https://github.com/google/guava/guava: Guava is a suite of core and expanded libraries that include utility classes, google's collections, io classes, and much much more. 
/*
 * Copyright (C) 2011 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.hash;

import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;

import com.google.common.annotations.Beta;
import com.google.errorprone.annotations.Immutable;
import java.security.Key;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Iterator;
import java.util.List;
import java.util.zip.Adler32;
import java.util.zip.CRC32;
import java.util.zip.Checksum;
import javax.crypto.spec.SecretKeySpec;
import org.checkerframework.checker.nullness.qual.Nullable;


Static methods to obtain HashFunction instances, and other static hashing-related utilities. 
A comparison of the various hash functions can be found here.

Author:Kevin Bourrillion, Dimitris Andreou, Kurt Alfred Kluever
Since:11.0
/**
 * Static methods to obtain {@link HashFunction} instances, and other static hashing-related
 * utilities.
 *
 * <p>A comparison of the various hash functions can be found <a
 * href="http://goo.gl/jS7HH">here</a>.
 *
 * @author Kevin Bourrillion
 * @author Dimitris Andreou
 * @author Kurt Alfred Kluever
 * @since 11.0
 */
@Beta
public final class Hashing {
  
Returns a general-purpose, temporary-use, non-cryptographic hash function. The algorithm
the returned function implements is unspecified and subject to change without notice.

Warning: a new random seed for these functions is chosen each time the 
Hashing class is loaded. Do not use this method if hash codes may escape the current process in any way, for example being sent over RPC, or saved to disk. For a general-purpose, non-cryptographic hash function that will never change behavior, we suggest murmur3_128. 
Repeated calls to this method on the same loaded Hashing class, using the same value for minimumBits, will return identically-behaving HashFunction instances. 
Params:
  • minimumBits – a positive integer (can be arbitrarily large)
Returns:a hash function, described above, that produces hash codes of length 
    minimumBits or greater
/**
   * Returns a general-purpose, <b>temporary-use</b>, non-cryptographic hash function. The algorithm
   * the returned function implements is unspecified and subject to change without notice.
   *
   * <p><b>Warning:</b> a new random seed for these functions is chosen each time the {@code
   * Hashing} class is loaded. <b>Do not use this method</b> if hash codes may escape the current
   * process in any way, for example being sent over RPC, or saved to disk. For a general-purpose,
   * non-cryptographic hash function that will never change behavior, we suggest {@link
   * #murmur3_128}.
   *
   * <p>Repeated calls to this method on the same loaded {@code Hashing} class, using the same value
   * for {@code minimumBits}, will return identically-behaving {@link HashFunction} instances.
   *
   * @param minimumBits a positive integer (can be arbitrarily large)
   * @return a hash function, described above, that produces hash codes of length {@code
   *     minimumBits} or greater
   */
  public static HashFunction goodFastHash(int minimumBits) {
    int bits = checkPositiveAndMakeMultipleOf32(minimumBits);

    if (bits == 32) {
      return Murmur3_32HashFunction.GOOD_FAST_HASH_32;
    }
    if (bits <= 128) {
      return Murmur3_128HashFunction.GOOD_FAST_HASH_128;
    }

    // Otherwise, join together some 128-bit murmur3s
    int hashFunctionsNeeded = (bits + 127) / 128;
    HashFunction[] hashFunctions = new HashFunction[hashFunctionsNeeded];
    hashFunctions[0] = Murmur3_128HashFunction.GOOD_FAST_HASH_128;
    int seed = GOOD_FAST_HASH_SEED;
    for (int i = 1; i < hashFunctionsNeeded; i++) {
      seed += 1500450271; // a prime; shouldn't matter
      hashFunctions[i] = murmur3_128(seed);
    }
    return new ConcatenatedHashFunction(hashFunctions);
  }

  
Used to randomize goodFastHash instances, so that programs which persist anything dependent on the hash codes they produce will fail sooner. 
/**
   * Used to randomize {@link #goodFastHash} instances, so that programs which persist anything
   * dependent on the hash codes they produce will fail sooner.
   */
  static final int GOOD_FAST_HASH_SEED = (int) System.currentTimeMillis();

  
Returns a hash function implementing the 32-bit murmur3
algorithm, x86 variant (little-endian variant), using the given seed value.

The exact C++ equivalent is the MurmurHash3_x86_32 function (Murmur3A).

/**
   * Returns a hash function implementing the <a
   * href="https://github.com/aappleby/smhasher/blob/master/src/MurmurHash3.cpp">32-bit murmur3
   * algorithm, x86 variant</a> (little-endian variant), using the given seed value.
   *
   * <p>The exact C++ equivalent is the MurmurHash3_x86_32 function (Murmur3A).
   */
  public static HashFunction murmur3_32(int seed) {
    return new Murmur3_32HashFunction(seed);
  }

  
Returns a hash function implementing the 32-bit murmur3
algorithm, x86 variant (little-endian variant), using a seed value of zero.

The exact C++ equivalent is the MurmurHash3_x86_32 function (Murmur3A).

/**
   * Returns a hash function implementing the <a
   * href="https://github.com/aappleby/smhasher/blob/master/src/MurmurHash3.cpp">32-bit murmur3
   * algorithm, x86 variant</a> (little-endian variant), using a seed value of zero.
   *
   * <p>The exact C++ equivalent is the MurmurHash3_x86_32 function (Murmur3A).
   */
  public static HashFunction murmur3_32() {
    return Murmur3_32HashFunction.MURMUR3_32;
  }

  
Returns a hash function implementing the 128-bit murmur3
algorithm, x64 variant (little-endian variant), using the given seed value.

The exact C++ equivalent is the MurmurHash3_x64_128 function (Murmur3F).

/**
   * Returns a hash function implementing the <a
   * href="https://github.com/aappleby/smhasher/blob/master/src/MurmurHash3.cpp">128-bit murmur3
   * algorithm, x64 variant</a> (little-endian variant), using the given seed value.
   *
   * <p>The exact C++ equivalent is the MurmurHash3_x64_128 function (Murmur3F).
   */
  public static HashFunction murmur3_128(int seed) {
    return new Murmur3_128HashFunction(seed);
  }

  
Returns a hash function implementing the 128-bit murmur3
algorithm, x64 variant (little-endian variant), using a seed value of zero.

The exact C++ equivalent is the MurmurHash3_x64_128 function (Murmur3F).

/**
   * Returns a hash function implementing the <a
   * href="https://github.com/aappleby/smhasher/blob/master/src/MurmurHash3.cpp">128-bit murmur3
   * algorithm, x64 variant</a> (little-endian variant), using a seed value of zero.
   *
   * <p>The exact C++ equivalent is the MurmurHash3_x64_128 function (Murmur3F).
   */
  public static HashFunction murmur3_128() {
    return Murmur3_128HashFunction.MURMUR3_128;
  }

  
Returns a hash function implementing the 64-bit
SipHash-2-4 algorithm using a seed value of k = 00 01 02 .... 
Since:15.0
/**
   * Returns a hash function implementing the <a href="https://131002.net/siphash/">64-bit
   * SipHash-2-4 algorithm</a> using a seed value of {@code k = 00 01 02 ...}.
   *
   * @since 15.0
   */
  public static HashFunction sipHash24() {
    return SipHashFunction.SIP_HASH_24;
  }

  
Returns a hash function implementing the 64-bit
SipHash-2-4 algorithm using the given seed.

Since:15.0
/**
   * Returns a hash function implementing the <a href="https://131002.net/siphash/">64-bit
   * SipHash-2-4 algorithm</a> using the given seed.
   *
   * @since 15.0
   */
  public static HashFunction sipHash24(long k0, long k1) {
    return new SipHashFunction(2, 4, k0, k1);
  }

  
Returns a hash function implementing the MD5 hash algorithm (128 hash bits).

Deprecated:If you must interoperate with a system that requires MD5, then use this method,
    despite its deprecation. But if you can choose your hash function, avoid MD5, which is
    neither fast nor secure. As of January 2017, we suggest:
    
    
      
  • For security: sha256 or a higher-level API. 
  • For speed: goodFastHash, though see its docs for caveats. 
/**
   * Returns a hash function implementing the MD5 hash algorithm (128 hash bits).
   *
   * @deprecated If you must interoperate with a system that requires MD5, then use this method,
   *     despite its deprecation. But if you can choose your hash function, avoid MD5, which is
   *     neither fast nor secure. As of January 2017, we suggest:
   *     <ul>
   *       <li>For security:
   *           {@link Hashing#sha256} or a higher-level API.
   *       <li>For speed: {@link Hashing#goodFastHash}, though see its docs for caveats.
   *     </ul>
   */
  @Deprecated
  public static HashFunction md5() {
    return Md5Holder.MD5;
  }

  private static class Md5Holder {
    static final HashFunction MD5 = new MessageDigestHashFunction("MD5", "Hashing.md5()");
  }

  
Returns a hash function implementing the SHA-1 algorithm (160 hash bits).

Deprecated:If you must interoperate with a system that requires SHA-1, then use this method,
    despite its deprecation. But if you can choose your hash function, avoid SHA-1, which is
    neither fast nor secure. As of January 2017, we suggest:
    
    
      
  • For security: sha256 or a higher-level API. 
  • For speed: goodFastHash, though see its docs for caveats. 
/**
   * Returns a hash function implementing the SHA-1 algorithm (160 hash bits).
   *
   * @deprecated If you must interoperate with a system that requires SHA-1, then use this method,
   *     despite its deprecation. But if you can choose your hash function, avoid SHA-1, which is
   *     neither fast nor secure. As of January 2017, we suggest:
   *     <ul>
   *       <li>For security:
   *           {@link Hashing#sha256} or a higher-level API.
   *       <li>For speed: {@link Hashing#goodFastHash}, though see its docs for caveats.
   *     </ul>
   */
  @Deprecated
  public static HashFunction sha1() {
    return Sha1Holder.SHA_1;
  }

  private static class Sha1Holder {
    static final HashFunction SHA_1 = new MessageDigestHashFunction("SHA-1", "Hashing.sha1()");
  }

  
Returns a hash function implementing the SHA-256 algorithm (256 hash bits). 
/** Returns a hash function implementing the SHA-256 algorithm (256 hash bits). */
  public static HashFunction sha256() {
    return Sha256Holder.SHA_256;
  }

  private static class Sha256Holder {
    static final HashFunction SHA_256 =
        new MessageDigestHashFunction("SHA-256", "Hashing.sha256()");
  }

  
Returns a hash function implementing the SHA-384 algorithm (384 hash bits).

Since:19.0
/**
   * Returns a hash function implementing the SHA-384 algorithm (384 hash bits).
   *
   * @since 19.0
   */
  public static HashFunction sha384() {
    return Sha384Holder.SHA_384;
  }

  private static class Sha384Holder {
    static final HashFunction SHA_384 =
        new MessageDigestHashFunction("SHA-384", "Hashing.sha384()");
  }

  
Returns a hash function implementing the SHA-512 algorithm (512 hash bits). 
/** Returns a hash function implementing the SHA-512 algorithm (512 hash bits). */
  public static HashFunction sha512() {
    return Sha512Holder.SHA_512;
  }

  private static class Sha512Holder {
    static final HashFunction SHA_512 =
        new MessageDigestHashFunction("SHA-512", "Hashing.sha512()");
  }

  
Returns a hash function implementing the Message Authentication Code (MAC) algorithm, using the
MD5 (128 hash bits) hash function and the given secret key.

Params:
  • key – the secret key
Throws:
Since:20.0
/**
   * Returns a hash function implementing the Message Authentication Code (MAC) algorithm, using the
   * MD5 (128 hash bits) hash function and the given secret key.
   *
   *
   * @param key the secret key
   * @throws IllegalArgumentException if the given key is inappropriate for initializing this MAC
   * @since 20.0
   */
  public static HashFunction hmacMd5(Key key) {
    return new MacHashFunction("HmacMD5", key, hmacToString("hmacMd5", key));
  }

  
Returns a hash function implementing the Message Authentication Code (MAC) algorithm, using the MD5 (128 hash bits) hash function and a SecretKeySpec created from the given byte array and the MD5 algorithm. 
Params:
  • key – the key material of the secret key
Since:20.0
/**
   * Returns a hash function implementing the Message Authentication Code (MAC) algorithm, using the
   * MD5 (128 hash bits) hash function and a {@link SecretKeySpec} created from the given byte array
   * and the MD5 algorithm.
   *
   *
   * @param key the key material of the secret key
   * @since 20.0
   */
  public static HashFunction hmacMd5(byte[] key) {
    return hmacMd5(new SecretKeySpec(checkNotNull(key), "HmacMD5"));
  }

  
Returns a hash function implementing the Message Authentication Code (MAC) algorithm, using the
SHA-1 (160 hash bits) hash function and the given secret key.

Params:
  • key – the secret key
Throws:
Since:20.0
/**
   * Returns a hash function implementing the Message Authentication Code (MAC) algorithm, using the
   * SHA-1 (160 hash bits) hash function and the given secret key.
   *
   *
   * @param key the secret key
   * @throws IllegalArgumentException if the given key is inappropriate for initializing this MAC
   * @since 20.0
   */
  public static HashFunction hmacSha1(Key key) {
    return new MacHashFunction("HmacSHA1", key, hmacToString("hmacSha1", key));
  }

  
Returns a hash function implementing the Message Authentication Code (MAC) algorithm, using the SHA-1 (160 hash bits) hash function and a SecretKeySpec created from the given byte array and the SHA-1 algorithm. 
Params:
  • key – the key material of the secret key
Since:20.0
/**
   * Returns a hash function implementing the Message Authentication Code (MAC) algorithm, using the
   * SHA-1 (160 hash bits) hash function and a {@link SecretKeySpec} created from the given byte
   * array and the SHA-1 algorithm.
   *
   *
   * @param key the key material of the secret key
   * @since 20.0
   */
  public static HashFunction hmacSha1(byte[] key) {
    return hmacSha1(new SecretKeySpec(checkNotNull(key), "HmacSHA1"));
  }

  
Returns a hash function implementing the Message Authentication Code (MAC) algorithm, using the
SHA-256 (256 hash bits) hash function and the given secret key.

Params:
  • key – the secret key
Throws:
Since:20.0
/**
   * Returns a hash function implementing the Message Authentication Code (MAC) algorithm, using the
   * SHA-256 (256 hash bits) hash function and the given secret key.
   *
   *
   * @param key the secret key
   * @throws IllegalArgumentException if the given key is inappropriate for initializing this MAC
   * @since 20.0
   */
  public static HashFunction hmacSha256(Key key) {
    return new MacHashFunction("HmacSHA256", key, hmacToString("hmacSha256", key));
  }

  
Returns a hash function implementing the Message Authentication Code (MAC) algorithm, using the SHA-256 (256 hash bits) hash function and a SecretKeySpec created from the given byte array and the SHA-256 algorithm. 
Params:
  • key – the key material of the secret key
Since:20.0
/**
   * Returns a hash function implementing the Message Authentication Code (MAC) algorithm, using the
   * SHA-256 (256 hash bits) hash function and a {@link SecretKeySpec} created from the given byte
   * array and the SHA-256 algorithm.
   *
   *
   * @param key the key material of the secret key
   * @since 20.0
   */
  public static HashFunction hmacSha256(byte[] key) {
    return hmacSha256(new SecretKeySpec(checkNotNull(key), "HmacSHA256"));
  }

  
Returns a hash function implementing the Message Authentication Code (MAC) algorithm, using the
SHA-512 (512 hash bits) hash function and the given secret key.

Params:
  • key – the secret key
Throws:
Since:20.0
/**
   * Returns a hash function implementing the Message Authentication Code (MAC) algorithm, using the
   * SHA-512 (512 hash bits) hash function and the given secret key.
   *
   *
   * @param key the secret key
   * @throws IllegalArgumentException if the given key is inappropriate for initializing this MAC
   * @since 20.0
   */
  public static HashFunction hmacSha512(Key key) {
    return new MacHashFunction("HmacSHA512", key, hmacToString("hmacSha512", key));
  }

  
Returns a hash function implementing the Message Authentication Code (MAC) algorithm, using the SHA-512 (512 hash bits) hash function and a SecretKeySpec created from the given byte array and the SHA-512 algorithm. 
Params:
  • key – the key material of the secret key
Since:20.0
/**
   * Returns a hash function implementing the Message Authentication Code (MAC) algorithm, using the
   * SHA-512 (512 hash bits) hash function and a {@link SecretKeySpec} created from the given byte
   * array and the SHA-512 algorithm.
   *
   *
   * @param key the key material of the secret key
   * @since 20.0
   */
  public static HashFunction hmacSha512(byte[] key) {
    return hmacSha512(new SecretKeySpec(checkNotNull(key), "HmacSHA512"));
  }

  private static String hmacToString(String methodName, Key key) {
    return String.format(
        "Hashing.%s(Key[algorithm=%s, format=%s])",
        methodName, key.getAlgorithm(), key.getFormat());
  }

  
Returns a hash function implementing the CRC32C checksum algorithm (32 hash bits) as described
by RFC 3720, Section 12.1.

This function is best understood as a checksum rather than a true hash function.

Since:18.0
/**
   * Returns a hash function implementing the CRC32C checksum algorithm (32 hash bits) as described
   * by RFC 3720, Section 12.1.
   *
   * <p>This function is best understood as a <a
   * href="https://en.wikipedia.org/wiki/Checksum">checksum</a> rather than a true <a
   * href="https://en.wikipedia.org/wiki/Hash_function">hash function</a>.
   *
   * @since 18.0
   */
  public static HashFunction crc32c() {
    return Crc32cHashFunction.CRC_32_C;
  }

  
Returns a hash function implementing the CRC-32 checksum algorithm (32 hash bits).

To get the long value equivalent to Checksum.getValue() for a 
HashCode produced by this function, use HashCode.padToLong(). 
This function is best understood as a checksum rather than a true hash function.

Since:14.0
/**
   * Returns a hash function implementing the CRC-32 checksum algorithm (32 hash bits).
   *
   * <p>To get the {@code long} value equivalent to {@link Checksum#getValue()} for a {@code
   * HashCode} produced by this function, use {@link HashCode#padToLong()}.
   *
   * <p>This function is best understood as a <a
   * href="https://en.wikipedia.org/wiki/Checksum">checksum</a> rather than a true <a
   * href="https://en.wikipedia.org/wiki/Hash_function">hash function</a>.
   *
   * @since 14.0
   */
  public static HashFunction crc32() {
    return ChecksumType.CRC_32.hashFunction;
  }

  
Returns a hash function implementing the Adler-32 checksum algorithm (32 hash bits).

To get the long value equivalent to Checksum.getValue() for a 
HashCode produced by this function, use HashCode.padToLong(). 
This function is best understood as a checksum rather than a true hash function.

Since:14.0
/**
   * Returns a hash function implementing the Adler-32 checksum algorithm (32 hash bits).
   *
   * <p>To get the {@code long} value equivalent to {@link Checksum#getValue()} for a {@code
   * HashCode} produced by this function, use {@link HashCode#padToLong()}.
   *
   * <p>This function is best understood as a <a
   * href="https://en.wikipedia.org/wiki/Checksum">checksum</a> rather than a true <a
   * href="https://en.wikipedia.org/wiki/Hash_function">hash function</a>.
   *
   * @since 14.0
   */
  public static HashFunction adler32() {
    return ChecksumType.ADLER_32.hashFunction;
  }

  @Immutable
  enum ChecksumType implements ImmutableSupplier<Checksum> {
    CRC_32("Hashing.crc32()") {
      @Override
      public Checksum get() {
        return new CRC32();
      }
    },
    ADLER_32("Hashing.adler32()") {
      @Override
      public Checksum get() {
        return new Adler32();
      }
    };

    public final HashFunction hashFunction;

    ChecksumType(String toString) {
      this.hashFunction = new ChecksumHashFunction(this, 32, toString);
    }
  }

  
Returns a hash function implementing FarmHash's Fingerprint64, an open-source algorithm.

This is designed for generating persistent fingerprints of strings. It isn't
cryptographically secure, but it produces a high-quality hash with fewer collisions than some
alternatives we've used in the past.

FarmHash fingerprints are encoded by HashCode.asBytes in little-endian order. This means HashCode.asLong is guaranteed to return the same value that farmhash::Fingerprint64() would for the same input (when compared using UnsignedLongs's encoding of 64-bit unsigned numbers). 
This function is best understood as a fingerprint rather than a true
hash function.

Since:20.0
/**
   * Returns a hash function implementing FarmHash's Fingerprint64, an open-source algorithm.
   *
   * <p>This is designed for generating persistent fingerprints of strings. It isn't
   * cryptographically secure, but it produces a high-quality hash with fewer collisions than some
   * alternatives we've used in the past.
   *
   * <p>FarmHash fingerprints are encoded by {@link HashCode#asBytes} in little-endian order. This
   * means {@link HashCode#asLong} is guaranteed to return the same value that
   * farmhash::Fingerprint64() would for the same input (when compared using {@link
   * com.google.common.primitives.UnsignedLongs}'s encoding of 64-bit unsigned numbers).
   *
   * <p>This function is best understood as a <a
   * href="https://en.wikipedia.org/wiki/Fingerprint_(computing)">fingerprint</a> rather than a true
   * <a href="https://en.wikipedia.org/wiki/Hash_function">hash function</a>.
   *
   * @since 20.0
   */
  public static HashFunction farmHashFingerprint64() {
    return FarmHashFingerprint64.FARMHASH_FINGERPRINT_64;
  }

  
Assigns to hashCode a "bucket" in the range [0, buckets), in a uniform manner that minimizes the need for remapping as buckets grows. That is, 
consistentHash(h, n) equals: 
    
  
  • n - 1, with approximate probability 1/n 
  • consistentHash(h, n - 1), otherwise (probability 1 - 1/n) 


This method is suitable for the common use case of dividing work among buckets that meet the
following conditions:

    
  
  • You want to assign the same fraction of inputs to each bucket.
  
  • When you reduce the number of buckets, you can accept that the most recently added buckets will be removed first. More concretely, if you are dividing traffic among tasks, you can decrease the number of tasks from 15 and 10, killing off the final 5 tasks, and consistentHash will handle it. If, however, you are dividing traffic among servers alpha, bravo, and charlie and you occasionally need to take each of the servers offline, consistentHash will be a poor fit: It provides no way for you to specify which of the three buckets is disappearing. Thus, if your buckets change from [alpha, bravo, charlie] to [bravo, charlie], it will assign all the old alpha traffic to bravo and all the old bravo traffic to charlie, rather than letting bravo keep its traffic. 


See the Wikipedia article on
consistent hashing for more information.

/**
   * Assigns to {@code hashCode} a "bucket" in the range {@code [0, buckets)}, in a uniform manner
   * that minimizes the need for remapping as {@code buckets} grows. That is, {@code
   * consistentHash(h, n)} equals:
   *
   * <ul>
   *   <li>{@code n - 1}, with approximate probability {@code 1/n}
   *   <li>{@code consistentHash(h, n - 1)}, otherwise (probability {@code 1 - 1/n})
   * </ul>
   *
   * <p>This method is suitable for the common use case of dividing work among buckets that meet the
   * following conditions:
   *
   * <ul>
   *   <li>You want to assign the same fraction of inputs to each bucket.
   *   <li>When you reduce the number of buckets, you can accept that the most recently added
   *       buckets will be removed first. More concretely, if you are dividing traffic among tasks,
   *       you can decrease the number of tasks from 15 and 10, killing off the final 5 tasks, and
   *       {@code consistentHash} will handle it. If, however, you are dividing traffic among
   *       servers {@code alpha}, {@code bravo}, and {@code charlie} and you occasionally need to
   *       take each of the servers offline, {@code consistentHash} will be a poor fit: It provides
   *       no way for you to specify which of the three buckets is disappearing. Thus, if your
   *       buckets change from {@code [alpha, bravo, charlie]} to {@code [bravo, charlie]}, it will
   *       assign all the old {@code alpha} traffic to {@code bravo} and all the old {@code bravo}
   *       traffic to {@code charlie}, rather than letting {@code bravo} keep its traffic.
   * </ul>
   *
   *
   * <p>See the <a href="http://en.wikipedia.org/wiki/Consistent_hashing">Wikipedia article on
   * consistent hashing</a> for more information.
   */
  public static int consistentHash(HashCode hashCode, int buckets) {
    return consistentHash(hashCode.padToLong(), buckets);
  }

  
Assigns to input a "bucket" in the range [0, buckets), in a uniform manner that minimizes the need for remapping as buckets grows. That is, consistentHash(h,
n) equals: 
    
  
  • n - 1, with approximate probability 1/n 
  • consistentHash(h, n - 1), otherwise (probability 1 - 1/n) 


This method is suitable for the common use case of dividing work among buckets that meet the
following conditions:

    
  
  • You want to assign the same fraction of inputs to each bucket.
  
  • When you reduce the number of buckets, you can accept that the most recently added buckets will be removed first. More concretely, if you are dividing traffic among tasks, you can decrease the number of tasks from 15 and 10, killing off the final 5 tasks, and consistentHash will handle it. If, however, you are dividing traffic among servers alpha, bravo, and charlie and you occasionally need to take each of the servers offline, consistentHash will be a poor fit: It provides no way for you to specify which of the three buckets is disappearing. Thus, if your buckets change from [alpha, bravo, charlie] to [bravo, charlie], it will assign all the old alpha traffic to bravo and all the old bravo traffic to charlie, rather than letting bravo keep its traffic. 


See the Wikipedia article on
consistent hashing for more information.

/**
   * Assigns to {@code input} a "bucket" in the range {@code [0, buckets)}, in a uniform manner that
   * minimizes the need for remapping as {@code buckets} grows. That is, {@code consistentHash(h,
   * n)} equals:
   *
   * <ul>
   *   <li>{@code n - 1}, with approximate probability {@code 1/n}
   *   <li>{@code consistentHash(h, n - 1)}, otherwise (probability {@code 1 - 1/n})
   * </ul>
   *
   * <p>This method is suitable for the common use case of dividing work among buckets that meet the
   * following conditions:
   *
   * <ul>
   *   <li>You want to assign the same fraction of inputs to each bucket.
   *   <li>When you reduce the number of buckets, you can accept that the most recently added
   *       buckets will be removed first. More concretely, if you are dividing traffic among tasks,
   *       you can decrease the number of tasks from 15 and 10, killing off the final 5 tasks, and
   *       {@code consistentHash} will handle it. If, however, you are dividing traffic among
   *       servers {@code alpha}, {@code bravo}, and {@code charlie} and you occasionally need to
   *       take each of the servers offline, {@code consistentHash} will be a poor fit: It provides
   *       no way for you to specify which of the three buckets is disappearing. Thus, if your
   *       buckets change from {@code [alpha, bravo, charlie]} to {@code [bravo, charlie]}, it will
   *       assign all the old {@code alpha} traffic to {@code bravo} and all the old {@code bravo}
   *       traffic to {@code charlie}, rather than letting {@code bravo} keep its traffic.
   * </ul>
   *
   *
   * <p>See the <a href="http://en.wikipedia.org/wiki/Consistent_hashing">Wikipedia article on
   * consistent hashing</a> for more information.
   */
  public static int consistentHash(long input, int buckets) {
    checkArgument(buckets > 0, "buckets must be positive: %s", buckets);
    LinearCongruentialGenerator generator = new LinearCongruentialGenerator(input);
    int candidate = 0;
    int next;

    // Jump from bucket to bucket until we go out of range
    while (true) {
      next = (int) ((candidate + 1) / generator.nextDouble());
      if (next >= 0 && next < buckets) {
        candidate = next;
      } else {
        return candidate;
      }
    }
  }

  
Returns a hash code, having the same bit length as each of the input hash codes, that combines
the information of these hash codes in an ordered fashion. That is, whenever two equal hash
codes are produced by two calls to this method, it is as likely as possible that each
was computed from the same input hash codes in the same order.

Throws:
  • IllegalArgumentException – if hashCodes is empty, or the hash codes do not all have the same bit length
/**
   * Returns a hash code, having the same bit length as each of the input hash codes, that combines
   * the information of these hash codes in an ordered fashion. That is, whenever two equal hash
   * codes are produced by two calls to this method, it is <i>as likely as possible</i> that each
   * was computed from the <i>same</i> input hash codes in the <i>same</i> order.
   *
   * @throws IllegalArgumentException if {@code hashCodes} is empty, or the hash codes do not all
   *     have the same bit length
   */
  public static HashCode combineOrdered(Iterable<HashCode> hashCodes) {
    Iterator<HashCode> iterator = hashCodes.iterator();
    checkArgument(iterator.hasNext(), "Must be at least 1 hash code to combine.");
    int bits = iterator.next().bits();
    byte[] resultBytes = new byte[bits / 8];
    for (HashCode hashCode : hashCodes) {
      byte[] nextBytes = hashCode.asBytes();
      checkArgument(
          nextBytes.length == resultBytes.length, "All hashcodes must have the same bit length.");
      for (int i = 0; i < nextBytes.length; i++) {
        resultBytes[i] = (byte) (resultBytes[i] * 37 ^ nextBytes[i]);
      }
    }
    return HashCode.fromBytesNoCopy(resultBytes);
  }

  
Returns a hash code, having the same bit length as each of the input hash codes, that combines
the information of these hash codes in an unordered fashion. That is, whenever two equal hash
codes are produced by two calls to this method, it is as likely as possible that each
was computed from the same input hash codes in some order.

Throws:
  • IllegalArgumentException – if hashCodes is empty, or the hash codes do not all have the same bit length
/**
   * Returns a hash code, having the same bit length as each of the input hash codes, that combines
   * the information of these hash codes in an unordered fashion. That is, whenever two equal hash
   * codes are produced by two calls to this method, it is <i>as likely as possible</i> that each
   * was computed from the <i>same</i> input hash codes in <i>some</i> order.
   *
   * @throws IllegalArgumentException if {@code hashCodes} is empty, or the hash codes do not all
   *     have the same bit length
   */
  public static HashCode combineUnordered(Iterable<HashCode> hashCodes) {
    Iterator<HashCode> iterator = hashCodes.iterator();
    checkArgument(iterator.hasNext(), "Must be at least 1 hash code to combine.");
    byte[] resultBytes = new byte[iterator.next().bits() / 8];
    for (HashCode hashCode : hashCodes) {
      byte[] nextBytes = hashCode.asBytes();
      checkArgument(
          nextBytes.length == resultBytes.length, "All hashcodes must have the same bit length.");
      for (int i = 0; i < nextBytes.length; i++) {
        resultBytes[i] += nextBytes[i];
      }
    }
    return HashCode.fromBytesNoCopy(resultBytes);
  }

  
Checks that the passed argument is positive, and ceils it to a multiple of 32. 
/** Checks that the passed argument is positive, and ceils it to a multiple of 32. */
  static int checkPositiveAndMakeMultipleOf32(int bits) {
    checkArgument(bits > 0, "Number of bits must be positive");
    return (bits + 31) & ~31;
  }

  
Returns a hash function which computes its hash code by concatenating the hash codes of the
underlying hash functions together. This can be useful if you need to generate hash codes of a
specific length.

For example, if you need 1024-bit hash codes, you could join two sha512 hash functions together: Hashing.concatenating(Hashing.sha512(), Hashing.sha512()). 
Since:19.0
/**
   * Returns a hash function which computes its hash code by concatenating the hash codes of the
   * underlying hash functions together. This can be useful if you need to generate hash codes of a
   * 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) {
    // We can't use Lists.asList() here because there's no hash->collect dependency
    List<HashFunction> list = new ArrayList<>();
    list.add(first);
    list.add(second);
    list.addAll(Arrays.asList(rest));
    return new ConcatenatedHashFunction(list.toArray(new HashFunction[0]));
  }

  
Returns a hash function which computes its hash code by concatenating the hash codes of the
underlying hash functions together. This can be useful if you need to generate hash codes of a
specific length.

For example, if you need 1024-bit hash codes, you could join two sha512 hash functions together: Hashing.concatenating(Hashing.sha512(), Hashing.sha512()). 
Since:19.0
/**
   * Returns a hash function which computes its hash code by concatenating the hash codes of the
   * underlying hash functions together. This can be useful if you need to generate hash codes of a
   * 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(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);
    }
    checkArgument(list.size() > 0, "number of hash functions (%s) must be > 0", list.size());
    return new ConcatenatedHashFunction(list.toArray(new HashFunction[0]));
  }

  private static final class ConcatenatedHashFunction extends AbstractCompositeHashFunction {

    private ConcatenatedHashFunction(HashFunction... functions) {
      super(functions);
      for (HashFunction function : functions) {
        checkArgument(
            function.bits() % 8 == 0,
            "the number of bits (%s) in hashFunction (%s) must be divisible by 8",
            function.bits(),
            function);
      }
    }

    @Override
    HashCode makeHash(Hasher[] hashers) {
      byte[] bytes = new byte[bits() / 8];
      int i = 0;
      for (Hasher hasher : hashers) {
        HashCode newHash = hasher.hash();
        i += newHash.writeBytesTo(bytes, i, newHash.bits() / 8);
      }
      return HashCode.fromBytesNoCopy(bytes);
    }

    @Override
    public int bits() {
      int bitSum = 0;
      for (HashFunction function : functions) {
        bitSum += function.bits();
      }
      return bitSum;
    }

    @Override
    public boolean equals(@Nullable Object object) {
      if (object instanceof ConcatenatedHashFunction) {
        ConcatenatedHashFunction other = (ConcatenatedHashFunction) object;
        return Arrays.equals(functions, other.functions);
      }
      return false;
    }

    @Override
    public int hashCode() {
      return Arrays.hashCode(functions);
    }
  }

  
Linear CongruentialGenerator to use for consistent hashing. See
http://en.wikipedia.org/wiki/Linear_congruential_generator

/**
   * Linear CongruentialGenerator to use for consistent hashing. See
   * http://en.wikipedia.org/wiki/Linear_congruential_generator
   */
  private static final class LinearCongruentialGenerator {
    private long state;

    public LinearCongruentialGenerator(long seed) {
      this.state = seed;
    }

    public double nextDouble() {
      state = 2862933555777941757L * state + 1;
      return ((double) ((int) (state >>> 33) + 1)) / (0x1.0p31);
    }
  }

  private Hashing() {}
}