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/*
 * 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 hash code.
 *
 * 

Definition

* *

Unpacking this definition: * *

    *
  • block of data: 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. * *
  • hash code: 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}. * *
  • pure function: 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. * *
  • collision-averse: 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 some degree to make this unlikely. (Without this * condition, a function that always returns zero could be called a hash function. It * is not.) *
* *

Summarizing the last two points: "equal yield equal always; unequal yield * unequal often." This is the most important characteristic of all hash functions. * *

Desirable properties

* *

A high-quality hash function strives for some subset of the following virtues: * *

    *
  • collision-resistant: while the definition above requires making at least * some token attempt, one measure of the quality of a hash function is how * well it succeeds at this goal. Important note: it may be easy to achieve the * theoretical minimum collision rate when using completely random 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. * *
  • bit-dispersing: masking out any single bit from a hash code should * yield only the expected twofold 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, any eight bits could be consistently * used. * *
  • cryptographic: 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 any two distinct inputs that * yield the same result. These are called cryptographic hash functions. 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 all cryptographic hashes.) * *
  • fast: perhaps self-explanatory, but often the most important consideration. * We have published microbenchmark results for many * common hash functions. *
* *

Providing input to a hash function

* *

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.) * *

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. * *

Hasher accepts primitive data types, but can also accept any Object of type {@code * T} provided that you implement a {@link Funnel Funnel} to specify how to "feed" data * from that object into the function. (See {@linkplain Hasher#putObject an example} of * this.) * *

Compatibility note: Throughout this API, multibyte values are always * interpreted in little-endian 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. * *

Relationship to {@link Object#hashCode}

* *

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 other existing * poor-quality {@code hashCode} implementations, including those in many JDK classes. * *

{@code Object.hashCode} implementations tend to be very fast, but have weak * collision prevention and no 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:

   {@code
   *
   *   HashFunction hf = Hashing.md5();
   *   HashCode hc = hf.newHasher()
   *       .putLong(id)
   *       .putBoolean(isActive)
   *       .hash();}
*/ 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 might * 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 * might perform better than its longhand equivalent, but should not perform worse. */ HashCode hashLong(long input); /** * Shortcut for {@code newHasher().putBytes(input).hash()}. The implementation * might 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 * might 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 * might 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 might 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 * might perform better than its longhand equivalent, but should not perform worse. * * @since 14.0 */ HashCode hashObject(T instance, Funnel funnel); /** * Returns the number of bits (a multiple of 32) that each hash code produced by this * hash function has. */ int bits(); }




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