#!/usr/bin/perl

# multi.pl
# usage: multi.pl [max_tasks] < job_list
# max_task defaults to 4
#
# do all the jobs as quickly as possible in parallel, but no more than
#   max_tasks at a time.

use warnings;
use strict;

sub mysystem
{
	my $cmd = shift;
	my $pid;
	#print "forking: $cmd\n";
	if (($pid = fork()) == 0) { exec($cmd); } # child
	return $pid; # parent
}

# how many tasks can we do at once
my $max_tasks = 4;
if (defined($ARGV[0])) { $max_tasks = shift(@ARGV); }
# num of curr tasks
my $num_tasks = 0;
# pids running, and command run
my %pids = ();
my ($pid, $cmd);

# read tasks
while ($cmd = <>)
{
	chomp($cmd);
	# execute the task
	print "starting: $cmd\n";
	$pid = mysystem($cmd);
	#print "forked pid $pid\n";
	$pids{$pid} = $cmd;
	$num_tasks++;
	# if we're doing all we should, wait for 1 to end
	while ($num_tasks >= $max_tasks)
	{
		$pid = wait();
		#print "reaped pid $pid\n";
		last if ($pid == -1); # error?
		print "finished: $pids{$pid}\n";
		delete($pids{$pid});
		$num_tasks--;
	}
}

# wait for all children to end
while ($num_tasks > 0)
{
	$pid = wait();
	last if ($pid == -1); # error?
	print "finished: $pids{$pid}\n";
	delete($pids{$pid});
	$num_tasks--;
}

exit(0);

##</code><code>##

sleep  4
sleep  2
sleep  6
sleep  3
sleep  4
sleep  2