code: private int findFirstDifferingBit(String key1, String key2) {
          int bit = 0;
  
          while (keyMapper.isSet(bit, key1) == keyMapper.isSet(bit, key2)) {
              bit++;
          }
          return bit;
      }
docs: Returns the leftmost differing bit index when doing a bitwise comparison of key1 and key2
  
  @param key1  first key to compare
  @param key2  second key to compare
  @return bit index of first different bit