#!/bin/bash

echoerr() {
    >&2 echo "$@"
}

max=$(intel_reg read 0xC8254 2>/dev/null | awk '{print $3}')
max=${max:2:4}
max_dec=$(printf "%d" $((16#$max)))

cur_br=$(cat /sys/class/backlight/intel_backlight/brightness)
cur_per=$(($cur_br * 100 / $max_dec))

case "$1" in
    "+")
        new_per=$(($cur_per+10))
        ;;

    "-")
        new_per=$(($cur_per-10))
        ;;
    
    *)
        echoerr "command '$1' not supported"
        exit 1
        ;;
esac

if [ $new_per -lt 0 ]; then
    new_per=0
elif [ $new_per -gt 100 ]; then
    new_per=100
fi

new_val=$(($new_per * $max_dec / 100))
echo $new_val > /sys/class/backlight/intel_backlight/brightness