private void clean() {
        int offset = 0;
        for (int i = 0; i < size; ++i) {
            if (entries[i] == null) {
                ++offset;
            } else {
                entries[i - offset] = entries[i];
            }
        }
        size -= offset;
    }
Expose the index in the internal array of a given object. -1 is returned
if the object is not present in the internal array.

@param e
           an object to check whether exists in this set
@return the index of the argument e in the internal array, or -1 if the
        object is not present
vespajlib/src/main/java/com/yahoo/collections/TinyIdentitySet.java