// use sub-array of array2, fixed length of array1 = 1024 bits
static int countDifferingBits_1024bit(long[] array1, long[] array2, int i1) {
  int diff = 0;
  for (int i = 0; i < 1024/64; i++)
    diff += popCount(array1[i]^array2[i1+i]);
  ret diff;
}