#### WARNING: THIS CODE IS BROKEN. DO NOT USE ####
#! perl -slw
use strict;
use threads;
use threads::shared;

our $BUFSIZE	||= 2**20;

open my $fh, '+< :raw', $ARGV[ 0 ] or die $!;

our $buffer1 : shared = '';
our $buffer2 : shared =  '';
our $p1 :shared = 0;
our $p2 :shared = 0;
my $done : shared = 0;
{
	lock $buffer2;
	async {
		while( not $done ) {
			{
				lock $buffer1;
				$p1 = $p2 + length $buffer2;
				sysseek $fh, $p1, 0;
				$done = !sysread $fh, $buffer1, $BUFSIZE;
			}
			{
				lock $buffer2;
				$p2 = $p1 + length $buffer1;
				sysseek $fh, $p2, 0;
				$done = !sysread $fh, $buffer2, $BUFSIZE;
			}
		}
		print "Thread done: $done";
		$done = 1;
	}->detach;
}
sleep 1;

while( not $done ) {
	{
		lock $buffer1;
# A visible, reversable transformation for testing!
		$buffer1 =~ tr[0*][*0]; #
		sysseek $fh, $p1, 0;
		syswrite $fh, $buffer1;
	}
	{
		lock $buffer2;
		$buffer2 =~ tr[0*][*0];
		sysseek $fh, $p2, 0;
		syswrite $fh, $buffer2;
	}
}
close $fh;