Method distinctList

core/src/main/java/org/apache/calcite/util/Util.java:2353–2367

Converts a list into a list with unique elements. The order is preserved; the second and subsequent occurrences are removed. If the list is already unique it is returned unchanged.

(List<E> list)

   *
   * <p>If the list is already unique it is returned unchanged. */
  public static <E> List<E> distinctList(List<E> list) {
    // If the list is small, check for duplicates using pairwise comparison.
    if (list.size() < QUICK_DISTINCT) {
      if (isDistinct(list)) {
        return list;
      }
    } else {
      // Lists that have all the same element are common. Avoiding creating a
      // set.
      if (allSameElement(list)) {
        return ImmutableList.of(list.get(0));
      }
    }
    return ImmutableList.copyOf(new LinkedHashSet<>(list));
  }

  /** Returns whether all of the elements of a list are equal.
   * The list is assumed to be non-empty. */