BloomFilter64bit


1   /* BloomFilter
2   *
3   * $Id: BloomFilter64bit.java,v 1.2.16.1 2007/01/13 01:31:39 stack-sf Exp $
4   *
5   * Created on Jun 21, 2005
6   *
7   * Copyright (C) 2005 Internet Archive; a slight adaptation of
8   * LGPL work (C) Sebastiano Vigna
9   *
10  * This file is part of the Heritrix web crawler (crawler.archive.org).
11  *
12  * Heritrix is free software; you can redistribute it and/or modify
13  * it under the terms of the GNU Lesser Public License as published by
14  * the Free Software Foundation; either version 2.1 of the License, or
15  * any later version.
16  *
17  * Heritrix is distributed in the hope that it will be useful,
18  * but WITHOUT ANY WARRANTY; without even the implied warranty of
19  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
20  * GNU Lesser Public License for more details.
21  *
22  * You should have received a copy of the GNU Lesser Public License
23  * along with Heritrix; if not, write to the Free Software
24  * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
25  */
26  
27  package org.archive.util;
28  
29  import java.io.Serializable  ;
30  import java.security.SecureRandom  ;
31  
32  /** A Bloom filter.
33   *
34   * SLIGHTLY ADAPTED VERSION OF MG4J it.unimi.dsi.mg4j.util.BloomFilter
35   * 
36   * <p>KEY CHANGES:
37   *
38   * <ul>
39   * <li>NUMBER_OF_WEIGHTS is 2083, to better avoid collisions between 
40   * similar strings</li>
41   * <li>Removed dependence on cern.colt MersenneTwister (replaced with
42   * SecureRandom) and QuickBitVector (replaced with local methods).</li>
43   * <li>Adapted to allow long bit indices so long as the index/64 (used 
44   * an array index in bit vector) fits within Integer.MAX_VALUE</li>
45   * </ul>
46   * 
47   * <hr>
48   * 
49   * <P>Instances of this class represent a set of character sequences (with false positives)
50   * using a Bloom filter. Because of the way Bloom filters work,
51   * you cannot remove elements.
52   *
53   * <P>Bloom filters have an expected error rate, depending on the number
54   * of hash functions used, on the filter size and on the number of elements in the filter. This implementation
55   * uses a variable optimal number of hash functions, depending on the expected
56   * number of elements. More precisely, a Bloom
57   * filter for <var>n</var> character sequences with <var>d</var> hash functions will use
58   * ln 2 <var>d</var><var>n</var> &#8776; 1.44 <var>d</var><var>n</var> bits;
59   * false positives will happen with probability 2<sup>-<var>d</var></sup>.
60   *
61   * <P>Hash functions are generated at creation time using universal hashing. Each hash function
62   * uses {@link #NUMBER_OF_WEIGHTS} random integers, which are cyclically multiplied by
63   * the character codes in a character sequence. The resulting integers are XOR-ed together.
64   *
65   * <P>This class exports access methods that are very similar to those of {@link java.util.Set},
66   * but it does not implement that interface, as too many non-optional methods
67   * would be unimplementable (e.g., iterators).
68   *
69   * @author Sebastiano Vigna
70   */
71  public class BloomFilter64bit implements Serializable  , BloomFilter {
72  
73      private static final long serialVersionUID = 2317000663009608403L;
74  
75      /** The number of weights used to create hash functions. */
76      final public static int NUMBER_OF_WEIGHTS = 2083; // CHANGED FROM 16
77      /** The number of bits in this filter. */
78      final public long m;
79      /** The number of hash functions used by this filter. */
80      final public int d;
81      /** The underlying bit vector. */
82      final private long[] bits;
83      /** The random integers used to generate the hash functions. */
84      final private long[][] weight;
85  
86      /** The number of elements currently in the filter. It may be
87       * smaller than the actual number of additions of distinct character
88       * sequences because of false positives.
89       */
90      private int size;
91  
92      /** The natural logarithm of 2, used in the computation of the number of bits. */
93      private final static double NATURAL_LOG_OF_2 = Math.log( 2 );
94  
95      private final static boolean DEBUG = false;
96  
97      /** Creates a new Bloom filter with given number of hash functions and expected number of elements.
98       *
99       * @param n the expected number of elements.
100      * @param d the number of hash functions; if the filter add not more than <code>n</code> elements,
101      * false positives will happen with probability 2<sup>-<var>d</var></sup>.
102      */
103     public BloomFilter64bit( final int n, final int d ) {
104         this.d = d;
105         int len = (int)Math.ceil( ( (long)n * (long)d / NATURAL_LOG_OF_2 ) / 64L );
106         if ( len/64 > Integer.MAX_VALUE ) throw new IllegalArgumentException  ( "This filter would require " + len * 64L + " bits" );
107         bits = new long[ len ];
108         m = bits.length * 64;
109 
110         if ( DEBUG ) System.err.println( "Number of bits: " + m );
111 
112         // seeded for reproduceable behavior in repeated runs; BUT: 
113         // SecureRandom's default implementation (as of 1.5) 
114         // seems to mix in its own seeding.
115         final SecureRandom   random = new SecureRandom  (new byte[] {19,96});
116         weight = new long[ d ][];
117         for( int i = 0; i < d; i++ ) {
118             weight[ i ] = new long[ NUMBER_OF_WEIGHTS ];
119             for( int j = 0; j < NUMBER_OF_WEIGHTS; j++ )
120                  weight[ i ][ j ] = random.nextLong();
121         }
122     }
123 
124     /** The number of character sequences in the filter.
125      *
126      * @return the number of character sequences in the filter (but see {@link #contains(CharSequence)}).
127      */
128 
129     public int size() {
130         return size;
131     }
132 
133     /** Hashes the given sequence with the given hash function.
134      *
135      * @param s a character sequence.
136      * @param l the length of <code>s</code>.
137      * @param k a hash function index (smaller than {@link #d}).
138      * @return the position in the filter corresponding to <code>s</code> for the hash function <code>k</code>.
139      */
140 
141     private long hash( final CharSequence   s, final int l, final int k ) {
142         final long[] w = weight[ k ];
143         long h = 0;
144         int i = l;
145         while( i-- != 0 ) h ^= s.charAt( i ) * w[ i % NUMBER_OF_WEIGHTS ];
146         return ( h & 0x7FFFFFFFFFFFFFFFL ) % m;
147     }
148 
149     /** Checks whether the given character sequence is in this filter.
150      *
151      * <P>Note that this method may return true on a character sequence that is has
152      * not been added to the filter. This will happen with probability 2<sub>-<var>d</var></sub>,
153      * where <var>d</var> is the number of hash functions specified at creation time, if
154      * the number of the elements in the filter is less than <var>n</var>, the number
155      * of expected elements specified at creation time.
156      *
157      * @param s a character sequence.
158      * @return true if the sequence is in the filter (or if a sequence with the
159      * same hash sequence is in the filter).
160      */
161 
162     public boolean contains( final CharSequence   s ) {
163         int i = d, l = s.length();
164         while( i-- != 0 ) if ( ! getBit( hash( s, l, i ) ) ) return false;
165         return true;
166     }
167 
168     /** Adds a character sequence to the filter.
169      *
170      * @param s a character sequence.
171      * @return true if the character sequence was not in the filter (but see {@link #contains(CharSequence)}).
172      */
173 
174     public boolean add( final CharSequence   s ) {
175         boolean result = false;
176         int i = d, l = s.length();
177         long h;
178         while( i-- != 0 ) {
179             h = hash( s, l, i );
180             if ( ! getBit( h ) ) result = true;
181             setBit( h );
182         }
183         if ( result ) size++;
184         return result;
185     }
186     
187     protected final static long ADDRESS_BITS_PER_UNIT = 6; // 64=2^6
188     protected final static long BIT_INDEX_MASK = 63; // = BITS_PER_UNIT - 1;
189 
190     /**
191      * Returns from the local bitvector the value of the bit with 
192      * the specified index. The value is <tt>true</tt> if the bit 
193      * with the index <tt>bitIndex</tt> is currently set; otherwise, 
194      * returns <tt>false</tt>.
195      *
196      * (adapted from cern.colt.bitvector.QuickBitVector)
197      * 
198      * @param     bitIndex   the bit index.
199      * @return    the value of the bit with the specified index.
200      */
201     protected boolean getBit(long bitIndex) {
202         return ((bits[(int)(bitIndex >> ADDRESS_BITS_PER_UNIT)] & (1L << (bitIndex & BIT_INDEX_MASK))) != 0);
203     }
204 
205     /**
206      * Changes the bit with index <tt>bitIndex</tt> in local bitvector.
207      *
208      * (adapted from cern.colt.bitvector.QuickBitVector)
209      * 
210      * @param     bitIndex   the index of the bit to be set.
211      */
212     protected void setBit( long bitIndex) {
213             bits[(int)(bitIndex >> ADDRESS_BITS_PER_UNIT)] |= 1L << (bitIndex & BIT_INDEX_MASK);
214     }
215     
216     /* (non-Javadoc)
217      * @see org.archive.util.BloomFilter#getSizeBytes()
218      */
219     public long getSizeBytes() {
220         return bits.length*8;
221     }
222 }
223
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