Transversing an index

[Therefore]

We are developing a C++ application development tool that uses mongoDB as the underlying database. Say the user has developed a patient collection with fields _id & patient# with a unique ascending index on patient#. Say there are 10 patients with patient#s 1, 5, 7, 13, 14, 20, 21, 22, 23, 25. Patient# 20 is displayed fetched with limit(1). The user presses PageDown and patient# 21 is displayed -- easy with $gt with patient# = 20.

When the user presses PageUp, patient# 14 should be displayed. My (hopefully wrong) solution is to create a parallel descending index on patient# and $lt. That implies every collection a user creates requires both indexes on the primary key fields to get bidirectional movement. That would apply also to secondary indexes such as name.

Additionally, the user presses F5, is prompted "# of records to move", they enter 3, patient# 23 should be displayed. Or they enter -3, patient# 7 should be displayed. My idea: First use a covered query and return the following 3 patient#s from the index and then fetch the 3rd document. This isn't at all ideal when a less simplified application has hundreds of thousands of documents and the user wants to transverse by 10s of thousands of records. And, again, to achieve the backward movement, I believe I would need that second descending index.

Finally, I need a way to have the user navigate to the last index entry (patient #25). Going to the beginning is trivial. Again, second index?

My question: Is there a way to transverse the ascending index using (say) an iterator or pointer to the current index element and then use iterator/pointer arithmetic to achieve what I want? I.e., +1 will get me the "next" index element from which I could fetch the "next" document; -1 the "previous", +3 the third following, -3 the third previous; index size to the last.