Hi,
Am Mittwoch, den 06.01.2021, 10:56 +0100 schrieb Ludovic Courtès:
> Hi,
>
> Leo Prikler <leo.prik...@student.tugraz.at> skribis:
>
> > *gnu/system/shadow.scm (find-duplicates): New variable.
> > (assert-unique-account-names, assert-unique-group-names): New
> > variables.
> > (account-activation): Use them here.
>
> [...]
>
> > +(define (find-duplicates list =)
> > + (match list
> > + ('() '())
>
> This should be:
>
> (match list
> (() '())
> …)
>
> I’m surprised '() works as a pattern.
I think it's because matching literals works, but you're right.
> > + ((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?
> > +(define (assert-unique-account-names users)
> > + (for-each
> > + (lambda (account)
> > + (raise (condition
> > + (&message
> > + (message
> > + (format #f (G_ "account with name '~a' found
> > twice.")
> > + (user-account-name account)))))))
> > + (find-duplicates users (lambda (alice bob)
> > + (string=? (user-account-name alice)
> > + (user-account-name bob))))))
>
> ‘for-each’ looks awkward since we’ll stop on the first one. How
> about
> something like:
>
> (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. Of course we could always count occurrences by allocating a
local hash table and then do some fancy hash-map->list conversion. If
we do use hash-tables, perhaps this could even be a linear algorithm?
Regards,
Leo
