*/
  private static final SuccessorsFunction<Character> TWO_CYCLES_GRAPH =
      createDirectedGraph("ab", "ac", "bc", "cd", "da");

  /**
   * A tree-shaped graph that looks as follows (all edges are directed facing downwards):
   *
   * <pre>{@code
   *        h
   *       /|\
   *      / | \
   *     /  |  \
   *    d   e   g
   *   /|\      |
   *  / | \     |
   * a  b  c    f
   * }</pre>

      // For each listener we add a task that submits it to the executor directly for the blocking
      // get use case and another task that adds it as a listener to the future to exercise both
      // racing addListener calls and addListener calls completing after the future completes.
      final Runnable listener =
          k % 2 == 0 ? collectResultsRunnable : collectResultsTimedGetRunnable;

  JAVA_HOME("java.home"),

  /** Java Virtual Machine specification version. */
  JAVA_VM_SPECIFICATION_VERSION("java.vm.specification.version"),

  /** Java Virtual Machine specification vendor. */
  JAVA_VM_SPECIFICATION_VENDOR("java.vm.specification.vendor"),

  /** Java Virtual Machine specification name. */
  JAVA_VM_SPECIFICATION_NAME("java.vm.specification.name"),

  /** Java Virtual Machine implementation version. */

   *       of nodes that have been seen but not yet visited, that is, the "horizon").
   * </ul>
   *
   * <p><b>Examples</b> (all edges are directed facing downwards)
   *
   * <p>The graph below would be valid input with start nodes of {@code a, f, c}. However, if {@code
   * b} were <i>also</i> a start node, then there would be multiple paths to reach {@code e} and

  JAVA_HOME("java.home"),

  /** Java Virtual Machine specification version. */
  JAVA_VM_SPECIFICATION_VERSION("java.vm.specification.version"),

  /** Java Virtual Machine specification vendor. */
  JAVA_VM_SPECIFICATION_VENDOR("java.vm.specification.vendor"),

  /** Java Virtual Machine specification name. */
  JAVA_VM_SPECIFICATION_NAME("java.vm.specification.name"),

  /** Java Virtual Machine implementation version. */

    }
  }


  @J2ktIncompatible // slow enough to cause flakiness
  @GwtIncompatible // far too slow
  public void testSqrtOfPerfectSquareAsDoubleIsPerfect() {
    // This takes just over a minute on my machine.
    for (long n = 0; n <= LongMath.FLOOR_SQRT_MAX_LONG; n++) {
      long actual = (long) Math.sqrt(n * n);
      assertTrue(actual == n);
    }
  }

  public void testSqrtOfLongIsAtMostFloorSqrtMaxLong() {

    if (parent == null) {
      /*
       * The given directory is a filesystem root. All zero of its ancestors exist. This doesn't
       * mean that the root itself exists -- consider x:\ on a Windows machine without such a drive
       * -- or even that the caller can create it, but this method makes no such guarantees even for
       * non-root files.
       */
      return;
    }
    parent.mkdirs();

   */
  private static final SuccessorsFunction<Character> TWO_CYCLES_GRAPH =
      createDirectedGraph("ab", "ac", "bc", "cd", "da");

  /**
   * A tree-shaped graph that looks as follows (all edges are directed facing downwards):
   *
   * <pre>{@code
   *        h
   *       /|\
   *      / | \
   *     /  |  \
   *    d   e   g
   *   /|\      |
   *  / | \     |
   * a  b  c    f
   * }</pre>

      // For each listener we add a task that submits it to the executor directly for the blocking
      // get use case and another task that adds it as a listener to the future to exercise both
      // racing addListener calls and addListener calls completing after the future completes.
      final Runnable listener =
          k % 2 == 0 ? collectResultsRunnable : collectResultsTimedGetRunnable;

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