reductio Max of a sum - crossfilter

Question

I have a dataset I'm working with that is buildings and electrical power use over time.
There are two aggregations on these buildings that are simple sums across the entire timespan and I have those written. They end up looking like:
var reducer = reductio();
// How much energy is used in the whole system
reducer.value("energy").sum(function (d) {
return +d.Energy;
});
These work great.
The third aggregation, however, is giving me some trouble. I need to find the point that the sum of all the buildings is at its greatest. I need the max of the sum and the time it happened.
I wrote:
reducer.value("power").sum(function (d) {
return +d.Power;
}).max(function (d) {
return +d.Power;
}).aliasProp({
time: function (d, v) {
return v.Timestamp;
}
});
But, this is not necessarily the biggest power use. I'm pretty sure this returns the sum and the time when any individual building used the most power.
So if the power values were 1, 1, 1, 15. I would end up with 18, when there might be a different moment when the values were 5, 5, 5, 5 for a total of 20. The 20 is what I need.
I am at a loss for how to get the maximum of a sum. Any advice?

Answer 1

Just to restate: You are grouping on time, so your group keys are time periods of some sort. What you want is to find the time period (group) for which power use is greatest.
If I'm right that this is what you want, then you would not do this in your reducer, but rather by sorting the groups. You can order groups by using the group.order method: https://github.com/crossfilter/crossfilter/wiki/API-Reference#group_order
// During group setup
group.order(function(p) { return p.power.sum; })
// Later, when you want to grab the top power group
group.top(1)
Reductio's max aggregation should just give you the maximum value that occurs within the group. So given a group with values 1,1,1,15, you would get back the value 15. It sounds like that's not what you want.
Hopefully I understood properly. If not, please comment. If you can put together an example with toy data that is public and where you can tell me what you would like to see vs what you are getting, I should be able to help out.
Update based on example:
So, what you want (based on the description in the example) is to find the maximum power usage for any given time within the selected time period. So you would do the following:
var timeDim = buildings.dimension(function(d) { return d.Timestamp })
var timeGrp = timeDim.group().reduceSum(function(d) { return d.Power })
var maxResults = timeGrp.top(1)
Whenever you want to find the max power usage time for your current filter, just call timeGrp.top(1) and the key of that group will be the time with the maximum power.
Note: Don't filter on timeDim as the filters on a dimension are not applied to groups defined on that dimension.
Here's an updated JSFiddle that writes out the maximum group to the console: https://jsfiddle.net/esjewett/1o3robm3/1/