bug#19896: 23.1; reverse-region is slow compared to sort-columns

[Ivan Shmakov]

>>>>> asparagus  <asparagus@comcast.net> writes:

 > The emacs manual recommends 'M-x reverse-region' as a companion to
 > 'M-x sort-columns' to achieve a descending-order column sort.

        This could be a bug in the manual, as sort-columns has an
        optional (prefix) ‘reverse’ argument since at least 1990
        (d32200ac.)

 > However, for large regions (> 100k lines), 'M-x reverse-region' is
 > noticeably slower than 'M-x sort-columns', making descending sort
 > much slower than ascending sort.

        I don’t seem to see this on the current master; at a glance,
        M-x reverse-region takes less time to complete than
        M-x sort-columns.

        The current stable version of Emacs is 24.4, with the pretest
        for 24.5 being available since 17th [1].  Could you please check
        if this issue also manifests with either of these versions?

        TIA.

[1] news:87mw4c6gjn.fsf@petton.fr
    http://permalink.gmane.org/gmane.emacs.devel/183187

 > A workaround on linux: use 'C-u M-x shell-command-on-region' with tac
 > (the opposite of cat) instead of using 'M-x reverse-region'.

        On a GNU system (as well as on any other system featuring
        GNU Coreutils), it should be possible to just use sort(1) for
        the purpose.  For instance, the “reverse” option is ‘-r’.

[…]

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