#!/usr/bin/perl
use strict;
use warnings;

use POSIX qw( ceil );
use List::Util qw( shuffle );

my $basepos = 0;

open my $fh, '<', $ARGV[ 0 ]
    or die "Couldn't open $ARGV[ 0 ] for reading: $!\n";

my $offs = pack 'L', 0;

while( my $advance = read $fh, $_, 128 * 1024 ) {
    s/(?=\n)/$offs .= pack 'L', $basepos + pos(); ""/eg;
    $basepos += advance;
}

# we will be looking for "start of following line", so this is
# a bogus entry to obviate need for special case at last line
$offs .= pack 'L', $basepos;

my $total_lines = length( $offs ) / 4;

my $been_here = "\0" x ceil( $total_lines / 8 );

my $i = 0;

while( $i < $total_lines ) {
    my $line = int rand $total_lines;
    next if vec $been_here, $line, 1;
    my ( $start, $end ) = unpack "x " . ( 4 * $line ) . " L L", $offs;
    seek $fh, $start, 0;
    read $fh, $_, $end - $start;
    print;
    ++$i;
    vec( $been_here, $line, 1 ) = 1;
}

##</code><code>##

#!/usr/bin/perl
use strict;
use warnings;

use POSIX qw( ceil );
use List::Util qw( shuffle );

my $basepos = 0;

open my $fh, '<', $ARGV[ 0 ]
    or die "Couldn't open $ARGV[ 0 ] for reading: $!\n";

my $offs;
vec( $offs, 0, 32 ) = 0;

my $total_lines = 1;

while( my $advance = read $fh, $_, 128 * 1024 ) {
    s/(?=\n)/vec( $offs, $total_lines++, 32 ) = $basepos + pos(); ""/eg;
    $basepos += advance;
}

# we will be looking for "start of following line", so this is
# a bogus entry to obviate need for special case at last line
vec( $offs, $total_lines, 32 ) = $basepos;

my $been_here = "\0" x ceil( $total_lines / 8 );

my $i = 0;

while( $i < $total_lines ) {
    my $line = int rand $total_lines;
    next if vec $been_here, $line, 1;
    my $start = vec $offs, $line, 32;
    my $end = vec $offs, $line + 1, 32;
    seek $fh, $start, 0;
    read $fh, $_, $end - $start;
    print;
    ++$i;
    vec( $been_here, $line, 1 ) = 1;
}