#!/usr/bin/perl

my $arg = $ARGV[0];
my $step = 10; # measured in percents

sub fsread($);  # read 1-st line from file
sub fswrite(@); # write string to file

my $brightnessfile = "/sys/class/backlight/acpi_video0/brightness";
my $maxbrightness = fsread("/sys/class/backlight/acpi_video0/max_brightness");
my $currbrightness = fsread($brightnessfile);

my $a = $maxbrightness / 100;
$step = int($step * $a);

if ( $arg eq "up" ) {
	if ($currbrightness != $maxbrightness) {
		my $projectedbrightness = $currbrightness + $step;

		if ($projectedbrightness > $maxbrightness) {
			$projectedbrightness = $maxbrightness;
		}

		fswrite($projectedbrightness, $brightnessfile);
	}
} elsif ( $arg eq "down" ) {
	if ($currbrightness != 0) {
		my $projectedbrightness = $currbrightness - $step;

		if ($projectedbrightness < 0) {
			$projectedbrightness = 0;
		}

		fswrite($projectedbrightness, $brightnessfile);
	}
}


sub fsread($) {
	my $fname = shift;
	open (F, $fname) || die;
	my $str = <F>;
	close F;
	return $str;
}

sub fswrite (@) {
	my $str = shift;
	my $fname = shift;
	open(OF, ">", $fname) || die;
	print OF $str;
	close OF;
}