#!/bin/sh

set -eu

get_b()
{
	local b=

	b=$(brightnessctl --class=backlight get) || return 1
	printf '%.0f\n' $(printf \
		'scale = 4; (7 * sqrt((2 - %d) * (%d - 3102))) / 1550\n' \
		${b} ${b} | bc)

	return 0
}

set_b()
{
	local b="${1}"
	shift 1

	brightnessctl --class=backlight set $(printf '%.0f\n' \
		$(printf 'scale = 4; 2 + (1 - sqrt(1 - (%d / 7)^2)) * 1550\n' \
		${b} | bc)) || return 1

	return 0
}

usage()
{
	printf 'Usage: %s {-|+}\n' "${0}"

	return 0
}

main()
{
	local dir=
	local b=

	if [ ${#} -ne 1 ]; then
		usage 1>&2
		return 1
	fi
	dir="${1}"
	shift 1

	case "${dir}" in
		-|+);;
		*)
			usage 1>&2
			return 1
			;;
	esac

	b=$(get_b) || return 1
	set_b $((${b} ${dir} 1)) || return 1

	return 0
}

main "${@}"