    1   /* BloomFilter32bit
    2   *
    3   * $Id: BloomFilter32bit.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>NUMBER_OF_WEIGHTS is 2083, to better avoid collisions between
    42   * similar strings</li>
    43   * <li>Removed dependence on cern.colt MersenneTwister (replaced with
    44   * SecureRandom) and QuickBitVector (replaced with local methods).</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 BloomFilter32bit implements Serializable, BloomFilter {
    72
    73      private static final long serialVersionUID = -1567837798979475689L;
    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 vectorS. */
    82      final private int[] bits;
    83      /*** The random integers used to generate the hash functions. */
    84      final private int[][] 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 BloomFilter32bit( final int n, final int d ) {
    104         this.d = d;
    105         int len =
    106         	(int)Math.ceil( ( (long)n * (long)d / NATURAL_LOG_OF_2 ) / 32 );
    107         this.m = len*32L;
    108         if ( m >= 1L<<32 ) {
    109         	throw new IllegalArgumentException( "This filter would require " + m + " bits" );
    110         }
    111         bits = new int[ len ];
    112
    113         if ( DEBUG ) System.err.println( "Number of bits: " + m );
    114
    115         // seeded for reproduceable behavior in repeated runs; BUT:
    116         // SecureRandom's default implementation (as of 1.5)
    117         // seems to mix in its own seeding.
    118         final SecureRandom random = new SecureRandom(new byte[] {19,96});
    119         weight = new int[ d ][];
    120         for( int i = 0; i < d; i++ ) {
    121             weight[ i ] = new int[ NUMBER_OF_WEIGHTS ];
    122             for( int j = 0; j < NUMBER_OF_WEIGHTS; j++ )
    123                  weight[ i ][ j ] = random.nextInt();
    124         }
    125     }
    126
    127     /*** The number of character sequences in the filter.
    128      *
    129      * @return the number of character sequences in the filter (but see {@link #contains(CharSequence)}).
    130      */
    131
    132     public int size() {
    133         return size;
    134     }
    135
    136     /*** Hashes the given sequence with the given hash function.
    137      *
    138      * @param s a character sequence.
    139      * @param l the length of <code>s</code>.
    140      * @param k a hash function index (smaller than {@link #d}).
    141      * @return the position in the filter corresponding to <code>s</code> for the hash function <code>k</code>.
    142      */
    143 	private long hash( final CharSequence s, final int l, final int k ) {
    144 		final int[] w = weight[ k ];
    145 		int h = 0, i = l;
    146 		while( i-- != 0 ) h ^= s.charAt( i ) * w[ i % NUMBER_OF_WEIGHTS ];
    147 		return ((long)h-Integer.MIN_VALUE) % m;
    148 	}
    149
    150     /*** Checks whether the given character sequence is in this filter.
    151      *
    152      * <P>Note that this method may return true on a character sequence that is has
    153      * not been added to the filter. This will happen with probability 2<sub>-<var>d</var></sub>,
    154      * where <var>d</var> is the number of hash functions specified at creation time, if
    155      * the number of the elements in the filter is less than <var>n</var>, the number
    156      * of expected elements specified at creation time.
    157      *
    158      * @param s a character sequence.
    159      * @return true if the sequence is in the filter (or if a sequence with the
    160      * same hash sequence is in the filter).
    161      */
    162
    163     public boolean contains( final CharSequence s ) {
    164         int i = d, l = s.length();
    165         while( i-- != 0 ) if ( ! getBit( hash( s, l, i ) ) ) return false;
    166         return true;
    167     }
    168
    169     /*** Adds a character sequence to the filter.
    170      *
    171      * @param s a character sequence.
    172      * @return true if the character sequence was not in the filter (but see {@link #contains(CharSequence)}).
    173      */
    174
    175     public boolean add( final CharSequence s ) {
    176         boolean result = false;
    177         int i = d, l = s.length();
    178         long h;
    179         while( i-- != 0 ) {
    180             h = hash( s, l, i );
    181             if ( ! getBit( h ) ) result = true;
    182             setBit( h );
    183         }
    184         if ( result ) size++;
    185         return result;
    186     }
    187
    188     protected final static long ADDRESS_BITS_PER_UNIT = 5; // 32=2^5
    189     protected final static long BIT_INDEX_MASK = 31; // = BITS_PER_UNIT - 1;
    190
    191     /***
    192      * Returns from the local bitvector the value of the bit with
    193      * the specified index. The value is <tt>true</tt> if the bit
    194      * with the index <tt>bitIndex</tt> is currently set; otherwise,
    195      * returns <tt>false</tt>.
    196      *
    197      * (adapted from cern.colt.bitvector.QuickBitVector)
    198      *
    199      * @param     bitIndex   the bit index.
    200      * @return    the value of the bit with the specified index.
    201      */
    202     protected boolean getBit(long bitIndex) {
    203         return ((bits[(int)(bitIndex >> ADDRESS_BITS_PER_UNIT)] & (1 << (bitIndex & BIT_INDEX_MASK))) != 0);
    204     }
    205
    206     /***
    207      * Changes the bit with index <tt>bitIndex</tt> in local bitvector.
    208      *
    209      * (adapted from cern.colt.bitvector.QuickBitVector)
    210      *
    211      * @param     bitIndex   the index of the bit to be set.
    212      */
    213     protected void setBit(long bitIndex) {
    214             bits[(int)(bitIndex >> ADDRESS_BITS_PER_UNIT)] |= 1 << (bitIndex & BIT_INDEX_MASK);
    215     }
    216
    217 	/* (non-Javadoc)
    218 	 * @see org.archive.util.BloomFilter#getSizeBytes()
    219 	 */
    220 	public long getSizeBytes() {
    221 		return bits.length*4;
    222 	}
    223 }