#include <stdio.h>
#include <stdlib.h>

void print_usage(char *argv[])
{
  fprintf(stderr, "Usage: %s <value>\n"
                  "\n"
                  "value:\n"
                  "\t +percent - increase brightness by percent\n"
                  "\t -percent - decrease brightness by percent\n",
          argv[0]);
}

int read_file(const char *path)
{
  FILE *file = fopen(path, "r");
  int value;
  int read = fscanf(file, "%d", &value);
  fclose(file);
  if (read != 1)
  {
    printf("Error reading %s\n", path);
    exit(1);
  }
  return value;
}

int max_brightness()
{
  return read_file("/sys/class/backlight/intel_backlight/max_brightness");
}

int brightness()
{
  return read_file("/sys/class/backlight/intel_backlight/brightness");
}

int brightness_percent()
{
  return (brightness() * 100) / max_brightness();
}

int main(int argc, char *argv[])
{
  if (argc < 2)
  {
    printf("%d\n", brightness_percent());
    return 0;
  }

  if (argc > 3)
  {
    print_usage(argv);
    exit(2);
  }

  int positive = 0;
  if (argv[1][0] == '-')
  {
    positive = 0;
  }
  else if (argv[1][0] == '+')
  {
    positive = 1;
  }
  else
  {
    print_usage(argv);
    exit(2);
  }

  int percent;
  char *number_str = argv[1] + 1;
  int read = sscanf(number_str, "%d", &percent);
  if (read != 1)
  {
    print_usage(argv);
    exit(3);
  }

  int current = brightness();
  int max = max_brightness();
  int change = (max * percent) / 100 * (positive ? 1 : -1);
  FILE *file = fopen("/sys/class/backlight/intel_backlight/brightness", "w");

  int set_to = current + change;
  // clamp
  if (set_to > max)
  {
    set_to = max;
  }
  if (set_to < 0)
  {
    set_to = 1;
  }
  fprintf(file, "%d", set_to);
}