Filter data being passed into a crossfilter group without using dimensional filters - grouping

Question

I'm trying to figure out how to add a filter onto a crossfilter group that is not related to a dimensional filter. Let's look at an example:
var livingThings = crossfilter({
// Fact data.
{ name: “Rusty”, type: “human”, legs: 2 },
{ name: “Alex”, type: “human”, legs: 2 },
{ name: “Lassie”, type: “dog”, legs: 4 },
{ name: “Spot”, type: “dog”, legs: 4 },
{ name: “Polly”, type: “bird”, legs: 2 },
{ name: “Fiona”, type: “plant”, legs: 0 }
}); //taken from http://blog.rusty.io/2012/09/17/crossfilter-tutorial/
if we were to make a dimension on type and a group of that dimension:
var typeDim = livingThings.dimension(function(d){return d.type});
var typeGroup = typeDim.group();
we would expect typeGroup.top(Infinity) to output
{{human:2},
{dog:2},
{bird:1},
{plant:1}}
My question is how can we filter the data such that they include only 4 legged creatures in this grouping? I also don't want to use dimension.filter... because i don't want this filter to be global, just for this one grouping. In other words
var filterDim = livingThings.dimension(function(d){return d.legs}).filterExact(4);
is not allowed.
I'm thinking of something similar to what I did to post-filter dimensions as in https://stackoverflow.com/a/30467216/4624663
basically I want to go into the internals of the typeDim dimension, and filter the data before it is passed into the groups. Creating a fake group that calls typeDim.group().top() will most likely not work as the individual livingThings records are already grouped by that point. I know this is tricky: thanks for any help.
V

Answer 1

Probably best to use the reduceSum functionality to create a pseudo-count group that only counts records with 4 or more legs:
var livingThings = crossfilter({
// Fact data.
{ name: “Rusty”, type: “human”, legs: 2 },
{ name: “Alex”, type: “human”, legs: 2 },
{ name: “Lassie”, type: “dog”, legs: 4 },
{ name: “Spot”, type: “dog”, legs: 4 },
{ name: “Polly”, type: “bird”, legs: 2 },
{ name: “Fiona”, type: “plant”, legs: 0 }
}); //taken from http://blog.rusty.io/2012/09/17/crossfilter-tutorial/
var typeDim = livingThings.dimension(function(d){return d.type});
var typeGroup = typeDim.group().reduceSum(function(d) {
return d.legs === 4 ? 1 : 0;
});
That will sum across a calculated value that will be 1 for records with 4 legs and 0 for records with ... not 4 legs. In other words, it should just count 4-legged creatures.

Answer 2

I think, this is what you are looking for. Comment back if I'm wrong.
var dimByLegs = livingThings.dimension(function(d){return d.legs});
dimByLegs.filterExact(4);
var dogs = dimByLegs.group();
dimByLegs.top(Infinity).forEach(function(d){console.log(d.type, d.legs);});
dimByLegs.dispose();