View Javadoc

    1   /* BloomFilter
    2   *
    3   * $Id: BloomFilter32bp2.java 4644 2006-09-20 22:40:21Z paul_jack $
    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>Adapted to use 32bit ops as much as possible... may be slightly
    40   * faster on 32bit hardware/OS</li>
    41   * <li>Changed to use bitfield that is a power-of-two in size, allowing
    42   * hash() to use bitshifting rather than modulus... may be slightly
    43   * faster</li>
    44   * <li>NUMBER_OF_WEIGHTS is 2083, to better avoid collisions between
    45   * similar strings</li>
    46   * <li>Removed dependence on cern.colt MersenneTwister (replaced with
    47   * SecureRandom) and QuickBitVector (replaced with local methods).</li>
    48   * </ul>
    49   *
    50   * <hr>
    51   *
    52   * <P>Instances of this class represent a set of character sequences (with false positives)
    53   * using a Bloom filter. Because of the way Bloom filters work,
    54   * you cannot remove elements.
    55   *
    56   * <P>Bloom filters have an expected error rate, depending on the number
    57   * of hash functions used, on the filter size and on the number of elements in the filter. This implementation
    58   * uses a variable optimal number of hash functions, depending on the expected
    59   * number of elements. More precisely, a Bloom
    60   * filter for <var>n</var> character sequences with <var>d</var> hash functions will use
    61   * ln 2 <var>d</var><var>n</var> &#8776; 1.44 <var>d</var><var>n</var> bits;
    62   * false positives will happen with probability 2<sup>-<var>d</var></sup>.
    63   *
    64   * <P>Hash functions are generated at creation time using universal hashing. Each hash function
    65   * uses {@link #NUMBER_OF_WEIGHTS} random integers, which are cyclically multiplied by
    66   * the character codes in a character sequence. The resulting integers are XOR-ed together.
    67   *
    68   * <P>This class exports access methods that are very similar to those of {@link java.util.Set},
    69   * but it does not implement that interface, as too many non-optional methods
    70   * would be unimplementable (e.g., iterators).
    71   *
    72   * @author Sebastiano Vigna
    73   */
    74  public class BloomFilter32bp2 implements Serializable, BloomFilter {
    75
    76      private static final long serialVersionUID = -2292902803681146635L;
    77
    78      /*** The number of weights used to create hash functions. */
    79      final public static int NUMBER_OF_WEIGHTS = 2083; // CHANGED FROM 16
    80      /*** The number of bits in this filter. */
    81      final public long m;
    82      /*** the power-of-two that m is */
    83      final public long power; // 1<<power == m
    84      /*** The number of hash functions used by this filter. */
    85      final public int d;
    86      /*** The underlying bit vectorS. */
    87      final private int[] bits;
    88      /*** The random integers used to generate the hash functions. */
    89      final private int[][] weight;
    90
    91      /*** The number of elements currently in the filter. It may be
    92       * smaller than the actual number of additions of distinct character
    93       * sequences because of false positives.
    94       */
    95      private int size;
    96
    97      /*** The natural logarithm of 2, used in the computation of the number of bits. */
    98      private final static double NATURAL_LOG_OF_2 = Math.log( 2 );
    99
    100     private final static boolean DEBUG = false;
    101
    102     /*** Creates a new Bloom filter with given number of hash functions and expected number of elements.
    103      *
    104      * @param n the expected number of elements.
    105      * @param d the number of hash functions; if the filter add not more than <code>n</code> elements,
    106      * false positives will happen with probability 2<sup>-<var>d</var></sup>.
    107      */
    108     public BloomFilter32bp2( final int n, final int d ) {
    109         this.d = d;
    110         long minBits = (long) ((long)n * (long)d / NATURAL_LOG_OF_2);
    111         long pow = 0;
    112         while((1L<<pow) < minBits) {
    113         	pow++;
    114         }
    115         this.power = pow;
    116         this.m = 1L<<pow;
    117         int len = (int) (m / 32);
    118         if ( m > 1L<<32 ) {
    119         	throw new IllegalArgumentException( "This filter would require " + m + " bits" );
    120         }
    121         System.out.println("power "+power+" bits "+m+" len "+len);
    122
    123         bits = new int[ len ];
    124
    125         if ( DEBUG ) System.err.println( "Number of bits: " + m );
    126
    127         // seeded for reproduceable behavior in repeated runs; BUT:
    128         // SecureRandom's default implementation (as of 1.5)
    129         // seems to mix in its own seeding.
    130         final SecureRandom random = new SecureRandom(new byte[] {19,96});
    131         weight = new int[ d ][];
    132         for( int i = 0; i < d; i++ ) {
    133             weight[ i ] = new int[ NUMBER_OF_WEIGHTS ];
    134             for( int j = 0; j < NUMBER_OF_WEIGHTS; j++ )
    135                  weight[ i ][ j ] = random.nextInt();
    136         }
    137     }
    138
    139     /*** The number of character sequences in the filter.
    140      *
    141      * @return the number of character sequences in the filter (but see {@link #contains(CharSequence)}).
    142      */
    143
    144     public int size() {
    145         return size;
    146     }
    147
    148     /*** Hashes the given sequence with the given hash function.
    149      *
    150      * @param s a character sequence.
    151      * @param l the length of <code>s</code>.
    152      * @param k a hash function index (smaller than {@link #d}).
    153      * @return the position in the filter corresponding to <code>s</code> for the hash function <code>k</code>.
    154      */
    155 	private int hash( final CharSequence s, final int l, final int k ) {
    156 		final int[] w = weight[ k ];
    157 		int h = 0, i = l;
    158 		while( i-- != 0 ) h ^= s.charAt( i ) * w[ i % NUMBER_OF_WEIGHTS ];
    159 		return h >>> (32-power);
    160 	}
    161
    162     /*** Checks whether the given character sequence is in this filter.
    163      *
    164      * <P>Note that this method may return true on a character sequence that is has
    165      * not been added to the filter. This will happen with probability 2<sub>-<var>d</var></sub>,
    166      * where <var>d</var> is the number of hash functions specified at creation time, if
    167      * the number of the elements in the filter is less than <var>n</var>, the number
    168      * of expected elements specified at creation time.
    169      *
    170      * @param s a character sequence.
    171      * @return true if the sequence is in the filter (or if a sequence with the
    172      * same hash sequence is in the filter).
    173      */
    174
    175     public boolean contains( final CharSequence s ) {
    176         int i = d, l = s.length();
    177         while( i-- != 0 ) if ( ! getBit( hash( s, l, i ) ) ) return false;
    178         return true;
    179     }
    180
    181     /*** Adds a character sequence to the filter.
    182      *
    183      * @param s a character sequence.
    184      * @return true if the character sequence was not in the filter (but see {@link #contains(CharSequence)}).
    185      */
    186
    187     public boolean add( final CharSequence s ) {
    188         boolean result = false;
    189         int i = d, l = s.length();
    190         int h;
    191         while( i-- != 0 ) {
    192             h = hash( s, l, i );
    193             if ( ! getBit( h ) ) result = true;
    194             setBit( h );
    195         }
    196         if ( result ) size++;
    197         return result;
    198     }
    199
    200     protected final static int ADDRESS_BITS_PER_UNIT = 5; // 32=2^5
    201     protected final static int BIT_INDEX_MASK = 31; // = BITS_PER_UNIT - 1;
    202
    203     /***
    204      * Returns from the local bitvector the value of the bit with
    205      * the specified index. The value is <tt>true</tt> if the bit
    206      * with the index <tt>bitIndex</tt> is currently set; otherwise,
    207      * returns <tt>false</tt>.
    208      *
    209      * (adapted from cern.colt.bitvector.QuickBitVector)
    210      *
    211      * @param     bitIndex   the bit index.
    212      * @return    the value of the bit with the specified index.
    213      */
    214     protected boolean getBit(int bitIndex) {
    215         return ((bits[(int)(bitIndex >>> ADDRESS_BITS_PER_UNIT)] & (1 << (bitIndex & BIT_INDEX_MASK))) != 0);
    216     }
    217
    218     /***
    219      * Changes the bit with index <tt>bitIndex</tt> in local bitvector.
    220      *
    221      * (adapted from cern.colt.bitvector.QuickBitVector)
    222      *
    223      * @param     bitIndex   the index of the bit to be set.
    224      */
    225     protected void setBit(int bitIndex) {
    226             bits[(int)(bitIndex >>> ADDRESS_BITS_PER_UNIT)] |= 1 << (bitIndex & BIT_INDEX_MASK);
    227     }
    228
    229 	/* (non-Javadoc)
    230 	 * @see org.archive.util.BloomFilter#getSizeBytes()
    231 	 */
    232 	public long getSizeBytes() {
    233 		return bits.length*4;
    234 	}
    235 }