Java tutorial
/* * 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.common.base.Supplier; import java.security.Key; import java.security.MessageDigest; 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.annotation.CheckReturnValue; import javax.annotation.Nullable; import javax.crypto.spec.SecretKeySpec; /** * 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 @CheckReturnValue public final class Hashing { /** * 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. * * <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_32Holder.GOOD_FAST_HASH_FUNCTION_32; } if (bits <= 128) { return Murmur3_128Holder.GOOD_FAST_HASH_FUNCTION_128; } // Otherwise, join together some 128-bit murmur3s int hashFunctionsNeeded = (bits + 127) / 128; HashFunction[] hashFunctions = new HashFunction[hashFunctionsNeeded]; hashFunctions[0] = Murmur3_128Holder.GOOD_FAST_HASH_FUNCTION_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 {@link #goodFastHash} instances, so that programs which persist anything * dependent on the hash codes they produce will fail sooner. */ private static final int GOOD_FAST_HASH_SEED = (int) System.currentTimeMillis(); /** * Returns a hash function implementing the * <a href="http://smhasher.googlecode.com/svn/trunk/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 * <a href="http://smhasher.googlecode.com/svn/trunk/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_32Holder.MURMUR3_32; } private static class Murmur3_32Holder { static final HashFunction MURMUR3_32 = new Murmur3_32HashFunction(0); /** Returned by {@link #goodFastHash} when {@code minimumBits <= 32}. */ static final HashFunction GOOD_FAST_HASH_FUNCTION_32 = murmur3_32(GOOD_FAST_HASH_SEED); } /** * Returns a hash function implementing the * <a href="http://smhasher.googlecode.com/svn/trunk/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 * <a href="http://smhasher.googlecode.com/svn/trunk/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_128Holder.MURMUR3_128; } private static class Murmur3_128Holder { static final HashFunction MURMUR3_128 = new Murmur3_128HashFunction(0); /** Returned by {@link #goodFastHash} when {@code 32 < minimumBits <= 128}. */ static final HashFunction GOOD_FAST_HASH_FUNCTION_128 = murmur3_128(GOOD_FAST_HASH_SEED); } /** * 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 SipHash24Holder.SIP_HASH_24; } private static class SipHash24Holder { static final HashFunction SIP_HASH_24 = new SipHashFunction(2, 4, 0x0706050403020100L, 0x0f0e0d0c0b0a0908L); } /** * 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) by delegating to * the MD5 {@link MessageDigest}. */ 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) by delegating to the * SHA-1 {@link MessageDigest}. */ 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) by delegating to * the SHA-256 {@link MessageDigest}. */ 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) by delegating to * the SHA-384 {@link MessageDigest}. * * @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) by delegating to the * SHA-512 {@link MessageDigest}. */ 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. * * * @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 {@link SecretSpecKey} 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. * * * @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 {@link SecretSpecKey} 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. * * * @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 {@link SecretSpecKey} 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. * * * @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 {@link SecretSpecKey} 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. * * @since 18.0 */ public static HashFunction crc32c() { return Crc32cHolder.CRC_32_C; } private static final class Crc32cHolder { static final HashFunction CRC_32_C = new Crc32cHashFunction(); } /** * Returns a hash function implementing the CRC-32 checksum algorithm (32 hash bits) by delegating * to the {@link CRC32} {@link Checksum}. * * <p>To get the {@code long} value equivalent to {@link Checksum#getValue()} for a * {@code HashCode} produced by this function, use {@link HashCode#padToLong()}. * * @since 14.0 */ public static HashFunction crc32() { return Crc32Holder.CRC_32; } private static class Crc32Holder { static final HashFunction CRC_32 = checksumHashFunction(ChecksumType.CRC_32, "Hashing.crc32()"); } /** * Returns a hash function implementing the Adler-32 checksum algorithm (32 hash bits) by * delegating to the {@link Adler32} {@link Checksum}. * * <p>To get the {@code long} value equivalent to {@link Checksum#getValue()} for a * {@code HashCode} produced by this function, use {@link HashCode#padToLong()}. * * @since 14.0 */ public static HashFunction adler32() { return Adler32Holder.ADLER_32; } private static class Adler32Holder { static final HashFunction ADLER_32 = checksumHashFunction(ChecksumType.ADLER_32, "Hashing.adler32()"); } private static HashFunction checksumHashFunction(ChecksumType type, String toString) { return new ChecksumHashFunction(type, type.bits, toString); } enum ChecksumType implements Supplier<Checksum> { CRC_32(32) { @Override public Checksum get() { return new CRC32(); } }, ADLER_32(32) { @Override public Checksum get() { return new Adler32(); } }; private final int bits; ChecksumType(int bits) { this.bits = bits; } @Override public abstract Checksum get(); } /** * 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 {@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 <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 <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. */ 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. * * <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<HashFunction>(); list.add(first); list.add(second); for (HashFunction hashFunc : rest) { list.add(hashFunc); } 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. * * <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<HashFunction>(); 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 final int bits; private ConcatenatedHashFunction(HashFunction... functions) { super(functions); int bitSum = 0; for (HashFunction function : functions) { bitSum += function.bits(); checkArgument(function.bits() % 8 == 0, "the number of bits (%s) in hashFunction (%s) must be divisible by 8", function.bits(), function); } this.bits = bitSum; } @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() { return bits; } @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) * 31 + bits; } } /** * 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() { } }