Drools, accumulate sum and then max - drools

Question

My java class Task has member groupId(String) and duration(long). The sum of task durations of each group, say group1 has task1, task2, task3, corresponding durations are d1, d2, d3, so the sum s1=d1+d2+d3; I want to get the largest s of all groups, say for group1 the sum is s1, group2 is s2, I want to get the $smax = max(s1,s2,...) and then do something on $smax in "then". How can I do this? There is no group class, a String named groupId is used to identify the same group. How to specify the "same-group-id"? Do I have to introduce group class? And how to get the $smax? Many thanks.
when
accumulate(Task("same-group-id", $d: duration);
$s: sum($d))
accumulate($smax: max($s))
then
// do something with $smax
Further question. Roddy's answer works perfectly in drools. But in my case it's used in Optaplanner, so Task must show up in "when" otherwise the rule will not be triggered. Is there any way to merge Roddy's two rules into one? The goal is still to get the largest group duration in "then" so that something can be done on it. I can make changes in java if it's needed.

Answer 1

You've got a bit of a wall of text going on here, but I'll try to parse out what you're trying to do.
You have Task objects in working memory with String groupId and long duration members
You want to group Tasks by the groupId and then sum up the durations for each group. --> basically get the total duration for each group.
Finally you want to find the longest of those summed durations. --> Effectively, you want to know which group has the longest duration.
If I'm avoiding using inline custom code in my accumulate, I'd probably do something like this:
declare GroupDuration {
id : String
duration : long
}
rule "Find Group Durations"
salience 1
when
Task( $id: groupId )
not( GroupDuration( id == $id ))
accumulate( Task( groupId == $id, $d: duration);
$duration: sum($d))
then
GroupDuration d = new GroupDuration();
d.setId($id);
d.setDuration($duration);
insert(d)
end
rule "Find Longest Group Duration"
when
GroupDuration( $id: id, $duration: duration )
not( GroupDuration( duration > $duration ))
then
// $id is the longest
end
I start off by declaring a new type GroupDuration to track the id and total duration of the group. Then I populate working memory with those durations in the first rule, using accumulate to sum up all durations for tasks that have matching IDs. Finally, I find the instance with the largest duration and yank its id.