static int countDifferingBits(byte[] array1, byte[] array2) {
  int n = array1.length;
  int diff = 0;
  for i to n:
    diff += Int.bitCount(ubyteToInt(array1[i])^ubyteToInt(array2[i]));
  ret diff;
}

static int countDifferingBits(int[] array1, int[] array2) {
  int n = array1.length;
  int diff = 0;
  for i to n:
    diff += Int.bitCount(array1[i]^array2[i]);
  ret diff;
}

static int countDifferingBits(long[] array1, long[] array2) {
  int n = array1.length;
  int diff = 0;
  for i to n:
    diff += Long.bitCount(array1[i]^array2[i]);
  ret diff;
}

// use sub-array of array2
static int countDifferingBits(long[] array1, long[] array2, int i1) {
  int n = array1.length;
  int diff = 0;
  for i to n:
    diff += Long.bitCount(array1[i]^array2[i1+i]);
  ret diff;
}