Re^2: efficient way to read a file in reverse

Hi Tybalt89,

Glad you enjoyed it! I omitted the first bit of the code where I do the timestamps for brevity's sake, but yes, the purpose is to look for the last 24 hours of logs for any thermal throttling messages. Thanks for the code, I think that will be quicker for me to adapt than the original File::ReadBackwards (I'll dig into 'tie filehandle' when I get a chance sometime), and there's some things new to me in your example as well which look interesting.

Cheers,
Cam

Comment on Re^2: efficient way to read a file in reverse

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Re^3: efficient way to read a file in reverse by tybalt89 (Monsignor) on Jan 07, 2021 at 21:05 UTC
Since I haven't played with tied file handles before, here's the previous package with the ability to use it either as an object, or as a tied file handle. #!/usr/bin/perl use strict; # https://perlmonks.org/?node_id=11126426 use warnings; tie BACKWARDS, 'Tybalt89BackwardsHeReads', $0 or die 'tie failed'; while( <BACKWARDS> ) { print; } package Tybalt89BackwardsHeReads; #################################### +### BEGIN { TIEHANDLE = \&new; READLINE = \&line; } sub line { my ($self) = @_; while( @{ $self->{lines} } == 0 and $self->{where} ) { my $window = 1024; # window size, adjust to suit my $pos = $self->{where} - $window; $pos < 0 and $pos = 0; seek $self->{fh}, $pos, 0; read $self->{fh}, my $data, $self->{where} - $pos; $pos and $data =~ s/^\N\n(?=.)//s ? ($pos += $+[0]) : die "increase window size"; $self->{lines} = [ split /^/, $data ]; $self->{where} = $pos; } return pop @{ $self->{lines} }; } sub new { my ($self, $filename) = @_; open my $fh, '<', $filename or die "$! on $filename"; bless { where => ref $filename ? length $$filename : -s $filename, fh => $fh, lines => [] }, ref $self \|\| $self; } 1; # so if split off, package ends with true [download] Replace the $0 with the name of your file. I was just using it for debugging. Please use this latest version of "sub line", it solves a problem with the fetching the line from the beginning of the file when the window is too small.	[reply] [d/l]

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Re^3: efficient way to read a file in reverse
by tybalt89 (Monsignor) on Jan 07, 2021 at 21:05 UTC

Since I haven't played with tied file handles before, here's the previous package with the ability to use it either as an object, or as a tied file handle.

#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11126426
use warnings;

tie *BACKWARDS, 'Tybalt89BackwardsHeReads', $0 or die 'tie failed';

while( <BACKWARDS> )
  {
  print;
  }

package Tybalt89BackwardsHeReads; ####################################
+###

BEGIN
  {
  *TIEHANDLE = \&new;
  *READLINE = \&line;
  }

sub line
  {
  my ($self) = @_;
  while( @{ $self->{lines} } == 0 and $self->{where} )
    {
    my $window = 1024;                   # window size, adjust to suit
    my $pos = $self->{where} - $window;
    $pos < 0 and $pos = 0;
    seek $self->{fh}, $pos, 0;
    read $self->{fh}, my $data, $self->{where} - $pos;
    $pos and $data =~ s/^\N*\n(?=.)//s ? ($pos += $+[0]) :
      die "increase window size";
    $self->{lines} = [ split /^/, $data ];
    $self->{where} = $pos;
    }
  return pop @{ $self->{lines} };
  }

sub new
  {
  my ($self, $filename) = @_;
  open my $fh, '<', $filename or die "$! on $filename";
  bless { where => ref $filename ? length $$filename : -s $filename,
    fh => $fh, lines => [] }, ref $self || $self;
  }

1; # so if split off, package ends with true
[download]

Replace the $0 with the name of your file. I was just using it for debugging.

Please use this latest version of "sub line", it solves a problem with the fetching the line from the beginning of the file when the window is too small.

[reply]
[d/l]