/* Copyright (C) 2023 Aiden Gall

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program.  If not, see <http://www.gnu.org/licenses/>. */

#include <errno.h>
#include <grp.h>
#include <limits.h>
#include <stdarg.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

static const char VIDEO_GROUP[] = "video";

static const char BRIGHTNESS[] =
        "/sys/class/backlight/intel_backlight/brightness";
static const char MAX_BRIGHTNESS[] =
        "/sys/class/backlight/intel_backlight/max_brightness";

static void die(const char *fmt, ...);
static long strtonum(const char *arg, long minval, long maxval);

static int check_group(void);

static long read_sys_file_num(const char *path);
static void write_sys_file_num(const char *path, long val);

static void
die(const char *const fmt, ...)
{
	va_list ap;

	va_start(ap, fmt);
	vfprintf(stderr, fmt, ap);
	va_end(ap);

	exit(EXIT_FAILURE);
}

static long
strtonum(const char *const arg, const long minval, const long maxval)
{
	long ret;
	char *endptr;

	if (minval > maxval)
		die("strtonum: Invalid range\n");
	if (!arg)
		die("strtonum: Missing argument\n");

	errno = 0;
	ret = strtol(arg, &endptr, 10);
	if (errno)
		die("strtol: %s\n", strerror(errno));
	if (arg == endptr || *endptr)
		die("strtol: Invalid integer\n");

	if (ret < minval || ret > maxval)
		die("strtonum: Argument out of range\n");

	return ret;
}

static int
check_group(void)
{
	uid_t uid, euid;
	gid_t video_gid;

	gid_t *glist;
	int glist_size;

	struct group *grp;

	int i;

	uid = getuid();
	euid = geteuid();
	if (uid == 0)
		return 1;
	if (euid != 0)
		die("check_group: setuid is not set\n");

	errno = 0;
	grp = getgrnam(VIDEO_GROUP);
	if (errno)
		die("getgrnam: %s\n", strerror(errno));
	if (!grp)
		die("getgrnam: Group '%s' not found\n", VIDEO_GROUP);
	video_gid = grp->gr_gid;

	errno = 0;
	glist_size = getgroups(0, NULL);
	if (errno)
		die("getgroups: %s\n", strerror(errno));
	glist = calloc(glist_size, sizeof(*glist));
	if (!glist)
		die("check_group: Out of memory\n");

	errno = 0;
	if (getgroups(glist_size, glist) != glist_size)
		die("getgroups: Inconsistent number of groups\n");
	if (errno)
		die("getgroups: %s\n", strerror(errno));

	for (i = 0; i < glist_size; i++) {
		if (glist[i] == video_gid) {
			free(glist);
			return 1;
		}
	}

	free(glist);
	return 0;
}

static long
read_sys_file_num(const char *const path)
{
	FILE *fd;
	char buf[64];

	errno = 0;
	fd = fopen(path, "r");
	if (errno)
		die("fopen: %s\n", strerror(errno));
	if (!fd)
		die("fopen: Returned NULL\n");

	if (!fgets(buf, sizeof(buf), fd))
		die("fgets: Returned NULL\n");
	fclose(fd);
	buf[strcspn(buf, "\n")] = '\0';

	return strtonum(buf, 0, LONG_MAX/2);
}

static void
write_sys_file_num(const char *const path, const long val)
{
	FILE *fd;

	errno = 0;
	fd = fopen(path, "w");
	if (errno)
		die("fopen: %s\n", strerror(errno));
	if (!fd)
		die("fopen: Returned NULL\n");

	if (!fprintf(fd, "%ld\n", val))
		die("fprintf: Failed to write\n");
	fclose(fd);
}

int
main(int argc, char *argv[])
{
	long brightness, max_brightness;
	long val;

	if (!check_group())
		die("%s: User not in '%s' group\n", argv[0], VIDEO_GROUP);

	if (argc != 3)
		die("%s: Invalid number of arguments\n", argv[0]);

	val = strtonum(argv[2], 0, LONG_MAX/2);

	if (!argv[1][0] || argv[1][1])
		goto invalid_mode;
	switch (argv[1][0]) {
	case '-':
		val = -val;
		/* FALLTHROUGH */
	case '+':
		brightness = read_sys_file_num(BRIGHTNESS);
		val += brightness;
		/* FALLTHROUGH */
	case '=':
		max_brightness = read_sys_file_num(MAX_BRIGHTNESS);
		if (val < 0)
			val = 0;
		if (val > max_brightness)
			val = max_brightness;

		write_sys_file_num(BRIGHTNESS, val);
		break;
	default:
	invalid_mode:
		die("%s: Invalid mode\n", argv[0]);
	}

	return 0;
}