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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 static com.google.common.base.Preconditions.checkArgument;
 
 
Static methods to obtain HashFunction instances, and other static hashing-related utilities.

Author(s):
Kevin Bourrillion
Dimitris Andreou
Kurt Alfred Kluever
Since:
11.0
 
 public final class Hashing {
   private Hashing() {}

  
Used to randomize goodFastHash(int) instances, so that programs which persist anything dependent on hashcodes of those, will fail sooner than later.
 
   private static final int GOOD_FAST_HASH_SEED = (int) System.currentTimeMillis();
 
   // Used by goodFastHash when minimumBits == 32.
 
   // Used by goodFastHash when 32 < minimumBits <= 128.
Returns a general-purpose, non-cryptographic-strength, streaming hash function that produces hash codes of length at least minimumBits. Users without specific compatibility requirements and who do not persist the hash codes are encouraged to choose this hash function.

Repeated calls to goodFastHash(int) with the same minimumBits value will return HashFunction instances with identical behavior (but not necessarily the same instance) for the duration of the current virtual machine.

Warning: the implementation is unspecified and is subject to change.

Throws:
java.lang.IllegalArgumentException if minimumBits is not positive
 
   public static HashFunction goodFastHash(int minimumBits) {
     int bits = checkPositiveAndMakeMultipleOf32(minimumBits);
 
     if (bits == 32) {
       return ;
     }
     if (bits <= 128) {
       return ;
     }
 
     // Otherwise, join together some 128-bit murmur3s
     int hashFunctionsNeeded = (bits + 127) / 128;
     HashFunction[] hashFunctions = new HashFunction[hashFunctionsNeeded];
     hashFunctions[0] = ;
     int seed = ;
     for (int i = 1; i < hashFunctionsNeededi++) {
       seed += 1500450271; // a prime; shouldn't matter
       hashFunctions[i] = murmur3_128(seed);
     }
     return new ConcatenatedHashFunction(hashFunctions);
   }

  
Returns a hash function implementing the 32-bit murmur3 algorithm (little-endian variant), using the given seed value.
 
   public static HashFunction murmur3_32(int seed) {
     return new Murmur3_32HashFunction(seed);
   }

  
Returns a hash function implementing the 32-bit murmur3 algorithm (little-endian variant), using a seed value of zero.
  public static HashFunction murmur3_32() {
    return ;
  }
  private static final Murmur3_32HashFunction MURMUR3_32 = new Murmur3_32HashFunction(0);

  
Returns a hash function implementing the 128-bit murmur3 algorithm, x64 variant (little-endian variant), using the given seed value.
  public static HashFunction murmur3_128(int seed) {
    return new Murmur3_128HashFunction(seed);
  }

  
Returns a hash function implementing the 128-bit murmur3 algorithm, x64 variant (little-endian variant), using a seed value of zero.
  public static HashFunction murmur3_128() {
    return ;
  }
  private static final Murmur3_128HashFunction MURMUR3_128 = new Murmur3_128HashFunction(0);

  
Returns a hash function implementing the MD5 hash algorithm (128 hash bits) by delegating to the MD5 java.security.MessageDigest.
  public static HashFunction md5() {
    return ;
  }
  private static final HashFunction MD5 = new MessageDigestHashFunction("MD5");

  
Returns a hash function implementing the SHA-1 algorithm (160 hash bits) by delegating to the SHA-1 java.security.MessageDigest.
  public static HashFunction sha1() {
    return ;
  }
  private static final HashFunction SHA_1 = new MessageDigestHashFunction("SHA-1");

  
Returns a hash function implementing the SHA-256 algorithm (256 hash bits) by delegating to the SHA-256 java.security.MessageDigest.
  public static HashFunction sha256() {
    return ;
  }
  private static final HashFunction SHA_256 = new MessageDigestHashFunction("SHA-256");

  
Returns a hash function implementing the SHA-512 algorithm (512 hash bits) by delegating to the SHA-512 java.security.MessageDigest.
  public static HashFunction sha512() {
    return ;
  }
  private static final HashFunction SHA_512 = new MessageDigestHashFunction("SHA-512");
  // Lazy initiliazation holder class idiom.

  
If hashCode has enough bits, returns hashCode.asLong(), otherwise returns a long value with hashCode.asInt() as the least-significant four bytes and 0x00 as each of the most-significant four bytes.
  public static long padToLong(HashCode hashCode) {
    return (hashCode.bits() < 64) ? UnsignedInts.toLong(hashCode.asInt()) : hashCode.asLong();
  }

  
Assigns to hashCode a "bucket" in the range [0, buckets), in a uniform manner that minimizes the need for remapping as buckets grows. That is, consistentHash(h, n) equals:
  • n - 1, with approximate probability 1/n
  • consistentHash(h, n - 1), otherwise (probability 1 - 1/n)

See the wikipedia article on consistent hashing for more information.

If you might want to have weights for the buckets in the future, take a look at weightedConsistentHash.

  public static int consistentHash(HashCode hashCodeint buckets) {
    return consistentHash(padToLong(hashCode), buckets);
  }

  
Assigns to input a "bucket" in the range [0, buckets), in a uniform manner that minimizes the need for remapping as buckets grows. That is, consistentHash(h, n) equals:
  • n - 1, with approximate probability 1/n
  • consistentHash(h, n - 1), otherwise (probability 1 - 1/n)

See the wikipedia article on consistent hashing for more information.

If you might want to have weights for the buckets in the future, take a look at weightedConsistentHash.

  public static int consistentHash(long inputint buckets) {
    checkArgument(buckets > 0, "buckets must be positive: %s"buckets);
    int candidate = 0;
    int next;
    // Jump from bucket to bucket until we go out of range
    while (true) {
      next = (int) ((candidate + 1) / generator.nextDouble());
      if (next >= 0 && next < buckets) {
        candidate = next;
      } else {
        return candidate;
      }
    }
  }

  
Returns a hash code, having the same bit length as each of the input hash codes, that combines the information of these hash codes in an ordered fashion. That is, whenever two equal hash codes are produced by two calls to this method, it is as likely as possible that each was computed from the same input hash codes in the same order.

Throws:
java.lang.IllegalArgumentException if hashCodes is empty, or the hash codes do not all have the same bit length
  public static HashCode combineOrdered(Iterable<HashCodehashCodes) {
    Iterator<HashCodeiterator = hashCodes.iterator();
    checkArgument(iterator.hasNext(), "Must be at least 1 hash code to combine.");
    int bits = iterator.next().bits();
    byte[] resultBytes = new byte[bits / 8];
    for (HashCode hashCode : hashCodes) {
      byte[] nextBytes = hashCode.asBytes();
      checkArgument(nextBytes.length == resultBytes.length,
          "All hashcodes must have the same bit length.");
      for (int i = 0; i < nextBytes.lengthi++) {
        resultBytes[i] = (byte) (resultBytes[i] * 37 ^ nextBytes[i]);
      }
    }
    return HashCodes.fromBytesNoCopy(resultBytes);
  }

  
Returns a hash code, having the same bit length as each of the input hash codes, that combines the information of these hash codes in an unordered fashion. That is, whenever two equal hash codes are produced by two calls to this method, it is as likely as possible that each was computed from the same input hash codes in some order.

Throws:
java.lang.IllegalArgumentException if hashCodes is empty, or the hash codes do not all have the same bit length
  public static HashCode combineUnordered(Iterable<HashCodehashCodes) {
    Iterator<HashCodeiterator = hashCodes.iterator();
    checkArgument(iterator.hasNext(), "Must be at least 1 hash code to combine.");
    byte[] resultBytes = new byte[iterator.next().bits() / 8];
    for (HashCode hashCode : hashCodes) {
      byte[] nextBytes = hashCode.asBytes();
      checkArgument(nextBytes.length == resultBytes.length,
          "All hashcodes must have the same bit length.");
      for (int i = 0; i < nextBytes.lengthi++) {
        resultBytes[i] += nextBytes[i];
      }
    }
    return HashCodes.fromBytesNoCopy(resultBytes);
  }

  
Checks that the passed argument is positive, and ceils it to a multiple of 32.
  static int checkPositiveAndMakeMultipleOf32(int bits) {
    checkArgument(bits > 0, "Number of bits must be positive");
    return (bits + 31) & ~31;
  }
  // TODO(kevinb): Maybe expose this class via a static Hashing method?
  static final class ConcatenatedHashFunction extends AbstractCompositeHashFunction {
    private final int bits;
    ConcatenatedHashFunction(HashFunction... functions) {
      super(functions);
      int bitSum = 0;
      for (HashFunction function : functions) {
        bitSum += function.bits();
      }
      this. = bitSum;
    }
    @Override
    HashCode makeHash(Hasher[] hashers) {
      // TODO(user): Get rid of the ByteBuffer here?
      byte[] bytes = new byte[ / 8];
      ByteBuffer buffer = ByteBuffer.wrap(bytes);
      for (Hasher hasher : hashers) {
        buffer.put(hasher.hash().asBytes());
      }
      return HashCodes.fromBytesNoCopy(bytes);
    }
    @Override
    public int bits() {
      return ;
    }
  }
  private static final class LinearCongruentialGenerator {
    private long state;
    public LinearCongruentialGenerator(long seed) {
      this. = seed;
    }
    public double nextDouble() {
       = 2862933555777941757L *  + 1;
      return ((double) ((int) ( >>> 33) + 1)) / (0x1.0p31);
    }
  }
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