sub abs_url {
	my ( $relative, $base ) = @_;

	return $relative if $relative =~ m{ \A http:// }ix;

	my ( $host, $hostrelative_abs ) = $base =~ m{
		\A
		http:// # skip scheme
		([^/]*) # capture hostname
		/*      # skip front slashes
		(.*?)   # capture everything that follows, but
		[^/]*   # leave out the optional final non-directory component
		\z
	}ix;

	$hostrelative_abs = '' if $relative =~ m!^/!;

	my $abs_url = join '/', $host, $hostrelative_abs, $relative;

	# replace '//' or '/./' with '/'
	1 while $abs_url =~ s{ / \.? (?=/|\z) }{}x;

	# remove '/foo/..' (but be careful to skip '/../..')
	1 while $abs_url =~ s{ / (?!\.\.) [^/]+ / \.\. (?=/|\z) }{}x;

	return "http://$abs_url";
}