From: "Ludovic Courtès" <ludo@gnu.org>
To: Leo Prikler <leo.prikler@student.tugraz.at>
Cc: 45570@debbugs.gnu.org, conjaroy@gmail.com
Subject: bug#45570: [PATCH] system: Assert, that user and group names are unique.
Date: Thu, 07 Jan 2021 09:29:59 +0100 [thread overview]
Message-ID: <87im89jgjc.fsf@gnu.org> (raw)
In-Reply-To: <ae66ed3a5c4b862c9cc9ae2bdb6954a51434d79c.camel@student.tugraz.at> (Leo Prikler's message of "Wed, 06 Jan 2021 22:00:03 +0100")
Hi,
Leo Prikler <leo.prikler@student.tugraz.at> skribis:
> Am Mittwoch, den 06.01.2021, 14:32 +0100 schrieb Ludovic Courtès:
>> Hi,
>>
>> Leo Prikler <leo.prikler@student.tugraz.at> skribis:
>>
>> > > > + ((first . rest)
>> > > > + (if (member first rest =) ; (srfi srfi-1) member
>> > > > + (cons first (find-duplicates rest =))
>> > > > + (find-duplicates rest =)))))
>> > >
>> > > Note that this is quadratic; it’s fine as long as we don’t have
>> > > “too
>> > > many” users, which may be the case in general.
>> > It is indeed quadratic, but would there even be an n log n
>> > solution?
>> > I've once done an n log n sort+delete-duplicates!, perhaps that'd
>> > be a
>> > nicer solution here?
>>
>> You could first build a hash table or vhash or set with all the
>> names,
>> then traverse again the list of names and check whether they’re in
>> that
>> table. That’d be linear (assuming the table is well balanced), but
>> the
>> constant factor would be higher.
> Yeah, I think the hash table solution would make the most sense here.
> Since VHashes are based on VLists, they're not actually purely
> functional, are they?
Their implementation is not “purely functional” but it’s
inconsequential; it’s a persistent data structure, and that’s what
matters (info "(guile) VLists").
>> > > (define (assert-unique-account-names users)
>> > > (match (find-duplicates things …)
>> > > (() #t)
>> > > (lst
>> > > (raise (formatted-message (G_ "the following accounts
>> > > appear
>> > > more than once:~{ ~a~}~%"
>> > > lst))))))
>> > >
>> > > ?
>> > That'd be weird for duplicate duplicates, hence just reporting the
>> > first.
>>
>> You could do (delete-duplicates lst) in the message above?
> Sure, but that'd be O(n^2) on top of O(n^2), which is less than ideal.
Yes, but it’s a small ‘n’, typically one or two.
Ludo’.
next prev parent reply other threads:[~2021-01-07 8:31 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-31 18:14 bug#45570: operating-system definitions allow duplicate passwd and group entries Jason Conroy
2021-01-01 11:13 ` bug#45570: [PATCH] system: Assert, that user and group names are unique Leo Prikler
2021-01-02 1:16 ` Danny Milosavljevic
2021-01-02 5:57 ` Leo Prikler
2021-01-06 9:56 ` Ludovic Courtès
2021-01-06 12:34 ` Leo Prikler
2021-01-06 13:32 ` Ludovic Courtès
2021-01-06 21:00 ` Leo Prikler
2021-01-07 8:29 ` Ludovic Courtès [this message]
2021-01-06 21:21 ` bug#45570: [PATCH v2] " Leo Prikler
2021-01-07 8:35 ` Ludovic Courtès
2021-01-07 11:13 ` Leo Prikler
2021-01-07 11:10 ` bug#45570: [PATCH v3] " Leo Prikler
2021-01-11 13:09 ` Ludovic Courtès
2021-01-11 15:06 ` Leo Prikler
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