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From: Lars Ingebrigtsen <larsi@gnus.org>
To: Drew Adams <drew.adams@oracle.com>
Cc: 45539@debbugs.gnu.org
Subject: bug#45539: 26.3; `add-to-ordered-list': Add optional arg for :test predicate for hash table
Date: Thu, 31 Dec 2020 04:55:05 +0100	[thread overview]
Message-ID: <87v9ciociu.fsf@gnus.org> (raw)
In-Reply-To: <c2aa0ab5-ac89-4a52-b12a-339e9c6ca558@default> (Drew Adams's message of "Wed, 30 Dec 2020 09:55:55 -0800 (PST)")

Drew Adams <drew.adams@oracle.com> writes:

> What this function is really about, I think, is this: a list with
> unique elements - no duplicates.
>
> With your fix of this bug, "unique" will be defined by the new TEST
> arg (predicate).  Until then, "unique" is defined by `eq'.

My confusion here was due to not reading the code closely enough,
and because it's implemented how you'd think, it's just ... confusing.

> (defun add-to-ordered-list (list-var element &optional order)
>   "Add ELEMENT to the value of LIST-VAR if it isn't there yet.
> The test for presence of ELEMENT is done with `eq'.

> The resulting list is reordered so that the elements are in the
> order given by each element's numeric list order.  Elements
> without a numeric list order are placed at the end of the list.

What on earth is a "numeric list order" of an object?  This seems to
imply that ELEMENT should be a number.

> If the third optional argument ORDER is a number (integer or
> float), set the element's list order to the given value.  If
> ORDER is nil or omitted, do not change the numeric order of
> ELEMENT.  If ORDER has any other value, remove the numeric order
> of ELEMENT if it has one.

The actual ORDER bit is optional, which makes no sense for a function
that's allegedly about an ordered list.

    > (when order
    >   (puthash element (and (numberp order) order) ordering))

The hash table is only used to store this ORDER, and ORDER is silently
discarded unless it's a number.

    > (unless (memq element (symbol-value list-var))
    >   (set list-var (cons element (symbol-value list-var))))

This is the actual member check -- the hash table isn't used to keep
track of what's already in the list or not.

    > (set list-var (sort (symbol-value list-var)

The list is re-sorted upon every call, which is maximally inefficient.

			> (lambda (a b)
			>   (let ((oa (gethash a ordering))
			> 	(ob (gethash b ordering)))
			>     (if (and oa ob)
			> 	(< oa ob)
			>       oa)))))))

Finally, we sort based on the values stashed in the hash table, and the
ones with a (numerical) order gets sorted first.

It's a strange, maximally inefficient function.  I'll fix it up a bit
and add the test-function.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no





  reply	other threads:[~2020-12-31  3:55 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-29 21:04 bug#45539: 26.3; `add-to-ordered-list': Add optional arg for :test predicate for hash table Drew Adams
2020-12-30  3:48 ` Lars Ingebrigtsen
2020-12-30  7:01   ` Drew Adams
2020-12-30  7:07     ` Lars Ingebrigtsen
2020-12-30  8:43       ` Drew Adams
2020-12-30 17:55         ` Drew Adams
2020-12-31  3:55           ` Lars Ingebrigtsen [this message]
2020-12-31  4:30             ` Lars Ingebrigtsen
2020-12-31  5:47               ` Drew Adams
2020-12-31 16:40                 ` Drew Adams
2020-12-31  5:34             ` Drew Adams

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