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
015 package com.google.common.hash;
016
017 import com.google.common.annotations.Beta;
018 import com.google.common.primitives.Ints;
019
020 import java.nio.charset.Charset;
021
022 /**
023 * A hash function is a collision-averse pure function that maps an arbitrary block of
024 * data to a number called a <i>hash code</i>.
025 *
026 * <h3>Definition</h3>
027 *
028 * <p>Unpacking this definition:
029 *
030 * <ul>
031 * <li><b>block of data:</b> the input for a hash function is always, in concept, an
032 * ordered byte array. This hashing API accepts an arbitrary sequence of byte and
033 * multibyte values (via {@link Hasher}), but this is merely a convenience; these are
034 * always translated into raw byte sequences under the covers.
035 *
036 * <li><b>hash code:</b> each hash function always yields hash codes of the same fixed bit
037 * length (given by {@link #bits}). For example, {@link Hashing#sha1} produces a
038 * 160-bit number, while {@link Hashing#murmur3_32()} yields only 32 bits. Because a
039 * {@code long} value is clearly insufficient to hold all hash code values, this API
040 * represents a hash code as an instance of {@link HashCode}.
041 *
042 * <li><b>pure function:</b> the value produced must depend only on the input bytes, in
043 * the order they appear. Input data is never modified. {@link HashFunction} instances
044 * should always be stateless, and therefore thread-safe.
045 *
046 * <li><b>collision-averse:</b> while it can't be helped that a hash function will
047 * sometimes produce the same hash code for distinct inputs (a "collision"), every
048 * hash function strives to <i>some</i> degree to make this unlikely. (Without this
049 * condition, a function that always returns zero could be called a hash function. It
050 * is not.)
051 * </ul>
052 *
053 * <p>Summarizing the last two points: "equal yield equal <i>always</i>; unequal yield
054 * unequal <i>often</i>." This is the most important characteristic of all hash functions.
055 *
056 * <h3>Desirable properties</h3>
057 *
058 * <p>A high-quality hash function strives for some subset of the following virtues:
059 *
060 * <ul>
061 * <li><b>collision-resistant:</b> while the definition above requires making at least
062 * <i>some</i> token attempt, one measure of the quality of a hash function is <i>how
063 * well</i> it succeeds at this goal. Important note: it may be easy to achieve the
064 * theoretical minimum collision rate when using completely <i>random</i> sample
065 * input. The true test of a hash function is how it performs on representative
066 * real-world data, which tends to contain many hidden patterns and clumps. The goal
067 * of a good hash function is to stamp these patterns out as thoroughly as possible.
068 *
069 * <li><b>bit-dispersing:</b> masking out any <i>single bit</i> from a hash code should
070 * yield only the expected <i>twofold</i> increase to all collision rates. Informally,
071 * the "information" in the hash code should be as evenly "spread out" through the
072 * hash code's bits as possible. The result is that, for example, when choosing a
073 * bucket in a hash table of size 2^8, <i>any</i> eight bits could be consistently
074 * used.
075 *
076 * <li><b>cryptographic:</b> certain hash functions such as {@link Hashing#sha512} are
077 * designed to make it as infeasible as possible to reverse-engineer the input that
078 * produced a given hash code, or even to discover <i>any</i> two distinct inputs that
079 * yield the same result. These are called <i>cryptographic hash functions</i>. But,
080 * whenever it is learned that either of these feats has become computationally
081 * feasible, the function is deemed "broken" and should no longer be used for secure
082 * purposes. (This is the likely eventual fate of <i>all</i> cryptographic hashes.)
083 *
084 * <li><b>fast:</b> perhaps self-explanatory, but often the most important consideration.
085 * We have published <a href="#noWeHaventYet">microbenchmark results</a> for many
086 * common hash functions.
087 * </ul>
088 *
089 * <h3>Providing input to a hash function</h3>
090 *
091 * <p>The primary way to provide the data that your hash function should act on is via a
092 * {@link Hasher}. Obtain a new hasher from the hash function using {@link #newHasher},
093 * "push" the relevant data into it using methods like {@link Hasher#putBytes(byte[])},
094 * and finally ask for the {@code HashCode} when finished using {@link Hasher#hash}. (See
095 * an {@linkplain #newHasher example} of this.)
096 *
097 * <p>If all you want to hash is a single byte array, string or {@code long} value, there
098 * are convenient shortcut methods defined directly on {@link HashFunction} to make this
099 * easier.
100 *
101 * <p>Hasher accepts primitive data types, but can also accept any Object of type {@code
102 * T} provided that you implement a {@link Funnel Funnel<T>} to specify how to "feed" data
103 * from that object into the function. (See {@linkplain Hasher#putObject an example} of
104 * this.)
105 *
106 * <p><b>Compatibility note:</b> Throughout this API, multibyte values are always
107 * interpreted in <i>little-endian</i> order. That is, hashing the byte array {@code
108 * {0x01, 0x02, 0x03, 0x04}} is equivalent to hashing the {@code int} value {@code
109 * 0x04030201}. If this isn't what you need, methods such as {@link Integer#reverseBytes}
110 * and {@link Ints#toByteArray} will help.
111 *
112 * <h3>Relationship to {@link Object#hashCode}</h3>
113 *
114 * <p>Java's baked-in concept of hash codes is constrained to 32 bits, and provides no
115 * separation between hash algorithms and the data they act on, so alternate hash
116 * algorithms can't be easily substituted. Also, implementations of {@code hashCode} tend
117 * to be poor-quality, in part because they end up depending on <i>other</i> existing
118 * poor-quality {@code hashCode} implementations, including those in many JDK classes.
119 *
120 * <p>{@code Object.hashCode} implementations tend to be very fast, but have weak
121 * collision prevention and <i>no</i> expectation of bit dispersion. This leaves them
122 * perfectly suitable for use in hash tables, because extra collisions cause only a slight
123 * performance hit, while poor bit dispersion is easily corrected using a secondary hash
124 * function (which all reasonable hash table implementations in Java use). For the many
125 * uses of hash functions beyond data structures, however, {@code Object.hashCode} almost
126 * always falls short -- hence this library.
127 *
128 * @author Kevin Bourrillion
129 * @since 11.0
130 */
131 @Beta
132 public interface HashFunction {
133 /**
134 * Begins a new hash code computation by returning an initialized, stateful {@code
135 * Hasher} instance that is ready to receive data. Example: <pre> {@code
136 *
137 * HashFunction hf = Hashing.md5();
138 * HashCode hc = hf.newHasher()
139 * .putLong(id)
140 * .putString(name)
141 * .hash();}</pre>
142 */
143 Hasher newHasher();
144
145 /**
146 * Begins a new hash code computation as {@link #newHasher()}, but provides a hint of the
147 * expected size of the input (in bytes). This is only important for non-streaming hash
148 * functions (hash functions that need to buffer their whole input before processing any
149 * of it).
150 */
151 Hasher newHasher(int expectedInputSize);
152
153 /**
154 * Shortcut for {@code newHasher().putInt(input).hash()}; returns the hash code for the given
155 * {@code int} value, interpreted in little-endian byte order. The implementation <i>might</i>
156 * perform better than its longhand equivalent, but should not perform worse.
157 *
158 * @since 12.0
159 */
160 HashCode hashInt(int input);
161
162 /**
163 * Shortcut for {@code newHasher().putLong(input).hash()}; returns the hash code for the
164 * given {@code long} value, interpreted in little-endian byte order. The implementation
165 * <i>might</i> perform better than its longhand equivalent, but should not perform worse.
166 */
167 HashCode hashLong(long input);
168
169 /**
170 * Shortcut for {@code newHasher().putBytes(input).hash()}. The implementation
171 * <i>might</i> perform better than its longhand equivalent, but should not perform
172 * worse.
173 */
174 HashCode hashBytes(byte[] input);
175
176 /**
177 * Shortcut for {@code newHasher().putBytes(input, off, len).hash()}. The implementation
178 * <i>might</i> perform better than its longhand equivalent, but should not perform
179 * worse.
180 *
181 * @throws IndexOutOfBoundsException if {@code off < 0} or {@code off + len > bytes.length}
182 * or {@code len < 0}
183 */
184 HashCode hashBytes(byte[] input, int off, int len);
185
186 /**
187 * Shortcut for {@code newHasher().putString(input).hash()}. The implementation <i>might</i>
188 * perform better than its longhand equivalent, but should not perform worse. Note that no
189 * character encoding is performed; the low byte and high byte of each character are hashed
190 * directly (in that order). This is equivalent to using
191 * {@code hashString(input, Charsets.UTF_16LE)}.
192 */
193 HashCode hashString(CharSequence input);
194
195 /**
196 * Shortcut for {@code newHasher().putString(input, charset).hash()}. Characters are encoded
197 * using the given {@link Charset}. The implementation <i>might</i> perform better than its
198 * longhand equivalent, but should not perform worse.
199 */
200 HashCode hashString(CharSequence input, Charset charset);
201
202 /**
203 * Returns the number of bits (a multiple of 32) that each hash code produced by this
204 * hash function has.
205 */
206 int bits();
207 }