# Counting Events

`count(a)` is the number of Events of Rule `a` Confirmed so far, an Expression like any other. `.within(duration)` counts only those Confirmed in a sliding window ending now:

```java
Rule run = Rule.named("Run").when(power.above(500)).forDuration(Duration.ofSeconds(30));
Rule frequentStarts = Rule.named("FrequentStarts")
    .when(count(run).within(Duration.ofHours(1)).above(3));
Rule serviceDue = Rule.named("ServiceDue").when(count(run).above(10_000));
```

An Event counts from the moment it is Confirmed, so a running Event is included.

## Frequent starts

A machine's power is sampled once a minute. It runs for five minutes from minutes 10, 30, 45, 60, 150 and 200. A `Run` needs 30 seconds above 500 W, so each one is Confirmed at the next Sample, a minute after its start. `FrequentStarts` holds while more than the limit of Runs were Confirmed within the window.

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

- Window (`window`): 10 to 120 min in steps of 10, default 60
- Limit (`limit`): 0 to 6 Runs in steps of 1, default 3

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 power = Channel.named("power");

Rule run = Rule.named("Run").when(power.above(500)).forDuration(Duration.ofSeconds(30));
Rule frequentStarts = Rule.named("FrequentStarts")
    .when(count(run).within(Duration.ofMinutes(60)).above(3));
Detector detector = Detector.builder()
    .rules(run, frequentStarts)
    .build();

int[] starts = {10, 30, 45, 60, 150, 200};   // each run lasts five minutes
for (int m = 0; m <= 240; m++) {   // one Sample a minute
  double watts = 0;
  for (int start : starts) if (m >= start && m < start + 5) watts = 800;
  detector.push(m * 60_000L, Map.of(power, watts));
}
detector.close();
```

At the defaults, an hour's window and a limit of 3:

- The count rises by one each time a `Run` is Confirmed: 1 at minute 11, 2 at 31, 3 at 46.
- At minute 61 the fourth `Run` is Confirmed, while it is still running. The count is 4, more than 3, and `FrequentStarts` starts.
- At minute 72 the `Run` Confirmed at minute 11 lies more than an hour back and drops out of the window. The count falls to 3, and `FrequentStarts` Ends.
- The later Runs are too far apart to count up again.

`above(3)` means more than 3, so it takes the fourth start. Without `within`, as in `ServiceDue`, the count only grows.

### Things to try

- Narrow the window to 30 minutes: no four starts fall into it, and `FrequentStarts` never holds.
- Lower the limit to 2: the third `Run`, Confirmed at minute 46, is enough. `FrequentStarts` runs from 46 to 92, where the second `Run` drops out of the window.

The [Building services](https://romanhaendler.github.io/rbed/building-services/) example counts a compressor's starts the same way.

## Reference

- [count: how many Events](https://github.com/romanhaendler/rbed#count-how-many-events) in the README, the canonical text
- [EventCount](https://romanhaendler.github.io/rbed/api/io.github.romanhaendler.rbed/io/github/romanhaendler/rbed/EventCount.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), [`Conditions.count`](https://romanhaendler.github.io/rbed/api/io.github.romanhaendler.rbed/io/github/romanhaendler/rbed/Conditions.html#count%28io.github.romanhaendler.rbed.Rule%29), [`EventCount.within`](https://romanhaendler.github.io/rbed/api/io.github.romanhaendler.rbed/io/github/romanhaendler/rbed/EventCount.html#within%28java.time.Duration%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).