# During another Event

A Rule can refer to the Events of another Rule by passing that Rule itself. `during(a)` holds while an Event of Rule `a` is Confirmed and has not Ended. It turns an Event into a state that other Rules can use.

A Rule may only refer to Rules added before it with `rules(...)`, so it sees their state at the same Sample, and dependencies can never be circular. Since the reference is the Rule object, not a name, a misspelled reference does not compile, and `build()` rejects a reference to a Rule that was not added or is added later.

## Restricting a Rule to a phase

Many criteria only make sense in a certain operating state. Vibration of a stopped machine is irrelevant, and a Rule for it would produce an Event for every truck driving past. Define the phase once as a Rule and refer to it:

```java
Rule running = Rule.named("Running").when(rpm.above(100)).forDuration(Duration.ofSeconds(10));
Rule roughRunning = Rule.named("RoughRunning").when(during(running).and(vibration.above(4)));
```

`RoughRunning` Ends at the latest when `Running` Ends. The phase Rule brings its own timing, here ten seconds of start-up during which vibration is ignored, which a plain `rpm.above(100)` could not express.

## Combining Events with their own timing

Each part keeps its own duration, End Condition and hysteresis, and the combination holds while both are Confirmed. A patient's heart rate `hr` is above 100 from 40 s to 200 s, and once more from 230 s to 250 s. Their oxygen saturation `spo2` dips below 90 for one Sample at 20 s, from 100 s to 160 s, and from 230 s to 260 s. `Tachycardia` needs 30 seconds, `Desaturation` 10, and `HypoxicTachycardia` holds while both are Confirmed.

Parameters (in the page address as ?tachycardia=…):

- Tachycardia duration (`tachycardia`): 0 to 60 s in steps of 5, default 30
- Desaturation duration (`desaturation`): 0 to 30 s in steps of 5, default 10

The site cannot run the default parameters: (JavaScript) TypeError: Cannot read properties of undefined (reading 'length')

Java source at the default parameters:

```java
import static io.github.romanhaendler.rbed.Conditions.*;

import io.github.romanhaendler.rbed.*;
import java.time.Duration;
import java.util.Map;

Channel hr = Channel.named("hr");
Channel spo2 = Channel.named("spo2");

Rule tachycardia = Rule.named("Tachycardia")
    .when(hr.above(100)).forDuration(Duration.ofSeconds(30));
Rule desaturation = Rule.named("Desaturation")
    .when(spo2.below(90)).forDuration(Duration.ofSeconds(10));
Rule hypoxicTachycardia = Rule.named("HypoxicTachycardia")
    .when(during(tachycardia).and(during(desaturation)));
Detector detector = Detector.builder()
    .rules(tachycardia, desaturation, hypoxicTachycardia)
    .build();

for (int s = 0; s <= 300; s += 5) {   // one Sample every 5 s
  detector.push(s * 1_000L, Map.of(
      hr, s >= 40 && s < 200 ? 115.0 : s >= 230 && s < 250 ? 110.0 : 75.0,
      spo2, s == 20 ? 86.0 : s >= 100 && s < 160 ? 87.0 : s >= 230 && s < 260 ? 86.0 : 97.0));
}
detector.close();
```

At the default durations:

- `Tachycardia` goes Pending at 40 s and is Confirmed at 70 s. `Desaturation` goes Pending at 100 s and is Confirmed at 110 s, and so is `HypoxicTachycardia`: a Pending Event does not count for `during`.
- `HypoxicTachycardia` Ends at 160 s with `Desaturation`, although `Tachycardia` runs until 200 s. A combination Ends at the latest when one of its parts Ends.
- The dip at 20 s is too short for `Desaturation` and is Discarded.
- At 230 s both go Pending again. `Desaturation` is Confirmed at 240 s, but the heart rate falls at 250 s, before its 30 seconds are reached: `Tachycardia` is Discarded, and there is no `HypoxicTachycardia`.

### Things to try

- Shorten `Tachycardia`'s duration to 15 s: it is Confirmed at 245 s, so `HypoxicTachycardia` holds from 245 s until `Tachycardia` Ends at 250 s.
- Set `Desaturation`'s duration to 0 s: `HypoxicTachycardia` starts at 100 s, as soon as the saturation drops, and even the dip at 20 s is a `Desaturation` of its own.

The [Healthcare](https://romanhaendler.github.io/rbed/healthcare/) example uses these Rules on a longer recording.

## Reference

- [during: while another Event runs](https://github.com/romanhaendler/rbed#during-while-another-event-runs) and [Rule order](https://github.com/romanhaendler/rbed#rule-order) in the README, the canonical text
- [Conditions](https://romanhaendler.github.io/rbed/api/io.github.romanhaendler.rbed/io/github/romanhaendler/rbed/Conditions.html) in the Javadoc

Methods used: [`Channel.named`](https://romanhaendler.github.io/rbed/api/io.github.romanhaendler.rbed/io/github/romanhaendler/rbed/Channel.html#named%28java.lang.String%29), [`Rule.named`](https://romanhaendler.github.io/rbed/api/io.github.romanhaendler.rbed/io/github/romanhaendler/rbed/Rule.html#named%28java.lang.String%29), [`Rule.when`](https://romanhaendler.github.io/rbed/api/io.github.romanhaendler.rbed/io/github/romanhaendler/rbed/Rule.html#when%28io.github.romanhaendler.rbed.Condition%29), [`Expression.above`](https://romanhaendler.github.io/rbed/api/io.github.romanhaendler.rbed/io/github/romanhaendler/rbed/Expression.html#above%28double%29), [`Rule.forDuration`](https://romanhaendler.github.io/rbed/api/io.github.romanhaendler.rbed/io/github/romanhaendler/rbed/Rule.html#forDuration%28java.time.Duration%29), [`Expression.below`](https://romanhaendler.github.io/rbed/api/io.github.romanhaendler.rbed/io/github/romanhaendler/rbed/Expression.html#below%28double%29), [`Conditions.during`](https://romanhaendler.github.io/rbed/api/io.github.romanhaendler.rbed/io/github/romanhaendler/rbed/Conditions.html#during%28io.github.romanhaendler.rbed.Rule%29), [`Condition.and`](https://romanhaendler.github.io/rbed/api/io.github.romanhaendler.rbed/io/github/romanhaendler/rbed/Condition.html#and%28io.github.romanhaendler.rbed.Condition%29), [`Detector.builder`](https://romanhaendler.github.io/rbed/api/io.github.romanhaendler.rbed/io/github/romanhaendler/rbed/Detector.html#builder%28%29), [`Detector.Builder.rules`](https://romanhaendler.github.io/rbed/api/io.github.romanhaendler.rbed/io/github/romanhaendler/rbed/Detector.Builder.html#rules%28io.github.romanhaendler.rbed.Rule...%29), [`Detector.Builder.build`](https://romanhaendler.github.io/rbed/api/io.github.romanhaendler.rbed/io/github/romanhaendler/rbed/Detector.Builder.html#build%28%29), [`Detector.push`](https://romanhaendler.github.io/rbed/api/io.github.romanhaendler.rbed/io/github/romanhaendler/rbed/Detector.html#push%28long,java.util.Map%29), [`Detector.close`](https://romanhaendler.github.io/rbed/api/io.github.romanhaendler.rbed/io/github/romanhaendler/rbed/Detector.html#close%28%29).