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

From: Junio C Hamano <gitster@pobox.com>
Date: Apr 10, 2013, 18:19 UTC
Message-ID: <7vr4iimenm.fsf@alter.siamese.dyndns.org>
In-Reply-To: <7vk3oao3e5.fsf@alter.siamese.dyndns.org>

Junio C Hamano <gitster@pobox.com> writes:

Show 32 quoted lines

> 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>

Note that with the above specification, a possible solution is to show all the items with Y=Unspecified before showing others, but that would not be ideal for the intended use case; pretending Y=U as if Y=max_range is not a usable workaround.

This is "I create a stream of items with specified Y in descending order. There are some items with Y=Unspecified and I want to find appropriate places to mix the latter into that stream".

Because the desired ordering is not a total order, I need to go to the "pair of priority list" route, I think.

Show 14 quoted lines

>
> 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.