int
factdigsum (unsigned border)
{
  #define GROUPING 1000000
  unsigned dig [border + 1]; /* digits grouped by log10 (GROUPING) */
  unsigned first = 0, last = 0, carry, n, x, sum = 0;

  dig [0] = 1;

  for (n = 2; n != border; ++n)
    {
      carry = 0;

      /* multiplying */
      for (x = first; x < last + 1; ++x)
        {
          unsigned nxt;
          nxt = dig [x] * n + carry;
          dig [x] = nxt % GROUPING;
          if ((x == first) && ((nxt % GROUPING) == 0))
            {
              first += 1;
            }
          carry = nxt / GROUPING;
        }

      /* opening new digit group */
      if (carry != 0)
        {
          last += 1;
          dig [last] = carry;
        }
    }

  /* now we may calculate sum of digits */
  for (x = first; x < last + 1; ++x)
    {
      unsigned g = GROUPING;
      while (g > 9)
        {
          g = g / 10;
          sum += (dig [x] / g) % 10;
        }
    }

  return sum;
}