public void clear(int index) {
    int wordNum = index >> 6;
    int bit = index & 0x03f;
    long bitmask = 1L << bit;
    bits[wordNum / PAGE_SIZE][wordNum % PAGE_SIZE] &= ~bitmask;
    // hmmm, it takes one more instruction to clear than it does to set... any
    // way to work around this?  If there were only 63 bits per word, we could
    // use a right shift of 10111111...111 in binary to position the 0 in the
    // correct place (using sign extension).
    // Could also use Long.rotateRight() or rotateLeft() *if* they were converted
    // by the JVM into a native instruction.
    // bits[word] &= Long.rotateLeft(0xfffffffe,bit);
  }
clears a bit.
The index should be less than the OpenBitSet size.
ambry-utils/src/main/java/com/github/ambry/utils/OpenBitSet.java