Merge #821 live TimeAverage/Total late-start with Uncertain first sample matches HistoryRead (Part 13 partial interval)

Claude-Session: https://claude.ai/code/session_01GT67AD99zQQqpLCVm3p7xk
2026-11-11 12:30:27 -05:00
parent cbd24d7a01 8a0032bfdf
commit fb44ec2a9c
11 changed files with 398 additions and 53 deletions
@@ -237,15 +237,16 @@ public abstract class QualityMixConformance : ConformanceBase
    public async Task Processed_reads_over_partial_data_match_the_Part_13_oracle(double from, double to, double intervalS)
    {
        // QP's start-of-data and end-of-data intervals against the oracle, forward and reverse (#821). Left out, each a documented or pinned divergence:
-        // TimeAverage2 / Total2 (the SDK's 1 ms end-of-data region), the min/max family (the SDK sets Partial on a Bad
-        // result), and TimeAverage / Total (pinned by Time_weighted_values_over_partial_data_cover_only_the_data below).
+        // TimeAverage2 / Total2 (the SDK's 1 ms end-of-data region) and the min/max family (the SDK sets Partial on a Bad
+        // result). TimeAverage / Total are in since #821: the live calculator no longer integrates the span before QP's
+        // Uncertain first sample (the hand-computed values are pinned below as well).
        await B.SeedAsync("QP", QP);
        await AssertMatchesOracle("QP", Rounded(QP), from, to, intervalS,
        [
            HistoryAggregateType.PercentGood, HistoryAggregateType.PercentBad, HistoryAggregateType.DurationGood,
            HistoryAggregateType.DurationBad, HistoryAggregateType.Count,
            HistoryAggregateType.Average, HistoryAggregateType.Start, HistoryAggregateType.End, HistoryAggregateType.Delta,
-            HistoryAggregateType.Interpolative,
+            HistoryAggregateType.Interpolative, HistoryAggregateType.TimeAverage, HistoryAggregateType.Total,
        ]);
    }

@@ -255,9 +256,9 @@ public abstract class QualityMixConformance : ConformanceBase
        // Part 13 §5.4.3.3/§5.4.3.8 over QP's [0,20) (data from +5): the interpolated-bounding signal is defined from +5
        // only — 10 → 11 over [5,12) (area 73.5) and 11 → 11 + 8/13 over [12,20) (area 4·(22 + 8/13)) — so
        // Total = 73.5 + 88 + 32/13 and TimeAverage = Total ÷ 15 s; Uncertain (an Uncertain value used, not fully covered),
-        // Partial (the data starts inside). Divergence found by #810: the 1.5.378.182 baseline, whose first sample is
-        // Uncertain here, integrates the uncovered [0,5) as a ramp from 0 (Total +25) — so these are pinned by hand, not
-        // against the oracle.
+        // Partial (the data starts inside). Found by #810: the untouched 1.5.378.182 baseline, whose first sample is
+        // Uncertain here, integrates the uncovered [0,5) as a ramp from 0 (Total +25); since #821 the live calculator (the
+        // oracle) does not.
        await B.SeedAsync("QP", QP);
        var src = B.CreateDataSource();
        const double total = 73.5 + 88 + 32d / 13;
@@ -268,8 +269,10 @@ public abstract class QualityMixConformance : ConformanceBase
        ((double)avg.Value!).ShouldBe(total / 15, 1e-6);
        avg.StatusCode.ShouldBe(UncertainCalc | Partial);

-        var sdk = SdkAggregateOracle.Run(HistoryAggregateType.Total, B.Round(At(0)), B.Round(At(20)), Twenty, Rounded(QP)).Single();
-        Convert.ToDouble(sdk.Value).ShouldBe(total + 25, 1e-6, "baseline divergence — if this moves, compare QP's TimeAverage/Total to the oracle");
+        var live = SdkAggregateOracle.Run(HistoryAggregateType.Total, B.Round(At(0)), B.Round(At(20)), Twenty, Rounded(QP)).Single();
+        Convert.ToDouble(live.Value).ShouldBe(total, 1e-6, "live AggregateFilter equals HistoryRead (#821)");
+        var before = SdkAggregateOracle.RunSdkAsIs(HistoryAggregateType.Total, B.Round(At(0)), B.Round(At(20)), Twenty, Rounded(QP)).Single();
+        Convert.ToDouble(before.Value).ShouldBe(total + 25, 1e-6, "the untouched baseline (live before #821)");
    }

    [Fact]
@@ -277,8 +280,8 @@ public abstract class QualityMixConformance : ConformanceBase
    {
        // #821: the reverse-order variant of the late-start case above — (0,20] read 20 → 0. The same Part 13 integral and
        // average over the covered [5,20); Uncertain; NOT Partial (a reverse read keys start-of-data on the latest sample,
-        // which is past the window — the SDK's rule, SdkAggregateParityTests); stamped with the later edge (#677). The
-        // baseline's reverse answer carries the same +25 ramp divergence.
+        // which is past the window — the SDK's rule, SdkAggregateParityTests); stamped with the later edge (#677). The live
+        // calculator agrees since #821; the untouched baseline's reverse answer carries the same +25 ramp.
        await B.SeedAsync("QP", QP);
        var src = B.CreateDataSource();
        const double total = 73.5 + 88 + 32d / 13;
@@ -290,8 +293,10 @@ public abstract class QualityMixConformance : ConformanceBase
        ((double)avg.Value!).ShouldBe(total / 15, 1e-6);
        avg.StatusCode.ShouldBe(UncertainCalc);

-        var sdk = SdkAggregateOracle.Run(HistoryAggregateType.Total, B.Round(At(20)), B.Round(At(0)), Twenty, Rounded(QP)).Single();
-        Convert.ToDouble(sdk.Value).ShouldBe(total + 25, 1e-6, "baseline divergence (reverse) — #821");
+        var live = SdkAggregateOracle.Run(HistoryAggregateType.Total, B.Round(At(20)), B.Round(At(0)), Twenty, Rounded(QP)).Single();
+        Convert.ToDouble(live.Value).ShouldBe(total, 1e-6, "live AggregateFilter equals HistoryRead (reverse) — #821");
+        var before = SdkAggregateOracle.RunSdkAsIs(HistoryAggregateType.Total, B.Round(At(20)), B.Round(At(0)), Twenty, Rounded(QP)).Single();
+        Convert.ToDouble(before.Value).ShouldBe(total + 25, 1e-6, "the untouched baseline (live before #821, reverse)");
    }

    private async Task AssertMatchesOracle(string tag, (DateTime, HistorianValue, uint)[] rows, double from, double to,
@@ -28,8 +28,15 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.Historian.Conformance;
 ///         <c>GoodOnlyStatusAggregateCalculator</c> (the same baseline region machinery — simple bounds, end-of-data 1 ms
 ///         region, status and Partial bits — counting only Good regions), so <see cref="Run"/> / <see cref="RunLive"/> are
 ///         exactly what a live AggregateFilter computes, for every aggregate. PercentBad / DurationBad count Bad regions only
-///         (unchanged by .182). <see cref="RunSdkAsIs"/> runs the untouched baseline (what live served before #809), for the
-///         tests that pin that divergence.
+///         (unchanged by .182). <see cref="RunSdkAsIs"/> runs the untouched baseline (what live served before #809 / #821),
+///         for the tests that pin those divergences.
+///     </para>
+///     <para>
+///         <b>TimeAverage / Total with no start bound (#821).</b> When the data starts inside an interval with an
+///         <b>Uncertain</b> first value, the baseline integrates the uncovered span before it as a ramp from 0 (its region
+///         machinery overwrites that region's Bad status with Uncertain). Part 13 ("No Start Bound: Calculate Partial
+///         Interval") and ours integrate only the covered part, as the baseline itself does after a Good first value. Since
+///         #821 the live calculators follow Part 13 here too (<c>CoveredTimeAverageAggregateCalculator</c>).
 ///     </para>
 ///     <para>
 ///         Not <c>100 − PercentBad</c> under TreatUncertainAsBad = true: that flag also changes the region machinery
@@ -56,8 +63,8 @@ public static class SdkAggregateOracle

    /// <summary>
    ///     Runs the Part 13 oracle for <paramref name="aggregate"/> over <paramref name="samples"/>: the live calculator
-    ///     (<see cref="RunLive"/>) — the baseline, except PercentGood / DurationGood, which count Good regions only (see the
-    ///     remarks, #796/#820/#809).
+    ///     (<see cref="RunLive"/>) — the baseline, except PercentGood / DurationGood, which count Good regions only, and
+    ///     TimeAverage / Total, which leave the span before an Uncertain first value out (see the remarks, #796/#820/#809/#821).
    /// </summary>
    /// <param name="aggregate">The aggregate.</param>
    /// <param name="start">Request start (later than <paramref name="end"/> for a reverse read).</param>
@@ -71,7 +78,8 @@ public static class SdkAggregateOracle

    /// <summary>
    ///     Runs exactly the calculator the server registers for live AggregateFilter monitored items
-    ///     (<c>CompatibilityAggregateFactory.Create</c>, the #809 Good-only PercentGood / DurationGood included).
+    ///     (<c>CompatibilityAggregateFactory.Create</c>, the #809 Good-only PercentGood / DurationGood and the #821
+    ///     covered-only TimeAverage / Total included).
    /// </summary>
    /// <param name="aggregate">The aggregate.</param>
    /// <param name="start">Request start (later than <paramref name="end"/> for a reverse read).</param>
@@ -89,7 +97,8 @@ public static class SdkAggregateOracle
    /// <summary>
    ///     Runs the untouched baseline (1.5.378.182) calculator for <paramref name="aggregate"/> exactly as it computes it,
    ///     under <see cref="DefaultConfiguration"/> — including its PercentGood / DurationGood rule (Uncertain counted as
-    ///     Good) that neither HistoryRead nor, since #809, the live calculators use.
+    ///     Good) and its TimeAverage / Total ramp before an Uncertain first value, which neither HistoryRead nor, since
+    ///     #809 / #821, the live calculators use.
    /// </summary>
    /// <param name="aggregate">The aggregate.</param>
    /// <param name="start">Request start (later than <paramref name="end"/> for a reverse read).</param>
@@ -525,17 +525,19 @@ public sealed class SdkAggregateParityTests
    public void An_Uncertain_first_sample_in_a_late_start_interval_makes_the_SDK_integrate_the_time_before_the_data(
        double from, double to)
    {
-        // #821, pinned divergence (pending the #821 owner decision — this test records, it does not choose).
-        // Part 13 §5.4.3.3/§5.4.3.8: the interpolated-bounding signal is defined from the first sample only — 10 → 11 over
-        // [5,12) (73.5) and 11 → 11 + 8/13 over [12,20) (4·(22 + 8/13)) ⇒ Total 163.96…, TimeAverage = Total ÷ 15 s.
+        // #821, decided 2026-11-11: live follows Part 13. Part 13 §5.4.3.6/§5.4.3.8 ("No Start Bound: Calculate Partial
+        // Interval"): the interpolated-bounding signal is defined from the first sample only — 10 → 11 over [5,12) (73.5)
+        // and 11 → 11 + 8/13 over [12,20) (4·(22 + 8/13)) ⇒ Total 163.96…, TimeAverage = Total ÷ 15 s.
        const double part13Total = 73.5 + 88 + 32d / 13;
        Pct(Ours(HistoryAggregateType.Total, from, to, 0, UncertainFirstLateStart)).ShouldBe(part13Total, 1e-9);
        Pct(Ours(HistoryAggregateType.TimeAverage, from, to, 0, UncertainFirstLateStart)).ShouldBe(part13Total / 15, 1e-9);
+        // The live AggregateFilter calculators (CompatibilityAggregateFactory — CoveredTimeAverageAggregateCalculator since
+        // #821; Sdk2AggregateCompatibilityTests proves the runtime manager serves the same code) answer the same.
+        Pct(Sdk(HistoryAggregateType.Total, from, to, 0, UncertainFirstLateStart)).ShouldBe(part13Total, 1e-9);
+        Pct(Sdk(HistoryAggregateType.TimeAverage, from, to, 0, UncertainFirstLateStart)).ShouldBe(part13Total / 15, 1e-9);

-        // The frozen 1.5.378.182 calculators — for Total / TimeAverage exactly what the server registers for live
-        // AggregateFilter items (CompatibilityAggregateFactory; Sdk2AggregateCompatibilityTests proves the runtime manager
-        // serves the same code)
-        // — integrate the uncovered [0,5) as a ramp 0 → 10 (+25) and divide TimeAverage by the WHOLE 20 s interval.
+        // The untouched frozen 1.5.378.182 calculators (live before #821) integrate the uncovered [0,5) as a ramp 0 → 10
+        // (+25) and divide TimeAverage by the WHOLE 20 s interval.
        Pct(AsIs(HistoryAggregateType.Total, from, to, UncertainFirstLateStart)).ShouldBe(part13Total + 25, 1e-9);
        Pct(AsIs(HistoryAggregateType.TimeAverage, from, to, UncertainFirstLateStart)).ShouldBe((part13Total + 25) / 20, 1e-9);
        // SDK 2.0.1's own stock calculators answer the same: the upgrade did not change this.
@@ -546,15 +548,18 @@ public sealed class SdkAggregateParityTests
        // Status agrees everywhere (Uncertain — an Uncertain value is used and the interval is not fully covered).
        (AsIs(HistoryAggregateType.Total, from, to, UncertainFirstLateStart).Single().StatusCode & StatusBits.CodeBitsMask)
            .ShouldBe(HistorianStatusCodes.UncertainDataSubNormal);
+        Sdk(HistoryAggregateType.Total, from, to, 0, UncertainFirstLateStart).Single().StatusCode
+            .ShouldBe(AsIs(HistoryAggregateType.Total, from, to, UncertainFirstLateStart).Single().StatusCode);
        (Ours(HistoryAggregateType.Total, from, to, 0, UncertainFirstLateStart).Single().StatusCode & StatusBits.CodeBitsMask)
            .ShouldBe(HistorianStatusCodes.UncertainDataSubNormal);

-        // Control: the SAME data with a Good first sample — all three agree on the Part 13 answer, so the trigger is the
+        // Control: the SAME data with a Good first sample — all agree on the Part 13 answer, so the trigger is the
        // Uncertain first sample, not the late start.
        foreach (var agg in new[] { HistoryAggregateType.Total, HistoryAggregateType.TimeAverage })
        {
            var expected = agg == HistoryAggregateType.Total ? part13Total : part13Total / 15;
            Pct(Ours(agg, from, to, 0, GoodFirstLateStart)).ShouldBe(expected, 1e-9);
+            Pct(Sdk(agg, from, to, 0, GoodFirstLateStart)).ShouldBe(expected, 1e-9);
            Pct(AsIs(agg, from, to, GoodFirstLateStart)).ShouldBe(expected, 1e-9);
            Stock201(agg, from, to, GoodFirstLateStart).Single().Value!.Value.ShouldBe(expected, 1e-9);
        }
@@ -661,6 +666,65 @@ public sealed class SdkAggregateParityTests
            .Select(r => Convert.ToDouble(r.Value)).ShouldBe([7000d, 10000, 10000, 10000, 5000, 10000, 10000, 7000], tolerance: 1e-9);
    }

+    /// <summary>
+    ///     #821: late-start data whose first usable sample is Uncertain (+7, after every window's start), with Good / Uncertain / Bad samples after it
+    ///     and a Good sample past every window's end (+90), so only the start of the data is in question. No sample sits on
+    ///     an interval edge of <see cref="WholeIntervalWindows"/> (the reverse on-edge divergence, class remarks).
+    /// </summary>
+    private static readonly StoredSample[] UncertainFirstLate =
+    [
+        S(7, HistorianValue.FromDouble(10), Uncertain),
+        S(12, HistorianValue.FromDouble(11), Good),
+        S(25, HistorianValue.FromDouble(12), Uncertain),
+        S(33, HistorianValue.FromDouble(14), Good),
+        S(47, HistorianValue.FromDouble(13), Bad),
+        S(52, HistorianValue.FromDouble(15), Good),
+        S(90, HistorianValue.FromDouble(16), Good),
+    ];
+
+    /// <summary>#821: the same, preceded by Bad samples only (an outage before the data — still no start bound).</summary>
+    private static readonly StoredSample[] BadThenUncertainFirstLate =
+    [
+        S(-10, HistorianValue.Empty, BadNoCommunication),
+        S(2, HistorianValue.FromDouble(7), Bad),
+        .. UncertainFirstLate,
+    ];
+
+    [Theory]
+    [MemberData(nameof(WholeIntervalWindows))]
+    public void Live_AggregateFilter_TimeAverage_and_Total_equal_HistoryRead_over_an_Uncertain_first_late_start(
+        double from, double to, double intervalS)
+    {
+        // #821: a live AggregateFilter monitored item and a HistoryRead-Processed (SqlServer / QuestDB share
+        // TimeWeightedAggregator) answer TimeAverage / Total identically when the data starts inside an interval with an
+        // Uncertain value — same values, timestamps, status and Partial / Calculated bits, NO documented-divergence mask.
+        // RunLive is exactly CompatibilityAggregateFactory.Create, the calculator the server registers.
+        var interval = intervalS > 0 ? TimeSpan.FromSeconds(intervalS) : (At(to) - At(from)).Duration();
+        var failures = new List<string>();
+        var discriminated = 0;
+        foreach (var agg in new[] { HistoryAggregateType.TimeAverage, HistoryAggregateType.Total })
+        foreach (var (name, data) in new[] { ("UncertainFirstLate", UncertainFirstLate), ("BadThenUncertainFirstLate", BadThenUncertainFirstLate) })
+        {
+            var rows = data.Select(x => (x.TimeUtc, x.Value, x.Status)).ToArray();
+            var live = SdkAggregateOracle.RunLive(agg, At(from), At(to), interval, rows);
+            try
+            {
+                SdkAggregateOracle.ShouldMatch(Ours(agg, from, to, intervalS, data), live, $"live {agg} {name} {from}→{to}/{intervalS}s");
+            }
+            catch (Exception ex)
+            {
+                failures.Add(ex.Message);
+            }
+            // The data must discriminate: the pre-#821 live calculator (the untouched baseline) integrates the span before
+            // the data, so it disagrees on the late-start interval — or this parity would pass against the old behaviour too.
+            var before = SdkAggregateOracle.RunSdkAsIs(agg, At(from), At(to), interval, rows);
+            if (before.Zip(live).Any(p => Math.Abs(Convert.ToDouble(p.First.Value) - Convert.ToDouble(p.Second.Value)) > 1e-9))
+                discriminated++;
+        }
+        failures.ShouldBeEmpty(string.Join("\n", failures));
+        discriminated.ShouldBe(4, "every aggregate × data set must differ from the pre-#821 live answer somewhere");
+    }
+
    /// <summary>#684: Good 10 / 30 with an Uncertain value BELOW the Good minimum (+4) and one ABOVE the maximum (+6).</summary>
    private static readonly StoredSample[] UncertainBeyond =
    [
@@ -6,10 +6,11 @@ namespace ZB.MOM.WW.OtOpcUa.OpcUaServer.IntegrationTests;

 /// <summary>
 ///     #821 / #809 facts, through the BOOTED server's runtime <c>AggregateManager</c> — the calculators a live
-///     AggregateFilter monitored item gets, under the server's own default configuration. The #821 Total / TimeAverage
-///     facts pin current behaviour for the owner decision; they do not choose. #809 is decided (2026-11-11): live
-///     PercentGood / DurationGood count Good time only, equal to HistoryRead. The HistoryRead (Part 13) answers and SDK
-///     2.0.1's stock answers for the same input are pinned beside it in <c>SdkAggregateParityTests</c>.
+///     AggregateFilter monitored item gets, under the server's own default configuration. Both are decided (owner,
+///     2026-11-11): live PercentGood / DurationGood count Good time only (#809), and live Total / TimeAverage integrate only
+///     the covered part of an interval the data starts inside, even with an Uncertain first sample (#821) — equal to
+///     HistoryRead. The HistoryRead (Part 13) answers, the untouched baseline's and SDK 2.0.1's stock answers for the same
+///     input are pinned beside it in <c>SdkAggregateParityTests</c>.
 /// </summary>
 public sealed class LiveAggregateLateStartFactsTests
 {
@@ -41,11 +42,12 @@ public sealed class LiveAggregateLateStartFactsTests
            return d;
        }

-        // Part 13 (HistoryRead on SqlServer/QuestDB): Total = 73.5 + 88 + 32/13 = 163.96…, TimeAverage = Total ÷ 15 s.
-        // The live calculators integrate the uncovered [0,5) as a ramp from 0 (+25) and divide by the whole 20 s.
+        // #821: Part 13 (HistoryRead on SqlServer/QuestDB): Total = 73.5 + 88 + 32/13 = 163.96…, TimeAverage = Total ÷ 15 s
+        // — live as in HistoryRead (before #821 the live calculator integrated the uncovered [0,5) as a ramp from 0 (+25)
+        // and divided by the whole 20 s: 188.96… / 9.448…).
        const double part13Total = 73.5 + 88 + 32d / 13;
-        One(ObjectIds.AggregateFunction_Total).ShouldBe(part13Total + 25, 1e-9);                 // 188.96…
-        One(ObjectIds.AggregateFunction_TimeAverage).ShouldBe((part13Total + 25) / 20, 1e-9);    // 9.448…  (Part 13: 10.93…)
+        One(ObjectIds.AggregateFunction_Total).ShouldBe(part13Total, 1e-9);                      // 163.96…
+        One(ObjectIds.AggregateFunction_TimeAverage).ShouldBe(part13Total / 15, 1e-9);           // 10.93…
        // #809: the Uncertain [5,12) is not Good — only [12,20) is, live as in HistoryRead (8 000 ms / 40 %; before #809
        // the live calculator counted the Uncertain time too: 15 000 ms / 75 %).
        One(ObjectIds.AggregateFunction_DurationGood).ShouldBe(8_000d, 1e-9);
@@ -15,6 +15,13 @@ namespace ZB.MOM.WW.OtOpcUa.OpcUaServer.IntegrationTests;
 ///     case computed by the baseline SDK with <c>TreatUncertainAsBad = true</c> (its "twin": there 1.5.378.182 already
 ///     counted Good regions only) — and each is pinned to that twin instead. The twin is baseline evidence, not output of
 ///     the application's calculator, so the pin never regenerates expectations from the implementation.
+///     <para>
+///         #821 (owner-approved 2026-11-11): TimeAverage / Total no longer integrate the span before an Uncertain first value
+///         when there is no start bound. No fixture vector changes (every data set has a usable value at −10 s, before every
+///         window, so no fixture interval lacks a start bound) — all 384 TimeAverage / Total vectors stay held to their own
+///         output. The change is pinned instead over late-start variants of every data set against a baseline-derived oracle
+///         (<see cref="Issue821_late_start_with_an_uncertain_first_sample_integrates_only_the_covered_part"/>) and by hand.
+///     </para>
 /// </remarks>
 public sealed class Sdk2AggregateCompatibilityTests
 {
@@ -22,6 +29,12 @@ public sealed class Sdk2AggregateCompatibilityTests
    private const string DurationGood = "AggregateFunction_DurationGood";
    private const int Issue809PercentGoodExemptions = 38;
    private const int Issue809DurationGoodExemptions = 26;
+    private const string TimeAverage = "AggregateFunction_TimeAverage";
+    private const string Total = "AggregateFunction_Total";
+    /// <summary>#821: fixture vectors changed — none (every data set has a usable value before the window; class remarks).</summary>
+    private const int Issue821FixtureExemptions = 0;
+    private const int Issue821Cut5Changed = 150;
+    private const int Issue821Cut25Changed = 142;

    [Theory]
    [InlineData(false)]
@@ -44,6 +57,8 @@ public sealed class Sdk2AggregateCompatibilityTests
        baseline.Vectors.Select(x => x.Aggregate).Distinct().Count().ShouldBe(37);
        exemptions.Keys.Count(x => x.Aggregate == PercentGood).ShouldBe(Issue809PercentGoodExemptions);
        exemptions.Keys.Count(x => x.Aggregate == DurationGood).ShouldBe(Issue809DurationGoodExemptions);
+        // #821: no TimeAverage / Total fixture vector is exempt — all 384 are held to their own baseline output above.
+        exemptions.Keys.Count(x => x.Aggregate is TimeAverage or Total).ShouldBe(Issue821FixtureExemptions);
        failures.ShouldBeEmpty("The application must retain baseline values, CLR types, complete status bits, timestamps and incremental processing (#809 exemptions: the uncertain-as-bad twin).");
    }

@@ -97,6 +112,104 @@ public sealed class Sdk2AggregateCompatibilityTests
        output.Select(x => double.Parse(x.Payload!, CultureInfo.InvariantCulture)).ShouldBe(expected);
    }

+    /// <summary>
+    ///     #821 pin over late-start variants of every fixture data set (the fixture itself has none — see the class remarks):
+    ///     each TimeAverage / Total vector's data set cut at <paramref name="cutSeconds"/> (no usable value before the data
+    ///     starts, so no start bound) with its first remaining non-Bad sample made Uncertain, run through the application's
+    ///     calculator under the vector's own direction, stepping, configuration and incremental draining.
+    ///     <para>
+    ///         The oracle is the untouched BASELINE, never the application's calculator: under TreatUncertainAsBad = false
+    ///         the baseline treats an Uncertain value exactly like a Good one except for statuses, and with a Good first
+    ///         value it already leaves the uncovered span out (the #821 control). So the expected value of each interval is
+    ///         the baseline's on the same variant with that first sample Good, and the expected status, timestamps and bits
+    ///         the baseline's on the variant itself (the region before the data is non-Good either way) — except an
+    ///         interval the Good twin answers Bad (nothing covered: only the uncovered span ever fed it), which is the twin
+    ///         whole. Stepped variables and TreatUncertainAsBad = true never reached the baseline's defect and must be the
+    ///         baseline byte for byte.
+    ///     </para>
+    /// </summary>
+    [Theory]
+    [InlineData(5, Issue821Cut5Changed)]
+    [InlineData(25, Issue821Cut25Changed)]
+    public void Issue821_late_start_with_an_uncertain_first_sample_integrates_only_the_covered_part(double cutSeconds, int expectedChanged)
+    {
+        var baseline = LoadBaseline();
+        var failures = new List<string>();
+        var changed = 0;
+        // Why the fixture needs no #821 exemption: every data set with a usable value has one before the window (−10 s), so
+        // no fixture interval lacks a start bound; the variants below remove it.
+        baseline.Data.Values.Where(d => d.Any(s => !StatusCode.IsBad(s.Status)))
+            .ShouldAllBe(d => d.Any(s => s.Seconds < 0 && !StatusCode.IsBad(s.Status)));
+        var vectors = baseline.Vectors.Where(v => v.Aggregate is TimeAverage or Total).ToList();
+        foreach (var vector in vectors)
+        {
+            var (variant, goodTwin) = LateStartVariant(baseline.Data[vector.DataSet], cutSeconds);
+            var asIs = Run(baseline, vector, rig: null, out var asIsError, variant, baselineAsIs: true);
+            var twin = Run(baseline, vector, rig: null, out var twinError, goodTwin, baselineAsIs: true);
+            var actual = Run(baseline, vector, rig: null, out var error, variant);
+            asIsError.ShouldBeNull(Describe(vector));
+            twinError.ShouldBeNull(Describe(vector));
+            twin.Count.ShouldBe(asIs.Count, Describe(vector));
+
+            Value[] expected = EffectiveTreatUncertainAsBad(vector) || vector.Stepped
+                ? [.. asIs]
+                : [.. asIs.Zip(twin, (a, g) =>
+                    {
+                        g.SourceTicks.ShouldBe(a.SourceTicks, Describe(vector));
+                        return StatusCode.IsBad(g.Status) ? g : a with { Type = g.Type, Payload = g.Payload };
+                    })];
+            if (!expected.SequenceEqual(asIs)) changed++;
+            if (error is not null || !actual.SequenceEqual(expected))
+                failures.Add($"{Describe(vector)} cut {cutSeconds}: error {error}; expected {JsonSerializer.Serialize(expected)} actual {JsonSerializer.Serialize(actual.ToArray())}");
+        }
+        vectors.Count.ShouldBe(384);
+        failures.ShouldBeEmpty("Live TimeAverage / Total must leave the span before an Uncertain first value out (Part 13 'No Start Bound: Calculate Partial Interval', #821).");
+        changed.ShouldBe(expectedChanged, "the late-start variants the #821 override changes from the untouched baseline");
+    }
+
+    /// <summary>
+    ///     #821 hand-computed pin, independent of every calculator: the "uncertain" data set from +5 (Uncertain 100 at 5,
+    ///     Good 18 at 15, Bad at 25, Good 19 at 35, …), forward, sloped, [0,20). Covered [5,20): 100 → 18 over 10 s = 590,
+    ///     then 18 → the bound at 20 (sloped 18 → 19 across the Bad sample: 18.25) over 5 s = 90.625 ⇒ Total 680.625,
+    ///     TimeAverage 680.625 ÷ 15 = 45.375. The baseline added the ramp 0 → 100 over [0,5) (+250): 930.625 and 46.53125.
+    ///     Uncertain either way (an Uncertain value is used, a Bad one skipped, the interval not covered).
+    /// </summary>
+    [Theory]
+    [InlineData(Total, 680.625, 930.625)]
+    [InlineData(TimeAverage, 45.375, 46.53125)]
+    public void Issue821_late_start_hand_computed(string aggregate, double part13, double baselineValue)
+    {
+        var baseline = LoadBaseline();
+        var vector = baseline.Vectors.Single(v => v.Aggregate == aggregate && v.DataSet == "uncertain" && !v.Reverse && !v.Stepped
+            && !v.UncertainAsBad && !v.Incremental && v.Configuration is null);
+        var (variant, _) = LateStartVariant(baseline.Data["uncertain"], 5);
+        variant[0].Status.ShouldBe(0x40A40000u, "the data set's own Uncertain 100 at +5");
+        var output = Run(baseline, vector, rig: null, out var error, variant);
+        var asIs = Run(baseline, vector, rig: null, out _, variant, baselineAsIs: true);
+        error.ShouldBeNull();
+        double.Parse(output[0].Payload!, CultureInfo.InvariantCulture).ShouldBe(part13, 1e-9);
+        double.Parse(asIs[0].Payload!, CultureInfo.InvariantCulture).ShouldBe(baselineValue, 1e-9);
+        (output[0].Status & 0xFFFF0000u).ShouldBe(StatusCodes.UncertainDataSubNormal.Code);
+        output[0].Status.ShouldBe(asIs[0].Status);
+        output.Skip(1).ShouldBe(asIs.Skip(1), "only the late-start interval changes");
+    }
+
+    /// <summary>
+    ///     A data set cut at <paramref name="cutSeconds"/> with its first non-Bad sample Uncertain (kept Uncertain if it already
+    ///     is), and the same with that sample Good.
+    /// </summary>
+    private static (Sample[] Variant, Sample[] GoodTwin) LateStartVariant(Sample[] data, double cutSeconds)
+    {
+        var cut = data.Where(s => s.Seconds >= cutSeconds).OrderBy(s => s.Seconds).ToArray();
+        var first = Array.FindIndex(cut, s => !StatusCode.IsBad(s.Status));
+        if (first < 0) return (cut, cut);
+        var variant = cut.ToArray();
+        var good = cut.ToArray();
+        variant[first] = cut[first] with { Status = StatusCode.IsUncertain(cut[first].Status) ? cut[first].Status : 0x40A40000u };
+        good[first] = cut[first] with { Status = 0u };
+        return (variant, good);
+    }
+
    private static Baseline LoadBaseline() =>
        JsonSerializer.Deserialize<Baseline>(File.ReadAllText(Path.Combine(AppContext.BaseDirectory, "Fixtures", "opcua-1.5.378.182-aggregates.json")), new JsonSerializerOptions { PropertyNameCaseInsensitive = true })!;

@@ -126,7 +239,8 @@ public sealed class Sdk2AggregateCompatibilityTests
            : null;
    }

-    private static RunOutput Run(Baseline baseline, Vector vector, ServerEventRig? rig, out string? error)
+    private static RunOutput Run(Baseline baseline, Vector vector, ServerEventRig? rig, out string? error,
+        Sample[]? data = null, bool baselineAsIs = false)
    {
        var telemetry = DefaultTelemetry.Create(_ => { });
        var epoch = new DateTime(baseline.EpochTicks, DateTimeKind.Utc);
@@ -142,13 +256,15 @@ public sealed class Sdk2AggregateCompatibilityTests
        };
        var calculator = rig is not null
            ? rig.Server.CurrentInstance.AggregateManager.CreateCalculator(id, start, end, baseline.IntervalMilliseconds, vector.Stepped, config)
-            : CompatibilityAggregateFactory.Create(id, start, end, baseline.IntervalMilliseconds, vector.Stepped, config, telemetry);
+            : baselineAsIs
+                ? new BaselineAdapter(Aggregators.CreateStandardCalculator(id, start, end, baseline.IntervalMilliseconds, vector.Stepped, config, telemetry)!)
+                : CompatibilityAggregateFactory.Create(id, start, end, baseline.IntervalMilliseconds, vector.Stepped, config, telemetry);
        calculator.ShouldNotBeNull($"Runtime registration must serve {vector.Aggregate}.");
        var output = new RunOutput();
        error = null;
        try
        {
-            var samples = baseline.Data[vector.DataSet].OrderBy(x => x.Seconds);
+            var samples = (data ?? baseline.Data[vector.DataSet]).OrderBy(x => x.Seconds);
            foreach (var sample in vector.Reverse ? samples.Reverse() : samples)
            {
                var time = epoch.AddSeconds(sample.Seconds);
@@ -185,6 +301,20 @@ public sealed class Sdk2AggregateCompatibilityTests
        "Double" => double.Parse(sample.Payload!, CultureInfo.InvariantCulture), "Int64" => long.Parse(sample.Payload!, CultureInfo.InvariantCulture), "Boolean" => bool.Parse(sample.Payload!), _ => null };
    static string? Format(object? value) => value switch { null => null, double number => number.ToString("R", CultureInfo.InvariantCulture), float number => number.ToString("R", CultureInfo.InvariantCulture), IFormattable f => f.ToString(null, CultureInfo.InvariantCulture), _ => value.ToString() };
    private sealed class RunOutput : List<Value> { public List<bool> Accepted { get; } = []; }
+
+    /// <summary>The untouched baseline calculator behind the SDK 2 protocol (what live served before #809 / #821).</summary>
+    private sealed class BaselineAdapter(AggregateCalculator calculator) : Opc.Ua.Server.IAggregateCalculator
+    {
+        public NodeId AggregateId => calculator.AggregateId;
+        public bool QueueRawValue(DataValue value) => calculator.QueueRawValue(LegacyDataValue.FromSdk(value));
+        public bool HasEndTimePassed(DateTimeUtc currentTime) => calculator.HasEndTimePassed((DateTime)currentTime);
+        public bool TryGetProcessedValue(bool returnPartial, out DataValue value)
+        {
+            var processed = calculator.GetProcessedValue(returnPartial);
+            value = processed is null ? default : processed.ToSdk();
+            return processed is not null;
+        }
+    }
    record Baseline(long EpochTicks, int IntervalMilliseconds, Dictionary<string,Sample[]> Data, Vector[] Vectors);
    record Sample(double Seconds,string Type,string? Payload,uint Status,ushort SourcePicoseconds = 0,ushort ServerPicoseconds = 0,long ServerOffsetTicks = 0);
    record Value(bool PartialRequested,string? Type,string? Payload,uint Status,long SourceTicks,long ServerTicks,ushort SourcePicoseconds,ushort ServerPicoseconds);