// TODO: optimize to x-(x%n) in case that's the same thing
// (or x-mod(x,n)?)
static int roundDownTo(int n, int x) {
  return x/n*n;
}

static long roundDownTo(long n, long x) {
  return x/n*n;
}

static double roundDownTo(double n, double x) {
  return floor(x/n)*n;
}