gnus: Ranges
11.7.5 Ranges
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GNUS introduced a concept that I found so useful that I’ve started using
it a lot and have elaborated on it greatly.
The question is simple: If you have a large amount of objects that
are identified by numbers (say, articles, to take a _wild_ example) that
you want to qualify as being “included”, a normal sequence isn’t very
useful. (A 200,000 length sequence is a bit long-winded.)
The solution is as simple as the question: You just collapse the
sequence.
(1 2 3 4 5 6 10 11 12)
is transformed into
((1 . 6) (10 . 12))
To avoid having those nasty ‘(13 . 13)’ elements to denote a lonesome
object, a ‘13’ is a valid element:
((1 . 6) 7 (10 . 12))
This means that comparing two ranges to find out whether they are
equal is slightly tricky:
((1 . 5) 7 8 (10 . 12))
and
((1 . 5) (7 . 8) (10 . 12))
are equal. In fact, any non-descending list is a range:
(1 2 3 4 5)
is a perfectly valid range, although a pretty long-winded one. This
is also valid:
(1 . 5)
and is equal to the previous range.
Here’s a BNF definition of ranges. Of course, one must remember the
semantic requirement that the numbers are non-descending. (Any number
of repetition of the same number is allowed, but apt to disappear in
range handling.)
range = simple-range / normal-range
simple-range = "(" number " . " number ")"
normal-range = "(" start-contents ")"
contents = "" / simple-range *[ " " contents ] /
number *[ " " contents ]
Gnus currently uses ranges to keep track of read articles and article
marks. I plan on implementing a number of range operators in C if The
Powers That Be are willing to let me. (I haven’t asked yet, because I
need to do some more thinking on what operators I need to make life
totally range-based without ever having to convert back to normal
sequences.)
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