Method firstDuplicate

core/src/main/java/org/apache/calcite/util/Util.java:2274–2302

Returns the ordinal of the first element in the list which is equal to a previous element in the list. For example, firstDuplicate(Arrays.asList("a", "b", "c", "b", "a")) returns 3, the ordinal of the 2nd "b". @param list List @return Ordinal of first duplicate, or -1 if not found

(List<E> list)

   * @return Ordinal of first duplicate, or -1 if not found
   */
  public static <E> int firstDuplicate(List<E> list) {
    final int size = list.size();
    if (size < 2) {
      // Lists of size 0 and 1 are always distinct.
      return -1;
    }
    if (size < QUICK_DISTINCT) {
      // For smaller lists, avoid the overhead of creating a set. Threshold
      // determined empirically using UtilTest.testIsDistinctBenchmark.
      for (int i = 1; i < size; i++) {
        E e = list.get(i);
        for (int j = i - 1; j >= 0; j--) {
          E e1 = list.get(j);
          if (Objects.equals(e, e1)) {
            return i;
          }
        }
      }
      return -1;
    }
    // we use HashMap here, because it is more efficient than HashSet.
    final Map<E, Object> set = new HashMap<>(size);
    for (E e : list) {
      if (set.put(e, "") != null) {
        return set.size();
      }
    }
    return -1;
  }

  /**
   * Returns whether the elements of {@code list} are definitely distinct