultipart("bnd", Heartbeat);
        using var client = NewClient(handler, new MTConnectDriverOptions { AgentUri = "http://agent:5000" });

        var chunks = await DrainAsync(client.SampleAsync(113, Ct));

        chunks.ShouldHaveSingleItem().Observations.ShouldBeEmpty();
        MTConnectAgentClient.IsSequenceGap(113, chunks[0]).ShouldBeFalse();
    }

    [Fact(Timeout = 30_000)]
    public async Task SampleAsync_faults_on_the_heartbeat_watchdog_rather_than_hanging_on_a_silent_agent()
    {
        // The R2-01 frozen-peer shape: the agent accepts the connection, sends one chunk, then goes
        // silent forever. Without a watchdog the pump wedges with no timeout ever firing.
        var prefix = StubHandler.MultipartBody("bnd", withContentLength: true, closing: false,
            await File.ReadAllTextAsync(SampleFixture, Ct));
        var handler = StubHandler.RespondingWithStallingStream("bnd", prefix);
        using var client = NewClient(handler, new MTConnectDriverOptions
        {
            AgentUri = "http://agent:5000",
            RequestTimeoutMs = 100,
            HeartbeatMs = 100
        });

        var started = Stopwatch.StartNew();
        await using var chunks = client.SampleAsync(108, Ct).GetAsyncEnumerator(Ct);

        (await chunks.MoveNextAsync()).ShouldBeTrue();
        chunks.Current.FirstSequence.ShouldBe(108);

        var ex = await Should.ThrowAsync<TimeoutException>(async () => await chunks.MoveNextAsync());
        ex.ShouldNotBeAssignableTo<OperationCanceledException>();
        ex.Message.ShouldContain("sample");
        started.Elapsed.ShouldBeLessThan(TimeSpan.FromSeconds(20));
    }

    [Fact(Timeout = 30_000)]
    public async Task SampleAsync_bounds_the_response_header_phase_by_the_request_timeout()
    {
        // A peer that accepts the TCP connection but never answers must not wedge the pump either.
        using var client = NewClient(
            StubHandler.Hanging(),
            new MTConnectDriverOptions { AgentUri = "http://agent:5000", RequestTimeoutMs = 50 });

        var ex = await Should.ThrowAsync<TimeoutException>(async () => await DrainAsync(client.SampleAsync(1, Ct)));

        ex.ShouldNotBeAssignableTo<OperationCanceledException>();
        ex.Message.ShouldContain("/sample");
    }

    [Fact]
    public async Task SampleAsync_throws_on_an_error_document_served_under_http_200()
    {
        // Not a multipart body at all — the agent answers a bad `from` with an error document.
        var handler = StubHandler.RespondingWith(
            """
            <MTConnectError><Errors><Error errorCode="OUT_OF_RANGE">bad from</Error></Errors></MTConnectError>
            """);
        using var client = NewClient(handler, new MTConnectDriverOptions { AgentUri = "http://agent:5000" });

        var ex = await Should.ThrowAsync<InvalidDataException>(
            async () => await DrainAsync(client.SampleAsync(1, Ct)));

        ex.Message.ShouldContain("OUT_OF_RANGE");
    }

    [Fact]
    public async Task SampleAsync_throws_on_a_non_success_status()
    {
        var handler = StubHandler.RespondingWith("nope", HttpStatusCode.BadRequest);
        using var client = NewClient(handler, new MTConnectDriverOptions { AgentUri = "http://agent:5000" });

        await Should.ThrowAsync<HttpRequestException>(async () => await DrainAsync(client.SampleAsync(1, Ct)));
    }

    [Fact(Timeout = 30_000)]
    public async Task SampleAsync_honours_the_callers_cancellation_token()
    {
        var handler = StubHandler.RespondingWithStallingStream("bnd", []);
        using var client = NewClient(handler, new MTConnectDriverOptions { AgentUri = "http://agent:5000" });
        using var cts = new CancellationTokenSource();
        await cts.CancelAsync();

        await Should.ThrowAsync<OperationCanceledException>(
            async () => await DrainAsync(client.SampleAsync(1, cts.Token)));
    }

    [Fact]
    public async Task SampleAsync_ends_cleanly_when_the_agent_closes_the_stream()
    {
        var handler = StubHandler.RespondingWithMultipart("bnd", await File.ReadAllTextAsync(SampleFixture, Ct));
        using var client = NewClient(handler, new MTConnectDriverOptions { AgentUri = "http://agent:5000" });

        var chunks = await DrainAsync(client.SampleAsync(108, Ct));

        chunks.ShouldHaveSingleItem();
    }

    [Fact]
    public void SampleAsync_opens_no_connection_until_it_is_enumerated()
    {
        var handler = StubHandler.Hanging();
        using var client = NewClient(handler, new MTConnectDriverOptions { AgentUri = "http://agent:5000" });

        _ = client.SampleAsync(1, CancellationToken.None);

        handler.RequestCount.ShouldBe(0);
    }

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

    private static CancellationToken Ct => TestContext.Current.CancellationToken;

    private static MTConnectAgentClient NewClient(HttpMessageHandler handler, MTConnectDriverOptions options) =>
        new(options, handler);

    private static async Task<List<MTConnectStreamsResult>> DrainAsync(IAsyncEnumerable<MTConnectStreamsResult> source)
    {
        var results = new List<MTConnectStreamsResult>();
        await foreach (var item in source)
        {
            results.Add(item);
        }

        return results;
    }

    private static Dictionary<string, MTConnectObservation> ParseCurrent() => ParseFixture(CurrentFixture);

    private static Dictionary<string, MTConnectObservation> ParseFixture(string path) =>
        MTConnectStreamsParser.Parse(File.ReadAllText(path)).Observations.ToDictionary(o => o.DataItemId);

    /// <summary>Wraps observation-container XML in a minimal but valid streams document.</summary>
    private static string StreamsDocument(string componentStreamBody) =>
        $"""
         <MTConnectStreams xmlns="urn:mtconnect.org:MTConnectStreams:1.3">
           <Header instanceId="1" firstSequence="1" lastSequence="1" nextSequence="2"/>
           <Streams>
             <DeviceStream name="D" uuid="d">
               <ComponentStream component="Path" componentId="d_path">{componentStreamBody}</ComponentStream>
             </DeviceStream>
           </Streams>
         </MTConnectStreams>
         """;

    /// <summary>A canned <see cref="HttpMessageHandler"/> — no socket is ever opened.</summary>
    private sealed class StubHandler : HttpMessageHandler
    {
        private readonly Func<HttpResponseMessage>? _responder;
        private readonly bool _hang;

        private StubHandler(Func<HttpResponseMessage>? responder, bool hang)
        {
            _responder = responder;
            _hang = hang;
        }

        public Uri? LastRequestUri { get; private set; }

        public int RequestCount { get; private set; }

        public static StubHandler RespondingWith(string body, HttpStatusCode status = HttpStatusCode.OK) =>
            new(() => new HttpResponseMessage(status)
            {
                Content = new StringContent(body, Encoding.UTF8, "text/xml")
            }, hang: false);

        public static StubHandler Hanging() => new(responder: null, hang: true);

        public static StubHandler RespondingWithMultipart(string boundary, params string[] parts) =>
            RespondingWithMultipart(boundary, withContentLength: true, parts);

        public static StubHandler RespondingWithMultipart(string boundary, bool withContentLength, params string[] parts)
        {
            var body = MultipartBody(boundary, withContentLength, closing: true, parts);

            return new StubHandler(() => Multipart(boundary, new ByteArrayContent(body)), hang: false);
        }

        public static StubHandler RespondingWithStallingStream(string boundary, byte[] prefix) =>
            new(() => Multipart(boundary, new StallingContent(prefix)), hang: false);

        /// <summary>Builds a <c>multipart/x-mixed-replace</c> body exactly as an Agent writes one.</summary>
        public static byte[] MultipartBody(string boundary, bool withContentLength, bool closing, params string[] parts)
        {
            var body = new List<byte>();
            foreach (var part in parts)
            {
                var payload = Encoding.UTF8.GetBytes(part);
                var header = new StringBuilder()
                    .Append("--").Append(boundary).Append("\r\n")
                    .Append("Content-type: text/xml\r\n");
                if (withContentLength)
                {
                    header.Append("Content-length: ").Append(payload.Length).Append("\r\n");
                }

                header.Append("\r\n");
                body.AddRange(Encoding.ASCII.GetBytes(header.ToString()));
                body.AddRange(payload);
                body.AddRange("\r\n"u8.ToArray());
            }

            if (closing)
            {
                body.AddRange(Encoding.ASCII.GetBytes($"--{boundary}--\r\n"));
            }

            return [.. body];
        }

        private static HttpResponseMessage Multipart(string boundary, HttpContent content)
        {
            content.Headers.ContentType =
                MediaTypeHeaderValue.Parse($"multipart/x-mixed-replace; boundary={boundary}");

            return new HttpResponseMessage(HttpStatusCode.OK) { Content = content };
        }

        protected override async Task<HttpResponseMessage> SendAsync(
            HttpRequestMessage request, CancellationToken cancellationToken)
        {
            LastRequestUri = request.RequestUri;
            RequestCount++;

            if (_hang)
            {
                await Task.Delay(Timeout.Infinite, cancellationToken);
            }

            return _responder!();
        }
    }

    /// <summary>Serves <paramref name="prefix"/>, then never completes another read.</summary>
    private sealed class StallingContent(byte[] prefix) : HttpContent
    {
        protected override Task<Stream> CreateContentReadStreamAsync() =>
            Task.FromResult<Stream>(new StallingStream(prefix));

        protected override Task SerializeToStreamAsync(Stream stream, TransportContext? context) =>
            Task.CompletedTask;

        protected override bool TryComputeLength(out long length)
        {
            length = -1;

            return false;
        }
    }

    private sealed class StallingStream(byte[] prefix) : Stream
    {
        private int _position;

        public override bool CanRead => true;

        public override bool CanSeek => false;

        public override bool CanWrite => false;

        public override long Length => throw new NotSupportedException();

        public override long Position
        {
            get => throw new NotSupportedException();
            set => throw new NotSupportedException();
        }

        public override async ValueTask<int> ReadAsync(Memory<byte> buffer, CancellationToken cancellationToken = default)
        {
            if (_position < prefix.Length)
            {
                var count = Math.Min(buffer.Length, prefix.Length - _position);
                prefix.AsMemory(_position, count).CopyTo(buffer);
                _position += count;

                return count;
            }

            await Task.Delay(Timeout.Infinite, cancellationToken);

            return 0;
        }

        public override Task<int> ReadAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken) =>
            ReadAsync(buffer.AsMemory(offset, count), cancellationToken).AsTask();

        public override void Flush()
        {
        }

        public override int Read(byte[] buffer, int offset, int count) => throw new NotSupportedException();

        public override long Seek(long offset, SeekOrigin origin) => throw new NotSupportedException();

        public override void SetLength(long value) => throw new NotSupportedException();

        public override void Write(byte[] buffer, int offset, int count) => throw new NotSupportedException();
    }
}