From d8fdc47628e4fda11c18a704b075b9a9cdc4cb95 Mon Sep 17 00:00:00 2001
From: clsr <clsr@clsr.net>
Date: Mon, 30 Dec 2019 22:37:38 +0100
Subject: Complete rewrite: make percentage changes more accurate

---
 bl | 194 ++++++++++++++++++++++++++++++++++++++++++++++++++++-----------------
 1 file changed, 147 insertions(+), 47 deletions(-)

diff --git a/bl b/bl
index d75ad3d..84f14d7 100755
--- a/bl
+++ b/bl
@@ -4,52 +4,152 @@
 # 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.
 
-set -e
-
-if [ -n "$BACKLIGHT" ]; then
-	bldir="/sys/class/backlight/$BACKLIGHT"
-else
-	for f in /sys/class/backlight/*; do
-		if [ -n "$bldir" ]; then
-			echo "too many backlights found, choose one with \$BACKLIGHT" >&2
-			exit 1
+# backlight brightness configuration utility
+#
+# Requirements: POSIX sh, backlight in /sys/class/backlight/*,
+#               write permission for $BACKLIGHT/brightness
+
+set -eu
+
+sys_backlight="/sys/class/backlight"
+
+get_bldir() {
+	bldir=
+	if [ -n "${BACKLIGHT:-}" ]; then
+		bldir="$sys_backlight/$BACKLIGHT"
+	else
+		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
+	fi
+}
+
+read_num() {
+	read -r data <"$1"
+	if ! is_num "$data"; then
+		printf "expected a number reading '%s', but got '%s'\n" "$1" "$data" >&2
+		exit 1
+	fi
+	printf %d "$data"
+}
+
+is_num() {
+	case "$1" in
+		[0-9]) return 0 ;;
+		[0-9][0-9]) return 0 ;;
+		[0-9][0-9][0-9]) return 0 ;;
+		[0-9][0-9][0-9][0-9]) return 0 ;;
+		[0-9][0-9][0-9][0-9][0-9]) return 0 ;;
+		[0-9][0-9][0-9][0-9][0-9][0-9][0-9]) return 0 ;;
+		[0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9]) return 0 ;;
+		[0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9]) return 0 ;;
+		# hopefully it's not a >2^32 number
+	esac
+	return 1
+}
+
+get_brightness() {
+	raw_max="$(read_num "$bldir/max_brightness")"
+	raw_curr="$(read_num "$bldir/brightness")"
+	max=100
+	curr=$(( (raw_curr + (raw_max / 2 / max)) * max / raw_max ))
+}
+
+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
+		: $(( raw_new = new * raw_max / max ))
+	fi
+	if [ -n "$raw_new" ]; then
+		if [ "$raw_new" -gt "$raw_max" ]; then
+			raw_new="$raw_max"
 		fi
-		bldir="$f"
-	done
-fi
-if [ -z "$bldir" ] || ! [ -d "$bldir" ]; then
-	echo "no backlight found" >&1
-	exit 1
-fi
-curr=$(($(cat "$bldir/brightness")+0))
-max=$(($(cat "$bldir/max_brightness")+0))
-mod="$1"
-case "$mod" in
-	+|-)
-		new="$((curr${mod}max/100))"
-		;;
-	+[1-9]*|-[1-9]*)
-		new="$((curr$mod*max/100))"
-		;;
-	[0-9]*)
-		new="$((mod*max/100+1))"
-		;;
-	=[0-9]*)
-		new="${mod#=*}"
-		;;
-	=)
-		echo "$curr"
-		exit
-		;;
-	'')
-		echo "$((curr*100/max))"
-		exit
-		;;
-	*)
-		echo "usage: $(basename "$0") [ +-=][0-100]" >&2
+		if [ "$raw_new" -le 0 ]; then
+			# brightness 0 tends to completely turn off the backlight
+			raw_new=1
+		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" "$prog"
+	printf "\n"
+	printf "Operators:\n"
+	printf "  (none)     print current brightness in percentages\n"
+	printf "  PERC       set brightness to PERC percentages of max\n"
+	printf "  +PERC      increase brightness by PERC percentage points\n"
+	printf "  -PERC      decrease brightness by PERC percentage points\n"
+	printf "  =          print current brightness raw value\n"
+	printf "  VAL        set brightness to VAL raw value\n"
+	printf "  =+VAL      increase brightness by VAL raw value\n"
+	printf "  =-VAL      decrease brightness by VAL raw value\n"
+	printf "\n"
+	printf "Environment variables:\n"
+	printf "  BACKLIGHT  the backlight in %s/ to use\n" "$sys_backlight"
+	printf "             (unnecessary if only one exists)\n"
+}
+
+main() {
+	if [ $# -gt 1 ]; then
+		usage >&2
 		exit 2
-		;;
-esac
-[ "$new" -gt "$max" ] && new="$max"
-[ "$new" -lt 1 ] && new=1
-echo "$new" > "$bldir/brightness"
+	fi
+	if [ "${1:-}" = "-h" ] || [ "${1:-}" = "--help" ]; then
+		usage
+		exit
+	fi
+	get_bldir
+	get_brightness
+	op="${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 "$@"
-- 
cgit