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Subject: Re: [boost] [Container] Priority Deque - Refinement
From: tim (tim_at_[hidden])
Date: 2013-11-26 02:14:44


>> having fifo/max lifo/min would be quite reasonable. a fifo of both min
> and max sides could probably be achieved by using use stability
> An implementation using a single stability counter modifies the comparison
> (which must be a strict weak ordering) to break ties according to the order
> in which the elements were added.
> For a priority queue, newer objects are considered "less" than older
> objects.
> For a priority deque, breaking ties in favor of one direction precludes
> breaking of ties in the other. Thus, simultaneous FIFO max and LIFO min is
> trivial (use the same stability counter as used for the priority queue),
> but that particular method cannot be used efficiently for simultaneous FIFO
> max and FIFO min.
> I don't believe that using two stability counters would help, but if you've
> got ideas or (preferably) details about how it could be accomplished, I
> would be happy to read them.

one will have to change the way the stability counters are used: one
possibility is to have a list of elements for each key (priority) and
look for the maximum stability count for the minimum or maximum side.
this is more a node-based approach, which probably does not translate
very well to a container adaptor, though ...

i had been thinking about the chaining of identical keys when
implementing boost.heap ... it has the nice advantage that it slightly
simplifies the tree structures

> While I'm on the subject of stability counters, I believe I can improve on
> the method used in Boost.Heap's stable priority queue. In particular, the
> Boost.Heap queue does not handle integer overflow except by throwing an
> exception. A better way to handle overflow would, I believe, be to sort the
> underlying container according to its counters (O(n^2), O(n log n), or
> O(n), depending on sort used). Follow this by editing the counters to be
> consecutive from the minimum possible value (O(n)), and by re-building the
> heap (O(n)).

yes, the handling of the stability count is a bit fragile, though 64bit
integers are probably enough to prevent overflows in most applications.
a simple way to prevent the overflow could also be to simply subtract
the minimum value of all stability counts, which could be done in O(n)
by traversing the heap twice.


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