How to count all set bits in a bitmap before an index? about roaring

[lemire]

Good day,

They approach you describe would work. It should be reasonably fast.

It is clearly not optimal since it requires creating a new input object and then outputting yet another object. So, at a minimum, you have more memory allocations than you would like.

It is be possible to extend slightly the API to support your need, if it is in a performance-critical component of your code. I would be glad to implement this feature for you. It is not very hard to do. (You might even be able to do it on your own.) Of course, before I do so, I'd be interested in knowing more about your context as it is possible that this is not the right solution to your real problem.

• Daniel

[donovanhide]

Hi Daniel,

thank you very much for the reply. Much appreciated!

My use case, in detail, is:

I have a PublicKey type which is a 32 bytes long Go array. A slice of PublicKeys is maintained. The index of each PublicKey is important and can never change. Rarely, a PublicKey might need to be deleted, but the index of the following PublicKeys must remain the same.

To make this efficient, it seems sensible to keep track of these garbage ids in a compressed bitset. If PublicKey i is deleted, then position i in the bitset is set. If PublicKey with index i is subsequently requested, the bitset is first consulted for a count of set bits below i and PublicKey with position i-count is returned from the slice. The bitset is effectively a concise way to preserve the deleted ids and get the correct PublicKey from the slice after deletions have occurred.

Hope that makes sense! I want to use a slice of PublicKeys, rather than a tree structure to minimise overall memory usage.

Any advice or extensions to the API would be very gratefully received :-)

[lemire]

Ok. I have added a Rank function. It should do what you seek.

(The technical term describing what I think you want is "rank".)

I am closing this. If it is not what you were seeking or if the implementation is faulty, please reopen.

[donovanhide]

Many thanks, looks like exactly what I need, will test and report back if any issues.

Much appreciated!