inline double distance(double x, double y, double z, int bdyID) {

  double dist;
  switch (bdyID) {
    case 1: dist = fabs(x);
        break;
    case 2: dist = fabs(x - 1);
        break;
    case 3: dist = fabs(y);
        break;
    case 4: dist = fabs(y - 1);
        break;
    case 5: dist = fabs(z);
        break;
    case 6: dist = fabs(z - 1);
        break;
  }

  return dist;
}