* @since 11.0
 */
@Immutable
@ElementTypesAreNonnullByDefault
public interface HashFunction {
  /**
   * Begins a new hash code computation by returning an initialized, stateful {@code Hasher}
   * instance that is ready to receive data. Example:
   *
   * <pre>{@code
   * HashFunction hf = Hashing.md5();
   * HashCode hc = hf.newHasher()
   *     .putLong(id)
   *     .putBoolean(isActive)

 * @since 11.0
 */
@Immutable
@ElementTypesAreNonnullByDefault
public interface HashFunction {
  /**
   * Begins a new hash code computation by returning an initialized, stateful {@code Hasher}
   * instance that is ready to receive data. Example:
   *
   * <pre>{@code
   * HashFunction hf = Hashing.md5();
   * HashCode hc = hf.newHasher()
   *     .putLong(id)
   *     .putBoolean(isActive)

  }

  @Benchmark
  int hasher(int reps) {
    HashFunction hashFunction = hashFunctionEnum.getHashFunction();
    int result = 37;
    for (int i = 0; i < reps; i++) {
      result ^= hashFunction.newHasher().putBytes(testBytes).hash().asBytes()[0];
    }
    return result;
  }

  @Benchmark
  int hashFunction(int reps) {
    HashFunction hashFunction = hashFunctionEnum.getHashFunction();
    int result = 37;

      result = (byte) (result ^ checksum.asLong());
    }
    return result;
  }

  // Helpers + main

  private byte runHashFunction(int reps, HashFunction hashFunction) {
    byte result = 0x01;
    // Trick the JVM to prevent it from using the hash function non-polymorphically
    result ^= Hashing.crc32().hashInt(reps).asBytes()[0];

   *
   * @since 19.0
   */
  public static HashFunction concatenating(Iterable<HashFunction> hashFunctions) {
    checkNotNull(hashFunctions);
    // We can't use Iterables.toArray() here because there's no hash->collect dependency
    List<HashFunction> list = new ArrayList<>();
    for (HashFunction hashFunction : hashFunctions) {
      list.add(hashFunction);
    }

      if (shouldHaveKnownHashes(method)) {
        HashFunction hashFunction = (HashFunction) method.invoke(Hashing.class);
        assertTrue(
            "There should be at least 3 entries in KNOWN_HASHES for " + hashFunction,
            KNOWN_HASHES.row(hashFunction).size() >= 3);
      }
    }
  }

  public void testKnownUtf8Hashing() {
    for (Cell<HashFunction, String, String> cell : KNOWN_HASHES.cellSet()) {

    assertHashByteBufferMatchesBytes(hashFunction);
    assertHashByteBufferExhaustsBuffer(hashFunction);
    assertHashByteBufferPreservesByteOrder(hashFunction);
    assertHasherByteBufferPreservesByteOrder(hashFunction);
  }

  static void assertHashByteBufferMatchesBytes(HashFunction hashFunction) {
    Random rng = new Random(0L);
    byte[] bytes = new byte[rng.nextInt(256) + 1];

  private Hasher hasher;
  private HashFunction hashFunction;
  private final ByteArrayOutputStream buffer = new ByteArrayOutputStream();

  @SuppressWarnings("DoNotMock")
  @Override
  protected void setUp() throws Exception {
    super.setUp();
    hasher = mock(Hasher.class);
    hashFunction = mock(HashFunction.class);

    when(hashFunction.newHasher()).thenReturn(hasher);
  }

  private Hasher hasher;
  private HashFunction hashFunction;
  private static final byte[] testBytes = new byte[] {'y', 'a', 'm', 's'};
  private ByteArrayInputStream buffer;

  @SuppressWarnings("DoNotMock")
  @Override
  protected void setUp() throws Exception {
    super.setUp();
    hasher = mock(Hasher.class);
    hashFunction = mock(HashFunction.class);
    buffer = new ByteArrayInputStream(testBytes);

  private Hasher hasher;
  private HashFunction hashFunction;
  private static final byte[] testBytes = new byte[] {'y', 'a', 'm', 's'};
  private ByteArrayInputStream buffer;

  @SuppressWarnings("DoNotMock")
  @Override
  protected void setUp() throws Exception {
    super.setUp();
    hasher = mock(Hasher.class);
    hashFunction = mock(HashFunction.class);
    buffer = new ByteArrayInputStream(testBytes);