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Tighten the Sensor.timestamp and latest reading["timestamp"] definitions #469

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Aug 10, 2023
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Tighten the Sensor.timestamp and latest reading["timestamp"] definitions #469

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35 changes: 25 additions & 10 deletions index.bs
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Original file line number Diff line number Diff line change
Expand Up @@ -845,13 +845,10 @@ the [=sensor reading=] as arguments.

The [=latest reading=] [=ordered map|map=] contains an [=map/entry=] whose [=map/key=] is
"timestamp" and whose [=map/value=] is a high resolution timestamp that estimates the
[=reading timestamp=] expressed in milliseconds since the [=time origin=].
[=reading timestamp=] expressed in milliseconds as an [=monotonic clock/unsafe current time=].

Note: The accuracy of the [=reading timestamp=] estimate depends on the underlying
platform interfaces that expose it.

The [=latest reading=]["timestamp"] is initially set to null,
unless the [=latest reading=] [=ordered map|map=] caches a previous [=sensor readings|reading=].
[=Latest reading=]["timestamp"] is initially set to null, unless the [=latest reading=] [=map=]
caches a previous [=sensor readings|reading=].

The other [=map/entries=] of the [=latest reading=] [=ordered map|map=]
hold the values of the different quantities measured by the [=platform sensor=].
Expand Down Expand Up @@ -1146,11 +1143,14 @@ with the internal slots described in the following table:

### Sensor.timestamp ### {#sensor-timestamp}

The {{Sensor/timestamp!!attribute}} getter steps are to return
the result of invoking [=get value from latest reading=] with [=this=]
and "timestamp" as arguments.
<div algorithm="sensor timestamp">
The {{Sensor/timestamp}} getter steps are:

It represents a [=reading timestamp=].
1. Let |global| be [=this=]'s [=relevant global object=].
2. Let |unsafeTimestamp| be the result of invoking [=get value from latest reading=] with
[=this=] and "timestamp" as arguments.
3. Return [=relative high resolution time=] with |unsafeTimestamp| and |global|.
</div>

### Sensor.start() ### {#sensor-start}

Expand Down Expand Up @@ -1437,6 +1437,21 @@ to {{SensorErrorEventInit}}.
1. Let |result| be the result of invoking |type|'s [=threshold check algorithm=]
with |reading| and |sensor|'s [=latest reading=].
1. If |result| is false, then abort these steps.
1. If |reading|["timestamp"] [=map/exists=]:
<!-- The language below is similar to what is found in
https://w3c.github.io/hr-time/#timeorigin-attribute -->
1. Set |reading|["timestamp"] to the result of converting its current value in an
[=implementation-defined=] way to an [=monotonic clock/unsafe current time=] using the
same [=monotonic clock=] that is shared by [=time origins=].

Note: The goal of this step is to ensure that a timestamp that may have been relative to
a different time origin is converted to a value that can be used in computations with
the same [=monotonic clock=] used by the operations described in [[HR-TIME]].
1. Otherwise, [=map/set=] |reading|["timestamp"] to the [=unsafe shared current time=].

Note: In neither case is an [=monotonic clock/unsafe current time=] ever exposed to script.
{{Sensor/timestamp|Sensor.timestamp}} always returns a [=moment/coarsened moment=] relative
to a [=time origin=].
1. [=map/For each=] |key| → <var ignore>value</var> of [=latest reading=].
1. [=map/Set=] [=latest reading=][|key|] to the corresponding
value of |reading|.
Expand Down
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