Calculate Delta - Aggregate Filter

[karnamonkster (Karan)]

Figured out..
Posting the solution, may help someone.

filter {
        aggregate {
        task_id => "%{uniquedeviceid}"
        code => "
                map['next'] ||= event.get('reading');
                event.set('delta', (event.get('reading') - map['next']))"
                timeout_task_id_field => "uniquedeviceid"
        }