bug#26540: 25.2; [PATCH] Add cl-set-equal to test for set equality

[Michael Heerdegen <michael_heerdegen@web.de>]

Damien Cassou <damien@cassou.me> writes:

> Damien Cassou <damien@cassou.me> writes:

> > it makes sense and I will try this way. Nevertheless, it also means
> > giving up on the :key feature. I guess it's ok.

OTOH I see no reason not to support it.  There is no reason to provide a
function in a library specializing on sequences with less features than
in some other lib.  Just my personal opinion.  Of course you can get the
effect of :key by adopting the TESTFN, but also note my other comment:

> here it is. Any feedback? 

It might be worth it to try to optimize things a bit for the most usual
TESTFNs `eq' and `equal'.  For example, try

#+begin_src emacs-lisp
(let ((s1 (number-sequence 1 10000))
      (s2 (number-sequence 1 10000)))
  (seq-set-equal s1 s2))
#+end_src

vs.

#+begin_src emacs-lisp
(let ((s1 (number-sequence 1 10000))
      (s2 (number-sequence 1 10000)))
  (seq-set-equal-2 s1 s2))
#+end_src

with this implementation using hash-tables:

#+begin_src emacs-lisp
(defun seq-set-equal-2 (sequence1 sequence2)
  (let ((table1 (make-hash-table :size (length sequence1)))
        (table2 (make-hash-table :size (length sequence2))))
    (seq-doseq (elt sequence1) (puthash elt t table1))
    (seq-doseq (elt sequence2) (puthash elt t table2))
    (and (seq-every-p (lambda (elt) (gethash elt table2)) sequence1)
         (seq-every-p (lambda (elt) (gethash elt table1)) sequence2))))
#+end_src

I guess other functions in seq.el could be optimized as well,
e.g. `seq-difference'.


Regards,

Michael.