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Extracting the function makes it a bit easier to
test and use in a generic way.
Also, I wanted this to live in stream_data, so that
it's easier to find if we change how we model
subscriber data.
Finally, I use _.every to do the subset check
instead of `_.difference`, since _.difference
is actually N-squared:
_.difference = restArguments(function(array, rest) {
rest = flatten(rest, true, true);
return _.filter(array, function(value){
return !_.contains(rest, value);
});
});
And we don't actually want to build a list only
to check that it's zero vs. nonzero length.
We now do this, which short circuits as soon
as it finds any key that is only in sub1:
return _.every(sub1.subscribers.keys(), (key) => {
return sub2_set.has(key);
});