// By Tom Smeding (2016)
// modified for new laptop (2017)

#include <stdio.h>
#include <stdlib.h>
#include <stdarg.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>

#define INCREMENT 100

__attribute__((noreturn))
__attribute__((format (printf, 1, 2)))
void die(const char *format,...){
	va_list ap;
	va_start(ap,format);
	vprintf(format,ap);
	va_end(ap);
	putchar('\n');
	exit(1);
}

int get_max_brightness(void){
	FILE *f=fopen("/sys/class/backlight/intel_backlight/max_brightness","r");
	if(!f)die("Could not open max_brightness file");
	int n;
	fscanf(f,"%d",&n);
	fclose(f);
	return n;
}

int get_brightness(void){
	FILE *f=fopen("/sys/class/backlight/intel_backlight/brightness","r");
	if(!f)die("Could not open brightness file");
	int n;
	fscanf(f,"%d",&n);
	fclose(f);
	return n;
}

void set_brightness(int n){
	FILE *f=fopen("/sys/class/backlight/intel_backlight/brightness","w");
	if(!f)die("Could not open brightness file");
	fprintf(f,"%d",n);
	fflush(f);
	fclose(f);
}

int main(int argc,char **argv){
	uid_t uid=geteuid();
	if(uid!=0){
		printf("brightness will only work with root.\n");
		return 1;
	}

	if(argc==1){
		printf("max: %d\n",get_max_brightness());
		printf("current: %d\n",get_brightness());
	} else if(argc==2){
		int n;
		if(strcmp(argv[1],"-")==0){
			n=get_brightness()-INCREMENT;
			if(n<0)n=0;
		} else if(strcmp(argv[1],"+")==0){
			n=get_brightness()+INCREMENT;
			int mx=get_max_brightness();
			if(n>mx)n=mx;
		} else {
			char *endp;
			n=strtol(argv[1],&endp,10);
			if(argv[1][0]=='\0'||*endp!='\0')die("Invalid number '%s'",argv[1]);
		}
		set_brightness(n);
	}
}