[rfh] do I need to use something more complex to do this?

I have set of items with two attributes, <X,Y>, and would like to keep them in some data structure in such a way that it is efficient to (1) add a new item to the data structure, and (2) pick an item in a specific order. There can be multiple items that share the same value for X, or Y, or both X and Y, and it does not matter in what order items comes out among those that share the same <X,Y>.

The type of X is totally ordered. The type of Y also usually is, but Y can take a special value U(nspecified).

Now on to the "specific" order I want to pick an item. I'd like to take the item with the largest value of Y in general, and tiebreaking on the value of X which also I prefer to take from larger to smaller.

But with a twist.

When I am picking an item <X=n,Y=m>, there should be no item remaining in the data store with a value of Y that is smaller than m (duplicates are allowed, so there can still be items with Y=m), and also when I am picking <X=n,Y=m>, there should be no item with Y=Unspecified that has a value of X that is equal or smaller than n.

E.g. if I have these 6 items (ignore the lines between the items for now):

            <104,U>--<105,U>--<106,4>
           /
    <101,U>--<100,U>--<102,3>--<104,4>

I would want to pick them up in this order:

    <106,4> <105,U> <104,U> <104,4> <102,3> <101,U> <100,U>

I see how this can easily be done by using a two priority lists, i.e. one for items with Y=Unspecified that is sorted by X, and the other for all other items that is sorted by <Y,X>. Peek the top of both, and pick the top of the former until its X is smaller than the value of X of the top of the latter, otherwise pick the top of the latter. I am wondering if I can use less complex data structure, like a single ordered sorted array, with a clever comparison function.

For the curious, the items in the above picture represents commits, and lines are ancestry chains between them. I am thinking how we can extend the still_interesting() function with an optional generation number.