Get average after groupby for a fair balancing

Question

I would like to test if the count (after grouping by technician, numberOfTasksByTechnician) above average for a good balance of tasks for technicians (every technician must do the same number of tasks), how can I get the average (hard coded value 4)

Constraint technicianCharge(ConstraintFactory constraintFactory)  {
        return constraintFactory.from(Task.class)
            .groupBy(
                    Task::getTechnician, ConstraintCollectors.countDistinct()
            )
            .filter((technician, count)-> {
                return count > 4; // return count > (?average)
            })
            .penalize("technicianCharge", HardSoftScore.ONE_SOFT);
}

Answer 1

At the moment, to collect average, you need to write a custom constraint collector. It's not hard to do, check out count() implementation for inspiration. When you have your constraint collector, your constraint stream would then look like this:

Constraint technicianCharge(ConstraintFactory constraintFactory)  {
    return constraintFactory.from(Task.class)
        .groupBy(
             Task::getTechnician, 
             ConstraintCollectors.countDistinct(),
             MyConstraintCollectors.average(task -> ...)
        )
        .filter((technician, count, average)-> {
            return count > average;
        })
        .penalize("technicianCharge", 
            HardSoftScore.ONE_SOFT,
            (technician, count, average) -> count - average);
}

In the future, we may provide the average constraint collector out-of-the-box .

As a footnote, since you mentioned fairness, you may want to see our blog on OptaPlanner load balancing and fairness .

Answer 2

As of OptaPlanner 8.11.0.Final, 'average' is available out of the box.

https://www.optaplanner.org/blog/2021/08/19/ConstraintStreamsSomeMoreLove.html

https://docs.optaplanner.org/8.11.0.Final/optaplanner-javadoc/org/optaplanner/core/api/score/stream/ConstraintCollectors.html

cheers :D