#define WORD_SIZE (8 * sizeof(unsigned int))

void poly_add(unsigned int* sum, unsigned int* a, unsigned int* b,
              unsigned int size)
{
  unsigned int nWords = (size + WORD_SIZE - 1) / WORD_SIZE;
  unsigned int carry = 0;
  unsigned int x,y,z;
  unsigned int i;
  for (i = 0; i < nWords; ++i)
  {
    x = a[i];
    y = b[i];
    z = x + y + carry;
    sum[i] = z;
    // carry if z is less than either of the arguments
    carry = (z < x || z < y) ? 1 : 0;
  }

  // prevent overflow into the unused bits 
  if ((size % WORD_SIZE) != 0)
    sum[nWords-1] &= (1 << (size % WORD_SIZE)) - 1;
}