Allows reduce() to accept object containing three functions

[wssbck]

This is a bit hacky but here is the intent. If reduce() could accept not only three functions but also an object containing three functions, it would be easier to build custom aggregations for crossfilter.

For example, a function:

weightedMean(freq, weight)

with "freq" being frequencies accessor function and "weight" being weights accessor function, would return something along the lines of:

{
    add : weightedMeanAdd,
    remove : weightedMeanRemove,
    initial : weightedMeanInitial
}

All fields of this object would be functions, similar to

crossfilter_reduceAdd(value), crossfilter_reduceSubtract(value), crossfilter_zero

[jasondavies]

Hello! Thanks for the contribution. However, I’m afraid this is a duplicate of existing requests for the same feature: #113, #115, and related #102. We’ve taken the decision not to extend the API to support this, as we’d prefer to keep it simple. As noted in these tickets, it’s possible to have a tiny helper function that unpacks an object and makes the appropriate reduce call, if that’s something that you really need.

[gordonwoodhull]

@wssbck, you might look at the way Ethan Jewett's reductio library does this: by generating the helper function @jasondavies mentions. https://github.com/esjewett/reductio

[wssbck]

OK, thanks for the tips, at first I will just focus on implementing as many reduction functions as possible.