/*
    * 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();
 }