Steve Howell c2af2c1fd1 refactor: Extract is_subscriber_subset(). ...

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);
    });

2020-01-14 13:19:49 -08:00

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