}

// mod returns true if the hash mod cycles == cycle.
// If cycles is 0 false is always returned.
// If cycles is 1 true is always returned (as expected).
func (h dataUsageHash) mod(cycle uint32, cycles uint32) bool {
	if cycles <= 1 {
		return cycles == 1
	}
	return uint32(xxhash.Sum64String(string(h)))%cycles == cycle%cycles
}

// modAlt returns true if the hash mod cycles == cycle.
// This is out of sync with mod.

 * </ul>
 *
 * <p>The locks created by a factory instance will detect lock acquisition cycles with locks created
 * by other {@code CycleDetectingLockFactory} instances (except those with {@code Policy.DISABLED}).
 * A lock's behavior when a cycle is detected, however, is defined by the {@code Policy} of the
 * factory that created it. This allows detection of cycles across components while delegating
 * control over lock behavior to individual components.
 *

   *
   * <pre>{@code
   * |--------------|
   * v              |
   * a -> b -> c -> d
   * |         ^
   * |---------|
   * }</pre>
   */
  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

   *
   * <pre>{@code
   * |--------------|
   * v              |
   * a -> b -> c -> d
   * |         ^
   * |---------|
   * }</pre>
   */
  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

	}
	createUsageTestFiles(t, base, bucket, files)
	err = os.RemoveAll(filepath.Join(base, bucket, "dir1/dira/dirasub/dcfile"))
	if err != nil {
		t.Fatal(err)
	}
	// Changed dir must be picked up in this many cycles.
	for i := 0; i < dataUsageUpdateDirCycles; i++ {
		got, err = scanDataFolder(context.Background(), nil, base, got, getSize, 0, weSleep)
		got.Info.NextCycle++
		if err != nil {
			t.Fatal(err)
		}
	}

 *       {@code $} in its name is a nested class</a>.
 * </ul>
 *
 * <h2>{@code ClassPath} and symlinks</h2>
 *
 * <p>In the case of directory classloaders, symlinks are supported but cycles are not traversed.
 * This guarantees discovery of each unique loadable resource. However, not all possible
 * aliases for resources on cyclic paths will be listed.
 *
 * @author Ben Yu
 * @since 14.0
 */

   *       number of nodes that have been seen but not yet visited, that is, the "horizon").
   * </ul>
   *
   * @param graph {@link SuccessorsFunction} representing a general graph that may have cycles.
   */
  public static <N> Traverser<N> forGraph(SuccessorsFunction<N> graph) {
    return new Traverser<N>(graph) {
      @Override
      Traversal<N> newTraversal() {
        return Traversal.inGraph(graph);

)

const (
	dataScannerSleepPerFolder        = time.Millisecond                 // Time to wait between folders.
	dataUsageUpdateDirCycles         = 16                               // Visit all folders every n cycles.
	dataScannerCompactLeastObject    = 500                              // Compact when there is less than this many objects in a branch.

    }
  }

  @AndroidIncompatible // Path (for symlink creation)

  public void testScanDirectory_symlinkCycle() throws IOException {
    if (isWindows()) {
      return; // TODO: b/136041958 - Can we detect cycles under Windows?
    }
    ClassLoader loader = ClassPathTest.class.getClassLoader();
    // directory with a cycle,
    // /root
    //    /left
    //       /[sibling -> right]
    //    /right

#### Optimizations

A naive implementation would simply regenerate all resources, of all subscribed types, for each client, on any configuration change. However, this scales poorly. As a result, we have many levels of optimizations to avoid doing this work.