001/* 002 * Copyright (C) 2011 The Guava Authors 003 * 004 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except 005 * in compliance with the License. You may obtain a copy of the License at 006 * 007 * http://www.apache.org/licenses/LICENSE-2.0 008 * 009 * Unless required by applicable law or agreed to in writing, software distributed under the License 010 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express 011 * or implied. See the License for the specific language governing permissions and limitations under 012 * the License. 013 */ 014 015package com.google.common.hash; 016 017import com.google.common.primitives.Ints; 018import com.google.errorprone.annotations.Immutable; 019import java.nio.ByteBuffer; 020import java.nio.charset.Charset; 021import org.checkerframework.checker.nullness.qual.Nullable; 022 023/** 024 * A hash function is a collision-averse pure function that maps an arbitrary block of data to a 025 * number called a <i>hash code</i>. 026 * 027 * <h3>Definition</h3> 028 * 029 * <p>Unpacking this definition: 030 * 031 * <ul> 032 * <li><b>block of data:</b> the input for a hash function is always, in concept, an ordered byte 033 * array. This hashing API accepts an arbitrary sequence of byte and multibyte values (via 034 * {@link Hasher}), but this is merely a convenience; these are always translated into raw 035 * byte sequences under the covers. 036 * <li><b>hash code:</b> each hash function always yields hash codes of the same fixed bit length 037 * (given by {@link #bits}). For example, {@link Hashing#sha1} produces a 160-bit number, 038 * while {@link Hashing#murmur3_32()} yields only 32 bits. Because a {@code long} value is 039 * clearly insufficient to hold all hash code values, this API represents a hash code as an 040 * instance of {@link HashCode}. 041 * <li><b>pure function:</b> the value produced must depend only on the input bytes, in the order 042 * they appear. Input data is never modified. {@link HashFunction} instances should always be 043 * stateless, and therefore thread-safe. 044 * <li><b>collision-averse:</b> while it can't be helped that a hash function will sometimes 045 * produce the same hash code for distinct inputs (a "collision"), every hash function strives 046 * to <i>some</i> degree to make this unlikely. (Without this condition, a function that 047 * always returns zero could be called a hash function. It is not.) 048 * </ul> 049 * 050 * <p>Summarizing the last two points: "equal yield equal <i>always</i>; unequal yield unequal 051 * <i>often</i>." This is the most important characteristic of all hash functions. 052 * 053 * <h3>Desirable properties</h3> 054 * 055 * <p>A high-quality hash function strives for some subset of the following virtues: 056 * 057 * <ul> 058 * <li><b>collision-resistant:</b> while the definition above requires making at least <i>some</i> 059 * token attempt, one measure of the quality of a hash function is <i>how well</i> it succeeds 060 * at this goal. Important note: it may be easy to achieve the theoretical minimum collision 061 * rate when using completely <i>random</i> sample input. The true test of a hash function is 062 * how it performs on representative real-world data, which tends to contain many hidden 063 * patterns and clumps. The goal of a good hash function is to stamp these patterns out as 064 * thoroughly as possible. 065 * <li><b>bit-dispersing:</b> masking out any <i>single bit</i> from a hash code should yield only 066 * the expected <i>twofold</i> increase to all collision rates. Informally, the "information" 067 * in the hash code should be as evenly "spread out" through the hash code's bits as possible. 068 * The result is that, for example, when choosing a bucket in a hash table of size 2^8, 069 * <i>any</i> eight bits could be consistently used. 070 * <li><b>cryptographic:</b> certain hash functions such as {@link Hashing#sha512} are designed to 071 * make it as infeasible as possible to reverse-engineer the input that produced a given hash 072 * code, or even to discover <i>any</i> two distinct inputs that yield the same result. These 073 * are called <i>cryptographic hash functions</i>. But, whenever it is learned that either of 074 * these feats has become computationally feasible, the function is deemed "broken" and should 075 * no longer be used for secure purposes. (This is the likely eventual fate of <i>all</i> 076 * cryptographic hashes.) 077 * <li><b>fast:</b> perhaps self-explanatory, but often the most important consideration. 078 * </ul> 079 * 080 * <h3>Providing input to a hash function</h3> 081 * 082 * <p>The primary way to provide the data that your hash function should act on is via a {@link 083 * Hasher}. Obtain a new hasher from the hash function using {@link #newHasher}, "push" the relevant 084 * data into it using methods like {@link Hasher#putBytes(byte[])}, and finally ask for the {@code 085 * HashCode} when finished using {@link Hasher#hash}. (See an {@linkplain #newHasher example} of 086 * this.) 087 * 088 * <p>If all you want to hash is a single byte array, string or {@code long} value, there are 089 * convenient shortcut methods defined directly on {@link HashFunction} to make this easier. 090 * 091 * <p>Hasher accepts primitive data types, but can also accept any Object of type {@code T} provided 092 * that you implement a {@link Funnel}{@code <T>} to specify how to "feed" data from that object 093 * into the function. (See {@linkplain Hasher#putObject an example} of this.) 094 * 095 * <p><b>Compatibility note:</b> Throughout this API, multibyte values are always interpreted in 096 * <i>little-endian</i> order. That is, hashing the byte array {@code {0x01, 0x02, 0x03, 0x04}} is 097 * equivalent to hashing the {@code int} value {@code 0x04030201}. If this isn't what you need, 098 * methods such as {@link Integer#reverseBytes} and {@link Ints#toByteArray} will help. 099 * 100 * <h3>Relationship to {@link Object#hashCode}</h3> 101 * 102 * <p>Java's baked-in concept of hash codes is constrained to 32 bits, and provides no separation 103 * between hash algorithms and the data they act on, so alternate hash algorithms can't be easily 104 * substituted. Also, implementations of {@code hashCode} tend to be poor-quality, in part because 105 * they end up depending on <i>other</i> existing poor-quality {@code hashCode} implementations, 106 * including those in many JDK classes. 107 * 108 * <p>{@code Object.hashCode} implementations tend to be very fast, but have weak collision 109 * prevention and <i>no</i> expectation of bit dispersion. This leaves them perfectly suitable for 110 * use in hash tables, because extra collisions cause only a slight performance hit, while poor bit 111 * dispersion is easily corrected using a secondary hash function (which all reasonable hash table 112 * implementations in Java use). For the many uses of hash functions beyond data structures, 113 * however, {@code Object.hashCode} almost always falls short -- hence this library. 114 * 115 * @author Kevin Bourrillion 116 * @since 11.0 117 */ 118@Immutable 119@ElementTypesAreNonnullByDefault 120public interface HashFunction { 121 /** 122 * Begins a new hash code computation by returning an initialized, stateful {@code Hasher} 123 * instance that is ready to receive data. Example: 124 * 125 * <pre>{@code 126 * HashFunction hf = Hashing.md5(); 127 * HashCode hc = hf.newHasher() 128 * .putLong(id) 129 * .putBoolean(isActive) 130 * .hash(); 131 * }</pre> 132 */ 133 Hasher newHasher(); 134 135 /** 136 * Begins a new hash code computation as {@link #newHasher()}, but provides a hint of the expected 137 * size of the input (in bytes). This is only important for non-streaming hash functions (hash 138 * functions that need to buffer their whole input before processing any of it). 139 */ 140 Hasher newHasher(int expectedInputSize); 141 142 /** 143 * Shortcut for {@code newHasher().putInt(input).hash()}; returns the hash code for the given 144 * {@code int} value, interpreted in little-endian byte order. The implementation <i>might</i> 145 * perform better than its longhand equivalent, but should not perform worse. 146 * 147 * @since 12.0 148 */ 149 HashCode hashInt(int input); 150 151 /** 152 * Shortcut for {@code newHasher().putLong(input).hash()}; returns the hash code for the given 153 * {@code long} value, interpreted in little-endian byte order. The implementation <i>might</i> 154 * perform better than its longhand equivalent, but should not perform worse. 155 */ 156 HashCode hashLong(long input); 157 158 /** 159 * Shortcut for {@code newHasher().putBytes(input).hash()}. The implementation <i>might</i> 160 * perform better than its longhand equivalent, but should not perform worse. 161 */ 162 HashCode hashBytes(byte[] input); 163 164 /** 165 * Shortcut for {@code newHasher().putBytes(input, off, len).hash()}. The implementation 166 * <i>might</i> perform better than its longhand equivalent, but should not perform worse. 167 * 168 * @throws IndexOutOfBoundsException if {@code off < 0} or {@code off + len > bytes.length} or 169 * {@code len < 0} 170 */ 171 HashCode hashBytes(byte[] input, int off, int len); 172 173 /** 174 * Shortcut for {@code newHasher().putBytes(input).hash()}. The implementation <i>might</i> 175 * perform better than its longhand equivalent, but should not perform worse. 176 * 177 * @since 23.0 178 */ 179 HashCode hashBytes(ByteBuffer input); 180 181 /** 182 * Shortcut for {@code newHasher().putUnencodedChars(input).hash()}. The implementation 183 * <i>might</i> perform better than its longhand equivalent, but should not perform worse. Note 184 * that no character encoding is performed; the low byte and high byte of each {@code char} are 185 * hashed directly (in that order). 186 * 187 * <p><b>Warning:</b> This method will produce different output than most other languages do when 188 * running the same hash function on the equivalent input. For cross-language compatibility, use 189 * {@link #hashString}, usually with a charset of UTF-8. For other use cases, use {@code 190 * hashUnencodedChars}. 191 * 192 * @since 15.0 (since 11.0 as hashString(CharSequence)). 193 */ 194 HashCode hashUnencodedChars(CharSequence input); 195 196 /** 197 * Shortcut for {@code newHasher().putString(input, charset).hash()}. Characters are encoded using 198 * the given {@link Charset}. The implementation <i>might</i> perform better than its longhand 199 * equivalent, but should not perform worse. 200 * 201 * <p><b>Warning:</b> This method, which reencodes the input before hashing it, is useful only for 202 * cross-language compatibility. For other use cases, prefer {@link #hashUnencodedChars}, which is 203 * faster, produces the same output across Java releases, and hashes every {@code char} in the 204 * input, even if some are invalid. 205 */ 206 HashCode hashString(CharSequence input, Charset charset); 207 208 /** 209 * Shortcut for {@code newHasher().putObject(instance, funnel).hash()}. The implementation 210 * <i>might</i> perform better than its longhand equivalent, but should not perform worse. 211 * 212 * @since 14.0 213 */ 214 <T extends @Nullable Object> HashCode hashObject( 215 @ParametricNullness T instance, Funnel<? super T> funnel); 216 217 /** 218 * Returns the number of bits (a multiple of 32) that each hash code produced by this hash 219 * function has. 220 */ 221 int bits(); 222}