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 static com.google.common.base.Preconditions.checkArgument;
018
019import com.google.common.annotations.Beta;
020import com.google.common.annotations.VisibleForTesting;
021import com.google.common.primitives.UnsignedInts;
022
023import java.nio.ByteBuffer;
024import java.security.MessageDigest;
025import java.util.Iterator;
026
027/**
028 * Static methods to obtain {@link HashFunction} instances, and other static
029 * hashing-related utilities.
030 *
031 * @author Kevin Bourrillion
032 * @author Dimitris Andreou
033 * @author Kurt Alfred Kluever
034 * @since 11.0
035 */
036@Beta
037public final class Hashing {
038  private Hashing() {}
039
040  /**
041   * Used to randomize {@link #goodFastHash} instances, so that programs which persist anything
042   * dependent on hashcodes of those, will fail sooner than later.
043   */
044  private static final int GOOD_FAST_HASH_SEED = (int) System.currentTimeMillis();
045
046  // Used by goodFastHash when minimumBits == 32.
047  private static final HashFunction GOOD_FAST_HASH_FUNCTION_32 = murmur3_32(GOOD_FAST_HASH_SEED);
048
049  // Used by goodFastHash when 32 < minimumBits <= 128.
050  private static final HashFunction GOOD_FAST_HASH_FUNCTION_128 = murmur3_128(GOOD_FAST_HASH_SEED);
051
052  /**
053   * Returns a general-purpose, <b>non-cryptographic-strength</b>, streaming hash function that
054   * produces hash codes of length at least {@code minimumBits}. Users without specific
055   * compatibility requirements and who do not persist the hash codes are encouraged to
056   * choose this hash function.
057   *
058   * <p>Repeated calls to {@link #goodFastHash} with the same {@code minimumBits} value will
059   * return {@link HashFunction} instances with identical behavior (but not necessarily the
060   * same instance) for the duration of the current virtual machine.
061   *
062   * <p><b>Warning: the implementation is unspecified and is subject to change.</b>
063   *
064   * @throws IllegalArgumentException if {@code minimumBits} is not positive
065   */
066  public static HashFunction goodFastHash(int minimumBits) {
067    int bits = checkPositiveAndMakeMultipleOf32(minimumBits);
068
069    if (bits == 32) {
070      return GOOD_FAST_HASH_FUNCTION_32;
071    }
072    if (bits <= 128) {
073      return GOOD_FAST_HASH_FUNCTION_128;
074    }
075
076    // Otherwise, join together some 128-bit murmur3s
077    int hashFunctionsNeeded = (bits + 127) / 128;
078    HashFunction[] hashFunctions = new HashFunction[hashFunctionsNeeded];
079    hashFunctions[0] = GOOD_FAST_HASH_FUNCTION_128;
080    int seed = GOOD_FAST_HASH_SEED;
081    for (int i = 1; i < hashFunctionsNeeded; i++) {
082      seed += 1500450271; // a prime; shouldn't matter
083      hashFunctions[i] = murmur3_128(seed);
084    }
085    return new ConcatenatedHashFunction(hashFunctions);
086  }
087
088  /**
089   * Returns a hash function implementing the
090   * <a href="http://smhasher.googlecode.com/svn/trunk/MurmurHash3.cpp">32-bit murmur3
091   * algorithm</a> (little-endian variant), using the given seed value.
092   */
093  public static HashFunction murmur3_32(int seed) {
094    return new Murmur3_32HashFunction(seed);
095  }
096
097  /**
098   * Returns a hash function implementing the
099   * <a href="http://smhasher.googlecode.com/svn/trunk/MurmurHash3.cpp">32-bit murmur3
100   * algorithm</a> (little-endian variant), using a seed value of zero.
101   */
102  public static HashFunction murmur3_32() {
103    return MURMUR3_32;
104  }
105
106  private static final Murmur3_32HashFunction MURMUR3_32 = new Murmur3_32HashFunction(0);
107
108  /**
109   * Returns a hash function implementing the
110   * <a href="http://smhasher.googlecode.com/svn/trunk/MurmurHash3.cpp">
111   * 128-bit murmur3 algorithm, x64 variant</a> (little-endian variant), using the given seed
112   * value.
113   */
114  public static HashFunction murmur3_128(int seed) {
115    return new Murmur3_128HashFunction(seed);
116  }
117
118  /**
119   * Returns a hash function implementing the
120   * <a href="http://smhasher.googlecode.com/svn/trunk/MurmurHash3.cpp">
121   * 128-bit murmur3 algorithm, x64 variant</a>  (little-endian variant), using a seed value
122   * of zero.
123   */
124  public static HashFunction murmur3_128() {
125    return MURMUR3_128;
126  }
127
128  private static final Murmur3_128HashFunction MURMUR3_128 = new Murmur3_128HashFunction(0);
129
130  /**
131   * Returns a hash function implementing the MD5 hash algorithm (128 hash bits) by delegating to
132   * the MD5 {@link MessageDigest}.
133   */
134  public static HashFunction md5() {
135    return MD5;
136  }
137
138  private static final HashFunction MD5 = new MessageDigestHashFunction("MD5");
139
140  /**
141   * Returns a hash function implementing the SHA-1 algorithm (160 hash bits) by delegating to the
142   * SHA-1 {@link MessageDigest}.
143   */
144  public static HashFunction sha1() {
145    return SHA_1;
146  }
147
148  private static final HashFunction SHA_1 = new MessageDigestHashFunction("SHA-1");
149
150  /**
151   * Returns a hash function implementing the SHA-256 algorithm (256 hash bits) by delegating to
152   * the SHA-256 {@link MessageDigest}.
153   */
154  public static HashFunction sha256() {
155    return SHA_256;
156  }
157
158  private static final HashFunction SHA_256 = new MessageDigestHashFunction("SHA-256");
159
160  /**
161   * Returns a hash function implementing the SHA-512 algorithm (512 hash bits) by delegating to the
162   * SHA-512 {@link MessageDigest}.
163   */
164  public static HashFunction sha512() {
165    return SHA_512;
166  }
167
168  private static final HashFunction SHA_512 = new MessageDigestHashFunction("SHA-512");
169
170  // Lazy initiliazation holder class idiom.
171
172  /**
173   * If {@code hashCode} has enough bits, returns {@code hashCode.asLong()}, otherwise
174   * returns a {@code long} value with {@code hashCode.asInt()} as the least-significant
175   * four bytes and {@code 0x00} as each of the most-significant four bytes.
176   */
177  public static long padToLong(HashCode hashCode) {
178    return (hashCode.bits() < 64) ? UnsignedInts.toLong(hashCode.asInt()) : hashCode.asLong();
179  }
180
181  /**
182   * Assigns to {@code hashCode} a "bucket" in the range {@code [0, buckets)}, in a uniform
183   * manner that minimizes the need for remapping as {@code buckets} grows. That is,
184   * {@code consistentHash(h, n)} equals:
185   *
186   * <ul>
187   * <li>{@code n - 1}, with approximate probability {@code 1/n}
188   * <li>{@code consistentHash(h, n - 1)}, otherwise (probability {@code 1 - 1/n})
189   * </ul>
190   *
191   * <p>See the <a href="http://en.wikipedia.org/wiki/Consistent_hashing">wikipedia
192   * article on consistent hashing</a> for more information.
193   * <p>
194   * If you might want to have weights for the buckets in the future, take a look at
195   * {@code weightedConsistentHash}.
196   */
197  public static int consistentHash(HashCode hashCode, int buckets) {
198    return consistentHash(padToLong(hashCode), buckets);
199  }
200
201  /**
202   * Assigns to {@code input} a "bucket" in the range {@code [0, buckets)}, in a uniform
203   * manner that minimizes the need for remapping as {@code buckets} grows. That is,
204   * {@code consistentHash(h, n)} equals:
205   *
206   * <ul>
207   * <li>{@code n - 1}, with approximate probability {@code 1/n}
208   * <li>{@code consistentHash(h, n - 1)}, otherwise (probability {@code 1 - 1/n})
209   * </ul>
210   *
211   * <p>See the <a href="http://en.wikipedia.org/wiki/Consistent_hashing">wikipedia
212   * article on consistent hashing</a> for more information.
213   * <p>
214   * If you might want to have weights for the buckets in the future, take a look at
215   * {@code weightedConsistentHash}.
216   */
217  public static int consistentHash(long input, int buckets) {
218    checkArgument(buckets > 0, "buckets must be positive: %s", buckets);
219    LinearCongruentialGenerator generator = new LinearCongruentialGenerator(input);
220    int candidate = 0;
221    int next;
222
223    // Jump from bucket to bucket until we go out of range
224    while (true) {
225      next = (int) ((candidate + 1) / generator.nextDouble());
226      if (next >= 0 && next < buckets) {
227        candidate = next;
228      } else {
229        return candidate;
230      }
231    }
232  }
233
234  /**
235   * Returns a hash code, having the same bit length as each of the input hash codes,
236   * that combines the information of these hash codes in an ordered fashion. That
237   * is, whenever two equal hash codes are produced by two calls to this method, it
238   * is <i>as likely as possible</i> that each was computed from the <i>same</i>
239   * input hash codes in the <i>same</i> order.
240   *
241   * @throws IllegalArgumentException if {@code hashCodes} is empty, or the hash codes
242   *     do not all have the same bit length
243   */
244  public static HashCode combineOrdered(Iterable<HashCode> hashCodes) {
245    Iterator<HashCode> iterator = hashCodes.iterator();
246    checkArgument(iterator.hasNext(), "Must be at least 1 hash code to combine.");
247    int bits = iterator.next().bits();
248    byte[] resultBytes = new byte[bits / 8];
249    for (HashCode hashCode : hashCodes) {
250      byte[] nextBytes = hashCode.asBytes();
251      checkArgument(nextBytes.length == resultBytes.length,
252          "All hashcodes must have the same bit length.");
253      for (int i = 0; i < nextBytes.length; i++) {
254        resultBytes[i] = (byte) (resultBytes[i] * 37 ^ nextBytes[i]);
255      }
256    }
257    return HashCodes.fromBytesNoCopy(resultBytes);
258  }
259
260  /**
261   * Returns a hash code, having the same bit length as each of the input hash codes,
262   * that combines the information of these hash codes in an unordered fashion. That
263   * is, whenever two equal hash codes are produced by two calls to this method, it
264   * is <i>as likely as possible</i> that each was computed from the <i>same</i>
265   * input hash codes in <i>some</i> order.
266   *
267   * @throws IllegalArgumentException if {@code hashCodes} is empty, or the hash codes
268   *     do not all have the same bit length
269   */
270  public static HashCode combineUnordered(Iterable<HashCode> hashCodes) {
271    Iterator<HashCode> iterator = hashCodes.iterator();
272    checkArgument(iterator.hasNext(), "Must be at least 1 hash code to combine.");
273    byte[] resultBytes = new byte[iterator.next().bits() / 8];
274    for (HashCode hashCode : hashCodes) {
275      byte[] nextBytes = hashCode.asBytes();
276      checkArgument(nextBytes.length == resultBytes.length,
277          "All hashcodes must have the same bit length.");
278      for (int i = 0; i < nextBytes.length; i++) {
279        resultBytes[i] += nextBytes[i];
280      }
281    }
282    return HashCodes.fromBytesNoCopy(resultBytes);
283  }
284
285  /**
286   * Checks that the passed argument is positive, and ceils it to a multiple of 32.
287   */
288  static int checkPositiveAndMakeMultipleOf32(int bits) {
289    checkArgument(bits > 0, "Number of bits must be positive");
290    return (bits + 31) & ~31;
291  }
292
293  // TODO(kevinb): Maybe expose this class via a static Hashing method?
294  @VisibleForTesting
295  static final class ConcatenatedHashFunction extends AbstractCompositeHashFunction {
296    private final int bits;
297
298    ConcatenatedHashFunction(HashFunction... functions) {
299      super(functions);
300      int bitSum = 0;
301      for (HashFunction function : functions) {
302        bitSum += function.bits();
303      }
304      this.bits = bitSum;
305    }
306
307    @Override
308    HashCode makeHash(Hasher[] hashers) {
309      // TODO(user): Get rid of the ByteBuffer here?
310      byte[] bytes = new byte[bits / 8];
311      ByteBuffer buffer = ByteBuffer.wrap(bytes);
312      for (Hasher hasher : hashers) {
313        buffer.put(hasher.hash().asBytes());
314      }
315      return HashCodes.fromBytesNoCopy(bytes);
316    }
317
318    @Override
319    public int bits() {
320      return bits;
321    }
322  }
323
324  private static final class LinearCongruentialGenerator {
325    private long state;
326
327    public LinearCongruentialGenerator(long seed) {
328      this.state = seed;
329    }
330
331    public double nextDouble() {
332      state = 2862933555777941757L * state + 1;
333      return ((double) ((int) (state >>> 33) + 1)) / (0x1.0p31);
334    }
335  }
336}