* specific length.
   *
   * <p>For example, if you need 1024-bit hash codes, you could join two {@link Hashing#sha512} hash
   * functions together: {@code Hashing.concatenating(Hashing.sha512(), Hashing.sha512())}.
   *
   * @since 19.0
   */
  public static HashFunction concatenating(
      HashFunction first, HashFunction second, HashFunction... rest) {

## Making your change

### Code change guidelines

All code contributions should contain the following:

* Create unit tests using [Spock](https://spockframework.org/spock/docs/2.0/index.html) for new classes or methods that you introduce.

   * objects per entry.  Instead we use an open-addressed hash table.  This design is basically
   * equivalent to RegularImmutableSet, save that instead of having a hash table containing the
   * elements directly and null for empty positions, we store indices of the keys in the hash table,
   * and ABSENT for empty positions.  We then look up the keys in alternatingKeysAndValues.
   *

      this.key = key;
      this.firstEntry = this;
      this.lastEntry = this;
      // Round expected values up to a power of 2 to get the table size.
      int tableSize = Hashing.closedTableSize(expectedValues, VALUE_SET_LOAD_FACTOR);

      @SuppressWarnings({"rawtypes", "unchecked"})
      @Nullable
      ValueEntry<K, V>[] hashTable = new @Nullable ValueEntry[tableSize];

  @Override
  public abstract int hashCode();

  /*
   * The implementation neither produces nor consumes any non-null instance of type C, so
   * casting the type parameter is safe.
   */
  @SuppressWarnings("unchecked")
  static <C extends Comparable> Cut<C> belowAll() {
    return (Cut<C>) BelowAll.INSTANCE;
  }

## Password hashing

"Hashing" means converting some content (a password in this case) into a sequence of bytes (just a string) that looks like gibberish.

Whenever you pass exactly the same content (exactly the same password) you get exactly the same gibberish.

But you cannot convert from the gibberish back to the password.

### Why use password hashing

// Add policy conditionals.
const (
	policyCondEqual         = "eq"
	policyCondStartsWith    = "starts-with"
	policyCondContentLength = "content-length-range"
)

// toString - Safely convert interface to string without causing panic.
func toString(val interface{}) string {
	switch v := val.(type) {
	case string:
		return v
	default:
		return ""
	}
}

// toLowerString - safely convert interface to lower string

   *       involving {@code view} will throw)
   *   <li>{@code hashCode()} does not throw
   *   <li>if {@code node} is re-added to the network after having been removed, {@code view}'s
   *       behavior is undefined
   * </ul>
   *
   * @throws IllegalArgumentException if {@code node} is not an element of this network
   */
  Set<N> adjacentNodes(N node);

  fun newBuilder(): Builder = commonNewBuilder()

  /**
   * Returns true if `other` is a `Headers` object with the same headers, with the same casing, in
   * the same order. Note that two headers instances may be *semantically* equal but not equal
   * according to this method. In particular, none of the following sets of headers are equal
   * according to this method:

  @Override
  public abstract int hashCode();

  /*
   * The implementation neither produces nor consumes any non-null instance of type C, so
   * casting the type parameter is safe.
   */
  @SuppressWarnings("unchecked")
  static <C extends Comparable> Cut<C> belowAll() {
    return (Cut<C>) BelowAll.INSTANCE;
  }