Re: empty-directory predicate, native implementation

[Arthur Miller <arthur.miller@live.com>]

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

> Sorry I'm coming to this late.  It's a long thread,
> and I've only checked your patch, not the messages.
>
> I don't understand why we would do this.  Is the
> real aim to be able to have an empty-directory
> predicate, i.e., to be able to see if there are
> no files in a directory?
Yes.
> If so, then why not do just that, witout
> bothering (changing the signature of)
> `directory-files'?
Because it is not possible to that efficiently with current elisp; how
would you do that directly? OS does not have such call, one have to
iterate through directories. At least I am not aware of such call. If
you know, please tell me, I would be happy to use it, at least for myself.

Fastest I am aware of is to get a buffered list with SYS_getdents and then
check for first non-dot file. That changes complexity from linear in
number of files in directory to constant (3 files at max). It works only
on Linux kernel, and I don't know if there is nt version of such call,
or what there is on macs.

I suggested such predicate, but to not re-iterate entire discussion, pls
check my first mail and followed 2 or 3 I think.

> Is there really some use case for getting only
> the first N files in a directory?  I can
> understand wanting to test whether a dir is
> empty.  I can't see why someone would want the
> first N files (however "first" might be defined).
Probably not; it is just a result of having count parameter. 

> The doc string itself seems to suggest that the
> (only?) use case is to test for emptiness:
>
>  You can use this function to short circuit evaluation
>  in case you are just interested to find if a directory
>  is empty or not (request one file and tell it to
>  ignore dot-files).
It does not say only, but it suggests it can be usd in that way. But
yes, that was the driver behind.

Check the attachment; maybe something you would like to have in Dired+?

I don't know if it could become a part of Dired itself, I believe it is
probably too niche and subjective.

> That need is clear enough.  But what else is
> COUNT useful for?  How about having the doc
> string also tell us what we can use a COUNT
> value > 1 for?  That's the case that really
> needs pointing out, not the empty-dir-p case.
>
> And what is the order that defines which file
> is first, second, etc.?  Without knowing that,
> what use is getting the "first" N?
OS.

There is no hard guarantee by OS:s that you will get files in
particular order. I had that sentence in docs, but it was a bit too
verse. I am not very literal of me, if you can phrase it better and more
concise, please go ahead, I would appreciate. Check all docs if you have
time and feel for it, I always need help with the language.

> If this has already been raised, discussed,
> and dismissed, fine - apologies for the noise.
> If not, what's the raison d'etre for adding
> optional parameter COUNT?
I hope you don't mind me snitching your regex for nodots. I was trying
to get one fast myself, and gave up after half hour, would never thought
it was so picky to get that one right.

> Why not just add a predicate `directory-empty-p'?
:-) Check my first mail. I implemented one with that name.

Best regards
/a


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(require 'dired)

(defvar nodots "^\\([^.]\\|\\.\\([^.]\\|\\..\\)\\).*")

(defun dired-go-to-first ()
    (interactive)
    (goto-char (point-min))
    (dired-next-line 1)
    (skip-chars-forward " \n\t"))

(defun directory-is-empty-p (directory-name)
  (null (directory-files directory-name nil nodots t 1)))

(defun dired-mark-empty-dirs ()
  (interactive)
  (when (equal major-mode 'dired-mode)
    (let ((curr-dir))
      (save-excursion
        (dired-go-to-first)
        
        (while (not (eobp))
          (setq curr-dir (dired-file-name-at-point))
          (cond ((or (null curr-dir)
                     (string= curr-dir ".")
                     (string= curr-dir ".."))
                     ;; do nothing here
                 )
                ((file-directory-p curr-dir)
                 (when (directory-is-empty-p curr-dir)
                   (dired-mark 1)
                   (dired-previous-line 1))))
          (dired-next-line 1))))))