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 com.google.common.annotations.Beta; import com.google.common.primitives.Ints; import java.nio.charset.Charset; /** * A hash function is a collision-averse pure function that maps an arbitrary block of * data to a number called a <i>hash code</i>. * * <h3>Definition</h3> * * <p>Unpacking this definition: * * <ul> * <li><b>block of data:</b> the input for a hash function is always, in concept, an * ordered byte array. This hashing API accepts an arbitrary sequence of byte and * multibyte values (via {@link Hasher}), but this is merely a convenience; these are * always translated into raw byte sequences under the covers. * * <li><b>hash code:</b> each hash function always yields hash codes of the same fixed bit * length (given by {@link #bits}). For example, {@link Hashing#sha1} produces a * 160-bit number, while {@link Hashing#murmur3_32()} yields only 32 bits. Because a * {@code long} value is clearly insufficient to hold all hash code values, this API * represents a hash code as an instance of {@link HashCode}. * * <li><b>pure function:</b> the value produced must depend only on the input bytes, in * the order they appear. Input data is never modified. {@link HashFunction} instances * should always be stateless, and therefore thread-safe. * * <li><b>collision-averse:</b> while it can't be helped that a hash function will * sometimes produce the same hash code for distinct inputs (a "collision"), every * hash function strives to <i>some</i> degree to make this unlikely. (Without this * condition, a function that always returns zero could be called a hash function. It * is not.) * </ul> * * <p>Summarizing the last two points: "equal yield equal <i>always</i>; unequal yield * unequal <i>often</i>." This is the most important characteristic of all hash functions. * * <h3>Desirable properties</h3> * * <p>A high-quality hash function strives for some subset of the following virtues: * * <ul> * <li><b>collision-resistant:</b> while the definition above requires making at least * <i>some</i> token attempt, one measure of the quality of a hash function is <i>how * well</i> it succeeds at this goal. Important note: it may be easy to achieve the * theoretical minimum collision rate when using completely <i>random</i> sample * input. The true test of a hash function is how it performs on representative * real-world data, which tends to contain many hidden patterns and clumps. The goal * of a good hash function is to stamp these patterns out as thoroughly as possible. * * <li><b>bit-dispersing:</b> masking out any <i>single bit</i> from a hash code should * yield only the expected <i>twofold</i> increase to all collision rates. Informally, * the "information" in the hash code should be as evenly "spread out" through the * hash code's bits as possible. The result is that, for example, when choosing a * bucket in a hash table of size 2^8, <i>any</i> eight bits could be consistently * used. * * <li><b>cryptographic:</b> certain hash functions such as {@link Hashing#sha512} are * designed to make it as infeasible as possible to reverse-engineer the input that * produced a given hash code, or even to discover <i>any</i> two distinct inputs that * yield the same result. These are called <i>cryptographic hash functions</i>. But, * whenever it is learned that either of these feats has become computationally * feasible, the function is deemed "broken" and should no longer be used for secure * purposes. (This is the likely eventual fate of <i>all</i> cryptographic hashes.) * * <li><b>fast:</b> perhaps self-explanatory, but often the most important consideration. * We have published <a href="#noWeHaventYet">microbenchmark results</a> for many * common hash functions. * </ul> * * <h3>Providing input to a hash function</h3> * * <p>The primary way to provide the data that your hash function should act on is via a * {@link Hasher}. Obtain a new hasher from the hash function using {@link #newHasher}, * "push" the relevant data into it using methods like {@link Hasher#putBytes(byte[])}, * and finally ask for the {@code HashCode} when finished using {@link Hasher#hash}. (See * an {@linkplain #newHasher example} of this.) * * <p>If all you want to hash is a single byte array, string or {@code long} value, there * are convenient shortcut methods defined directly on {@link HashFunction} to make this * easier. * * <p>Hasher accepts primitive data types, but can also accept any Object of type {@code * T} provided that you implement a {@link Funnel Funnel<T>} to specify how to "feed" data * from that object into the function. (See {@linkplain Hasher#putObject an example} of * this.) * * <p><b>Compatibility note:</b> Throughout this API, multibyte values are always * interpreted in <i>little-endian</i> order. That is, hashing the byte array {@code * {0x01, 0x02, 0x03, 0x04}} is equivalent to hashing the {@code int} value {@code * 0x04030201}. If this isn't what you need, methods such as {@link Integer#reverseBytes} * and {@link Ints#toByteArray} will help. * * <h3>Relationship to {@link Object#hashCode}</h3> * * <p>Java's baked-in concept of hash codes is constrained to 32 bits, and provides no * separation between hash algorithms and the data they act on, so alternate hash * algorithms can't be easily substituted. Also, implementations of {@code hashCode} tend * to be poor-quality, in part because they end up depending on <i>other</i> existing * poor-quality {@code hashCode} implementations, including those in many JDK classes. * * <p>{@code Object.hashCode} implementations tend to be very fast, but have weak * collision prevention and <i>no</i> expectation of bit dispersion. This leaves them * perfectly suitable for use in hash tables, because extra collisions cause only a slight * performance hit, while poor bit dispersion is easily corrected using a secondary hash * function (which all reasonable hash table implementations in Java use). For the many * uses of hash functions beyond data structures, however, {@code Object.hashCode} almost * always falls short -- hence this library. * * @author Kevin Bourrillion * @since 11.0 */ @Beta 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) * .hash();}</pre> */ Hasher newHasher(); /** * Begins a new hash code computation as {@link #newHasher()}, but provides a hint of the * expected size of the input (in bytes). This is only important for non-streaming hash * functions (hash functions that need to buffer their whole input before processing any * of it). */ Hasher newHasher(int expectedInputSize); /** * Shortcut for {@code newHasher().putInt(input).hash()}; returns the hash code for the given * {@code int} value, interpreted in little-endian byte order. The implementation <i>might</i> * perform better than its longhand equivalent, but should not perform worse. * * @since 12.0 */ HashCode hashInt(int input); /** * Shortcut for {@code newHasher().putLong(input).hash()}; returns the hash code for the * given {@code long} value, interpreted in little-endian byte order. The implementation * <i>might</i> perform better than its longhand equivalent, but should not perform worse. */ HashCode hashLong(long input); /** * Shortcut for {@code newHasher().putBytes(input).hash()}. The implementation * <i>might</i> perform better than its longhand equivalent, but should not perform * worse. */ HashCode hashBytes(byte[] input); /** * Shortcut for {@code newHasher().putBytes(input, off, len).hash()}. The implementation * <i>might</i> perform better than its longhand equivalent, but should not perform * worse. * * @throws IndexOutOfBoundsException if {@code off < 0} or {@code off + len > bytes.length} * or {@code len < 0} */ HashCode hashBytes(byte[] input, int off, int len); /** * Shortcut for {@code newHasher().putUnencodedChars(input).hash()}. The implementation * <i>might</i> perform better than its longhand equivalent, but should not perform worse. * Note that no character encoding is performed; the low byte and high byte of each {@code char} * are hashed directly (in that order). * * @since 15.0 (since 11.0 as hashString(CharSequence)). */ HashCode hashUnencodedChars(CharSequence input); /** * Shortcut for {@code newHasher().putString(input, charset).hash()}. Characters are encoded * using the given {@link Charset}. The implementation <i>might</i> perform better than its * longhand equivalent, but should not perform worse. */ HashCode hashString(CharSequence input, Charset charset); /** * Shortcut for {@code newHasher().putObject(instance, funnel).hash()}. The implementation * <i>might</i> perform better than its longhand equivalent, but should not perform worse. * * @since 14.0 */ <T> HashCode hashObject(T instance, Funnel<? super T> funnel); /** * Returns the number of bits (a multiple of 32) that each hash code produced by this * hash function has. */ int bits(); }