*
     * @param input the input object (String[], List, or String)
     * @return joined string with elements separated by spaces, or empty string if invalid input
     */
    public static String join(final Object input) {
        String[] values = null;
        if (input instanceof String[]) {
            values = (String[]) input;
        } else if (input instanceof List) {

    checkNotNull(keyFunction);
    checkNotNull(valuesFunction);
    return collectingAndThen(
        flatteningToMultimap(
            input -> checkNotNull(keyFunction.apply(input)),
            input -> valuesFunction.apply(input).peek(Preconditions::checkNotNull),
            MultimapBuilder.linkedHashKeys().arrayListValues()::<K, V>build),
        ImmutableListMultimap::copyOf);
  }

    checkNotNull(keyFunction);
    checkNotNull(valuesFunction);
    return collectingAndThen(
        flatteningToMultimap(
            input -> checkNotNull(keyFunction.apply(input)),
            input -> valuesFunction.apply(input).peek(Preconditions::checkNotNull),
            MultimapBuilder.linkedHashKeys().arrayListValues()::<K, V>build),
        ImmutableListMultimap::copyOf);
  }

@J2ktIncompatible
@GwtIncompatible
public interface Interner<E> {
  /**
   * Chooses and returns the representative instance for any of a collection of instances that are
   * equal to each other. If two {@linkplain Object#equals equal} inputs are given to this method,
   * both calls will return the same instance. That is, {@code intern(a).equals(a)} always holds,
   * and {@code intern(a) == intern(b)} if and only if {@code a.equals(b)}. Note that {@code

  }

  private static void testReverse(long[] input, long[] expectedOutput) {
    input = Arrays.copyOf(input, input.length);
    Longs.reverse(input);
    assertThat(input).isEqualTo(expectedOutput);
  }

  private static void testReverse(long[] input, int fromIndex, int toIndex, long[] expectedOutput) {
    input = Arrays.copyOf(input, input.length);
    Longs.reverse(input, fromIndex, toIndex);

  private static void testReverse(short[] input, short[] expectedOutput) {
    input = Arrays.copyOf(input, input.length);
    Shorts.reverse(input);
    assertThat(input).isEqualTo(expectedOutput);
  }

  private static void testReverse(
      short[] input, int fromIndex, int toIndex, short[] expectedOutput) {
    input = Arrays.copyOf(input, input.length);
    Shorts.reverse(input, fromIndex, toIndex);

    "input": "aa--",
    "output": "aa--"
  },
  {
    "input": "a†--",
    "output": "xn--a---kp0a"
  },
  {
    "input": "ab--c",
    "output": "ab--c"
  },
  {
    "comment": "Label with leading hyphen",
    "input": "-x",
    "output": "-x"
  },
  {
    "input": "-†",
    "output": "xn----xhn"
  },
  {
    "input": "-x.xn--zca",
    "output": "-x.xn--zca"

       - This is unfortunate, because actually it would be safe to declare
       the task as up-to-date, because these two files, which are based on
       the generated index.xml, are outputs, not inputs. We can be sure of
       this because they are deleted in the @BeforeEach method of the
       OsgiTest test class.

       - To enable the benefit of incremental builds, we can ask Gradle

  }

  private static void testSortDescending(byte[] input, byte[] expectedOutput) {
    input = Arrays.copyOf(input, input.length);
    SignedBytes.sortDescending(input);
    assertThat(input).isEqualTo(expectedOutput);
  }

  private static void testSortDescending(
      byte[] input, int fromIndex, int toIndex, byte[] expectedOutput) {
    input = Arrays.copyOf(input, input.length);

  }

  private static void testReverse(char[] input, char[] expectedOutput) {
    input = Arrays.copyOf(input, input.length);
    Chars.reverse(input);
    assertThat(input).isEqualTo(expectedOutput);
  }

  private static void testReverse(char[] input, int fromIndex, int toIndex, char[] expectedOutput) {
    input = Arrays.copyOf(input, input.length);
    Chars.reverse(input, fromIndex, toIndex);