Re: Reading a file from the back

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From	Eli the Bearded <*@eli.users.panix.com>
Newsgroups	comp.os.linux.misc
Subject	Re: Reading a file from the back
Date	Mon, 13 Apr 2020 01:03:16 +0000 (UTC)
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In comp.os.linux.misc, Eric Pozharski  <whynot@pozharski.name> wrote:
> Eli the Bearded wrote:
>> perl -ne '$all .= $_; END{ print scalar reverse $all }'
>> optimized the perl for readibility, not speed or code golf
> 
> perl's 'reverse' is pretty much speed optimized for many releases now.
> Unfoturnately I still don't have perl-zoo thus I can't show it.

That program will suck big time if the input exceeds the size of memory
and starts to swap.

If there are multiple files, reading them one at a time and reversing
those is probably a better idea.

For large *files*, reading block by block backwards from the file
itself and reversing those blocks is probably a huge win over filling
all of memory. For non-files (eg read from STDIN), using a temp file
to store data instead of using RAM is a good idea.

For medium files with very short lines, growing $all in a tight loop is
probably a lot worse than sucking the whole file in with a single
`$\ = undef;` read.

Need I go on?

It has nothing to do with if reverse() itself is fast or not, it simply
isn't optimized to deal with edge cases. 

Elijah
------
did you reply specifically to show off perl knowledge?

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Thread

Reading a file from the back Dieter Britz <dieterhansbritz@gmail.com> - 2020-04-07 12:01 +0000
  Re: Reading a file from the back Vasco Costa <vasco.costa@invalid.invalid> - 2020-04-07 12:12 +0000
  Re: Reading a file from the back Fenris <fenris@invalid.invalid> - 2020-04-07 12:12 +0000
  Re: Reading a file from the back Rich <rich@example.invalid> - 2020-04-07 15:18 +0000
    Re: Reading a file from the back Jim Jackson <jj@franjam.org.uk> - 2020-04-10 19:21 +0000
      Re: Reading a file from the back Eli the Bearded <*@eli.users.panix.com> - 2020-04-10 20:37 +0000
        Re: Reading a file from the back Eric Pozharski <whynot@pozharski.name> - 2020-04-11 15:22 +0300
          Re: Reading a file from the back Eli the Bearded <*@eli.users.panix.com> - 2020-04-13 01:03 +0000
            Re: Reading a file from the back Eric Pozharski <whynot@pozharski.name> - 2020-04-13 11:16 +0300
              Re: Reading a file from the back Eli the Bearded <*@eli.users.panix.com> - 2020-04-13 20:01 +0000
                Re: Reading a file from the back Eric Pozharski <whynot@pozharski.name> - 2020-04-14 12:14 +0300
                Re: Reading a file from the back Eli the Bearded <*@eli.users.panix.com> - 2020-04-14 16:50 +0000
      Re: Reading a file from the back Rich <rich@example.invalid> - 2020-04-10 21:40 +0000
  Re: Reading a file from the back The Natural Philosopher <tnp@invalid.invalid> - 2020-04-08 08:55 +0100

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