How can I compare current value in my stream with previous value in tickscript?

InfluxDB 2
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13

Hi ,
@adarsh_hatwar
talks about these relative alerts in chronograf in this post ,
https://community.influxdata.com/t/alerting-if-mean-of-a-value-is-above-a-threshold-value-for-a-certain-period-of-time/8969

I have tried it out for your script and chronograf came up with this …

var db = 'dbcpu'
var rp = 'autogen'
var measurement = 'cpu'
var groupBy = []
var whereFilter = lambda: ("cpu" == 'cpu-total')
var name = 'compare'
var idVar = name
var message = ''
var idTag = 'alertID'
var levelTag = 'level'
var messageField = 'message'
var durationField = 'duration'
var outputDB = 'chronograf'
var outputRP = 'autogen'
var outputMeasurement = 'alerts'
var triggerType = 'relative'
var shift = 5m
var crit = 0
var data = stream
    |from()
        .database(db)
        .retentionPolicy(rp)
        .measurement(measurement)
        .groupBy(groupBy)
        .where(whereFilter)
    |eval(lambda: "usage_idle")
        .as('value')

var past = data
    |shift(shift)

var current = data

var trigger = past
    |join(current)
        .as('past', 'current')
    |eval(lambda: float("current.value" - "past.value"))
        .keep()
        .as('value')
    |alert()
        .crit(lambda: "value" > crit)
        .message(message)
        .id(idVar)
        .idTag(idTag)
        .levelTag(levelTag)
        .messageField(messageField)
        .durationField(durationField)
        .stateChangesOnly()

trigger
    |eval(lambda: float("value"))
        .as('value')
        .keep()
    |influxDBOut()
        .create()
        .database(outputDB)
        .retentionPolicy(outputRP)
        .measurement(outputMeasurement)
        .tag('alertName', name)
        .tag('triggerType', triggerType)

trigger
    |httpOut('output')