docs: private static int prefixLen(String o1, String o2) {
      final int l1 = o1.length(), l2 = o2.length(), l = l1 < l2 ? l1 : l2;
      int prefix = 0;
      while(prefix < l && (o1.charAt(prefix) == o2.charAt(prefix))) {
        prefix++;
      }
      return prefix;
    }