How does Geofire calculate bounding geohashes for Firebase queries?

Question

Context

I've used Geofire along with Firebase for a while now and I got really curious how Geofire performs its queries. I understand that semantically, it's a function of coordinates and a radius that results in a minimum and maximum hashes. So the way I think it works in conjuncture with Firebase would look like this

ref.child("users").orderByChild("g").startAt(minHash).endAt(maxHash).on('child_added', function(snapshot) { /* retrieved snapshot contains the geohashes in range */ });

Where these two (min and max) geohashes are calculated off the given inputs. Now here comes the question

Question(s)

Assuming that what I stated above is correct, how are these two geohashes calculated? How do they return results within a certain circular region when geohashes typically represent bounding rectangles? And finally, how can two geohashes of different sizes have the same center?

To clarify on that last part: consider the following image

Since geohashing works by halving regions into smaller regions, how can two hashes of different sizes (min and max) have the same center points?

Assumptions

I thought that maybe it's as simple as increasing/decreasing the raw value of the hash, but that doesn't make much sense since the increase/decrease should be relative to the size of the hash (the "zoom" level so to speak) and the query radius, if I'm not mistaken.

Answer 1

GeoFire actually performs a range query of a rectangular region against the database. This range is the smallest rectangle that encompasses the range indicated in the query.

It then in the client-side code checks the actual distance of each key to the center of the query and only fires the key_entered/key_moved event for items inside the query.

The relevant code is here:

// Determine if the location is within this query
distanceFromCenter = GeoFire.distance(location, _center);
isInQuery = (distanceFromCenter <= _radius);

...

// Fire the "key_entered" event if the provided key has entered this query
if (isInQuery && !wasInQuery) {
  _fireCallbacksForKey("key_entered", key, location, distanceFromCenter);
} else if (isInQuery && oldLocation !== null && (location[0] !== oldLocation[0] || location[1] !== oldLocation[1])) {
  _fireCallbacksForKey("key_moved", key, location, distanceFromCenter);
} else if (!isInQuery && wasInQuery) {
  _fireCallbacksForKey("key_exited", key, location, distanceFromCenter);
}