public void clear(long index) {
    int wordNum = (int) (index >> 6); // div 64
    int bit = (int) index & 0x3f;     // mod 64
    long bitmask = 1L << bit;
    bits[wordNum / PAGE_SIZE][wordNum % PAGE_SIZE] &= ~bitmask;
  }
clears a bit.
The index should be less than the OpenBitSet size.
ambry-utils/src/main/java/com/github/ambry/utils/OpenBitSet.java