Thank you for your prompt response. I am evaluating if this would be a smart way:
- instead of array of “available bits” (array of n Integers, growing up to 32.000 Integers) I could store a raw bloom filter bitmap as a data, what should be negligible in size (about 5kB)
- in a cloud function I would need to serialise/deserialise it in to an array of “available bits” for the query to use indexing
- when the results would come to the client, the raw bitmap could get recalculated and updated (either on client device or directly in that cloud function (depending on how expensive it would be)
As I am total beginner with javaScript and this project is all self-learned in my free time, only thinking about writing such function is already making me already cry. 
Anyway, does that sound reasonable? I hear often that you should not recompute something that you can precompute (storing array of available bits), but if this would need a little cpu resources, perhaps it is the way. Any thoughts?
- For documentation purpose I tried to contact anyone from MongoDB forum here