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From: "John Wiegley" <johnw@gnu.org>
To: emacs-devel@gnu.org
Subject: New function for subr.el: sort-on
Date: Fri, 24 Nov 2017 09:36:43 -0800	[thread overview]
Message-ID: <m2h8tjpoic.fsf@newartisans.com> (raw)

This can be more memory efficient than using `sort' with a predicate, since it
computes each value used by the predicate only once. If the predicate only
needs to access sub-elements, and not do any computation, there may be no
advantage.

--8<---------------cut here---------------start------------->8---
(defsubst sort-on (seq predicate accessor)
  "Sort SEQ using PREDICATE applied to values returned by ACCESSOR.
This implements the so-called Schwartzian transform, which has
the performance advantage of applying ACCESSOR at most once per
element in the list, as opposed to using `sort' with a PREDICATE
that applies the ACCESSOR.
Note: this function is only a win over `sort' if ACCESSOR is
compute-intensive; otherwise, it uses more intermediate cons
cells than regular `sort', and so represents a memory for CPU
tradeoff."
  (mapcar #'cdr (sort (mapcar #'(lambda (x) (cons (funcall accessor x) x)) seq)
                      #'(lambda (x y) (funcall predicate (car x) (car y))))))

(defun sort-on* (seq predicate accessor)
  "Sort SEQ using PREDICATE applied to values returned by ACCESSOR.
This is a destructive version of `sort-on', which attempts to
reuse storage as much as possible."
  (let ((seq2 seq))
    (while seq2
      (setcar seq2 (cons (funcall accessor (car seq2)) (car seq2)))
      (setq seq2 (cdr seq2))))
  (setq seq (sort* seq #'(lambda (x y) (funcall predicate (car x) (car y)))))
  (let ((seq2 seq))
    (while seq2
      (setcar seq2 (cdar seq2))
      (setq seq2 (cdr seq2)))
    seq))
--8<---------------cut here---------------end--------------->8---

-- 
John Wiegley                  GPG fingerprint = 4710 CF98 AF9B 327B B80F
http://newartisans.com                          60E1 46C4 BD1A 7AC1 4BA2



             reply	other threads:[~2017-11-24 17:36 UTC|newest]

Thread overview: 2+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-11-24 17:36 John Wiegley [this message]
2017-11-25  1:09 ` New function for subr.el: sort-on Basil L. Contovounesios

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