#!/bin/sh

# This software is released into the public domain.
# It is provided "as is", without warranties or conditions of any kind.
# Anyone is free to use, modify, redistribute and do anything with this software.

# backlight brightness configuration utility
#
# Requirements: POSIX sh, backlight in /sys/class/backlight/*,
#               write permission for $BACKLIGHT/brightness

set -eu

sys_backlight="/sys/class/backlight"
sys_leds="/sys/class/leds"

find_backlight() {
	for f in "$sys_backlight"/*; do
		if [ "$f" = "$sys_backlight/*" ]; then
			printf "cannot find a backlight in %s\n" "$sys_backlight" >&2
			exit 1
		elif [ -n "$bldir" ]; then
			printf "multiple backlights found, choose one with \$BACKLIGHT\n" >&2
			exit 1
		fi
		bldir="$f"
	done
}

is_backlight() {
	[ -d "$1" ] && [ -f "$1/brightness" ] && [ -f "$1/max_brightness" ]
}

list_backlights() {
	for f in "$sys_backlight"/* "$sys_leds"/*; do
		if is_backlight "$f"; then
			printf "%s\n" "${f##*/}"
		fi
	done
}

get_bldir() {
	bldir=
	case "${BACKLIGHT:-}" in
		/*) bldir="$BACKLIGHT" ;;
		"") find_backlight ;;
		*)
			if is_backlight "$sys_backlight/$BACKLIGHT"; then
				bldir="$sys_backlight/$BACKLIGHT"
			elif is_backlight "$sys_leds/$BACKLIGHT"; then
				bldir="$sys_leds/$BACKLIGHT"
			else
				printf "no such backlight found: '%s'\n" "$BACKLIGHT" >&2
				exit 1
			fi
			;;
	esac
	blname="${bldir##*/}"
}

read_num() {
	read -r num <"$1"
	if ! is_num "$num"; then
		printf "expected a number reading '%s', but got '%s'\n" "$1" "$num" >&2
		exit 1
	fi
}

is_num() {
	case "$1" in
		[0-9]) return 0 ;;
		[0-9][0-9]) return 0 ;;
		[0-9][0-9][0-9]) return 0 ;;
		[0-9][0-9][0-9][0-9]) return 0 ;;
		[0-9][0-9][0-9][0-9][0-9]) return 0 ;;
		[0-9][0-9][0-9][0-9][0-9][0-9][0-9]) return 0 ;;
		[0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9]) return 0 ;;
		[0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9]) return 0 ;;
		# hopefully it's not a >2^32 number
	esac
	return 1
}

get_brightness() {
	read_num "$bldir/max_brightness"
	raw_max="$num"
	read_num "$bldir/brightness"
	raw_curr="$num"
	max=100
	curr=$(( (raw_curr + (raw_max / 2 / max)) * max / raw_max ))

	# brightness 0 tends to completely turn off the backlight
	raw_min=1
	case "$blname" in
		nvidia_[0-9]*) raw_min=0 ;; # nvidia_0 at 0 is still not off
	esac
}

get_raw_new() {
	op="$1"
	val="$2"
	raw_new=
	new=

	if ! is_num "$val"; then
		printf "expected a number for brightness, but got '%s'\n" "$val" >&2
		exit 2
	fi

	case "$op" in
		+) : $(( new = curr + val )) ;;
		-) : $(( new = curr - val )) ;;
		"") : $(( new = val )) ;;
		=+) : $(( raw_new = raw_curr + val )) ;;
		=-) : $(( raw_new = raw_curr - val )) ;;
		=) : $(( raw_new = val )) ;;
		*)
			printf "invalid operator '%s'\n" "$op" >&2
			exit 2
			;;
	esac
	if [ -n "$new" ]; then
		: $(( raw_new = new * raw_max / max ))
		if [ "$new" -lt "$curr" ] && [ "$raw_new" -ge "$raw_curr" ]; then
			: $(( raw_new = raw_curr - 1 ))
		elif [ "$new" -gt "$curr" ] && [ "$raw_new" -le "$raw_curr" ]; then
			: $(( raw_new = raw_curr + 1 ))
		fi
	fi
	if [ -n "$raw_new" ]; then
		if [ "$raw_new" -gt "$raw_max" ]; then
			raw_new="$raw_max"
		fi
		if [ "$raw_new" -lt "$raw_min" ]; then
			raw_new="$raw_min"
		fi
	fi
}

set_brightness() {
	get_raw_new "$1" "$2"
	printf %d "$raw_new" > "$bldir/brightness"
}

usage() {
	prog="$(basename "$0")"
	printf "%s: usage: %s OPERATOR\n" "$prog" "$prog"
	printf "\n"
	printf "Operators:\n"
	printf "  (none)      print current brightness in percentages\n"
	printf "  PERC        set brightness to PERC percentages of max\n"
	printf "  +PERC       increase brightness by PERC percentage points\n"
	printf "  -PERC       decrease brightness by PERC percentage points\n"
	printf "  =           print current brightness raw value\n"
	printf "  VAL         set brightness to VAL raw value\n"
	printf "  =+VAL       increase brightness by VAL raw value\n"
	printf "  =-VAL       decrease brightness by VAL raw value\n"
	printf "  -L, --list  list all found backlights\n"
	printf "\n"
	printf "Environment variables:\n"
	printf "  BACKLIGHT   the backlight to use (absolute path or a folder\n"
	printf "              in %s or %s)\n" "$sys_backlight" "$sys_leds"
}

main() {
	if [ $# -gt 1 ]; then
		usage >&2
		exit 2
	fi
	if [ "${1:-}" = "-h" ] || [ "${1:-}" = "--help" ]; then
		usage
		exit
	fi
	if [ "${1:-}" = "-L" ] || [ "${1:-}" = "--list" ]; then
		list_backlights
		exit
	fi
	get_bldir
	get_brightness
	op="${1:-}"
	case "$op" in
		"") printf "%d\n" "$curr" ;;
		+[0-9]*)  set_brightness +    "${op#+}" ;;
		-[0-9]*)  set_brightness -    "${op#-}" ;;
		[0-9]*)   set_brightness ""   "$op" ;;
		=) printf "%d\n" "$raw_curr" ;;
		=+[0-9]*) set_brightness "=+" "${op#=+}" ;;
		#+=[0-9]*) set_brightness "=+" "${op#+=}" ;;
		=-[0-9]*) set_brightness "=-" "${op#=-}" ;;
		#-=[0-9]*) set_brightness "=-" "${op#-=}" ;;
		=[0-9]*)  set_brightness "="  "${op#=}" ;;
		*)
			printf "invalid operator: '%s'\n" "$op" >&2
			exit 2
	esac
}

main "$@"