#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <fcntl.h>
#include <string.h>
#include <stdlib.h>
#include <limits.h>
#include <errno.h>
#include <sys/types.h>
#include <dirent.h>

#define BACKLIGHT_DIR "/sys/class/backlight"

void
die(const char *msg)
{
	puts(msg);
	exit(1);
}

void
err(const char *msg)
{
	perror(msg);
	exit(1);
}

void*
ecalloc(size_t nmemb, size_t size)
{
	void *ret = calloc(nmemb, size);
	if (!ret)
		err("ecalloc");
	return ret;
}

char*
appendStr(size_t len, const char *a, const char *b)
{
	char *buf = ecalloc(len, sizeof(char));
	int ret = snprintf(buf, len, "%s%s", a, b);
	if (ret == -1 || ret > len)
		err("snprintf");
	return buf;
}

/* returns errno */
int
maxBrightness(const char *prefix, long *max)
{
	int e;
	char *buf = appendStr(256, prefix, "/max_brightness");

	int fd = open(buf, O_RDONLY);
	if (fd == -1) {
		e = errno;
		free(buf);
		return e;
	}

	memset(buf, '\0', 256);

	int b = read(fd, buf, 256);
	if (b==-1) {
		e = errno;
		free(buf);
		close(fd);
		return e;
	}

	for (int i=0; i<256 ; i++) {
		if (buf[i] == '\n') {
			buf[i] = '\0';
			break;
		}
	}

	long n = strtol(buf, NULL, 10);
	if (n == LONG_MIN || n == LONG_MAX || n == 0)
		err("strtol");

	free(buf);
	buf = NULL;
	close(fd);
	*max = n;

	return 0;
}

int
setBacklight(const char *prefix, int percent)
{
	char *fn = appendStr(256, prefix, "/brightness");

	int fd = open(fn, O_WRONLY);
	if (fd == -1) {
		free(fn);
		return -1;
	}

	int n = dprintf(fd, "%d\n", percent);
	if (n == -1) {
		close(fd);
		free(fn);
		return -1;
	}
	
	close(fd);
	free(fn);
	fn = NULL;
}

int
main(int argc, char **argv)
{
	uid_t euid = geteuid();
	char **a = argv+1;

	if (euid != 0)
		die("Program must be run as root/setuid");

	if (!*a)
		die("Brightness percentage must be supplied");


	long percent = strtol(*a, NULL, 10);
	if (percent <= LONG_MIN || percent >= LONG_MAX)
		err("invalid percent");
	if (percent < 0 || percent > 100)
		die("percent must be between 0 and 100");

	DIR *d = opendir(BACKLIGHT_DIR);
	if (!d)
		err("opendir");

	errno = 0;
	for (;;) {
		struct dirent *dent = readdir(d);
		if(!dent&&errno!=0)
			err("readdir");

		if (!dent)
			break;

		if (strcmp("..", dent->d_name)==0 || strcmp(".", dent->d_name)==0)
			continue;

		char *prefix = appendStr(1024, BACKLIGHT_DIR"/", dent->d_name);

		printf("found backlight: %s\n", prefix);
	
		int ret;
		long max = 0;
		ret = maxBrightness(prefix, &max);
		if (ret != 0) {
			const char *msg = strerror(ret);
			printf("Error getting max brightness for: %s : %s\n",
					prefix, msg);
			continue;
		}

		float p = (double)max*((double)percent/100.0);

		ret = setBacklight(prefix, (int)p);
		if (ret != 0) {
			perror("setBacklight");
		}

		free(prefix);
		prefix = NULL;
	}
}