#! /bin/sh

set -e

fun="$1"
screen_brightness_device_path="/sys/class/backlight/intel_backlight"

# Padding with space because when fun="=10", awk fails to parse fun==10
new=$(
    awk -v fun=" $fun" '
        FILENAME ~ "/max_brightness$" {max = $1; next}
        FILENAME ~     "/brightness$" {cur = $1; next}

        END {
            unit  = max / 100
            oper  = substr(fun, 2, 1)
            units = substr(fun, 3, length(fun) - 1)
            if (oper == "=") {new = unit * units} else
            if (oper == "+") {new = cur + (unit * units)} else
            if (oper == "-") {new = cur - (unit * units)} else {
                printf("Unrecognized operator: %s\n", oper) > "/dev/stderr"
                exit(1)
            }
            print int(new)
        }
    ' \
    "$screen_brightness_device_path/max_brightness" \
    "$screen_brightness_device_path/brightness"
)

echo "${new}" | tee "${screen_brightness_device_path}/brightness"