title: Java fastest way to get cardinality of BitSet intersection
body: <p>The function below takes two <code>BitSets</code>, makes a copy of the first (it must not be overridden), intersects the copy with the second (bitwise AND) and returns the cardinality of the result.</p>
  
  <pre><code>public int getIntersectionSize(BitSet bits1, BitSet bits2) {
      BitSet copy = (BitSet) bits1.clone();
      copy.and(bits2);
      return copy.cardinality();
  }
  </code></pre>
  
  <p>I'm interested if this code can be sped up? This function is called billion of times so even a microsecond speed up makes sense plus I'm curious about the fastest possible code.</p>