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    1   /* BloomFilter32bit
    2   *
    3   * $Id: BloomFilter32bitSplit.java 5197 2007-06-06 01:31:46Z gojomo $
    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 BloomFilter32bitSplit implements Serializable, BloomFilter {
    72
    73      private static final long serialVersionUID = -164106965277863971L;
    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      final private int[][] bits;
    84      /*** The random integers used to generate the hash functions. */
    85      final private int[][] weight;
    86
    87      /*** The number of elements currently in the filter. It may be
    88       * smaller than the actual number of additions of distinct character
    89       * sequences because of false positives.
    90       */
    91      private int size;
    92
    93      /*** The natural logarithm of 2, used in the computation of the number of bits. */
    94      private final static double NATURAL_LOG_OF_2 = Math.log( 2 );
    95
    96      /*** number of ints in 1MB. */
    97      private final static int ONE_MB_INTS = 1 << 18; //
    98
    99      private final static boolean DEBUG = false;
    100
    101     /*** Creates a new Bloom filter with given number of hash functions and expected number of elements.
    102      *
    103      * @param n the expected number of elements.
    104      * @param d the number of hash functions; if the filter add not more than <code>n</code> elements,
    105      * false positives will happen with probability 2<sup>-<var>d</var></sup>.
    106      */
    107     public BloomFilter32bitSplit( final int n, final int d ) {
    108         this.d = d;
    109         int len =
    110         	(int)Math.ceil( ( (long)n * (long)d / NATURAL_LOG_OF_2 ) / 32 );
    111         // round up to ensure divisible into 1MiB chunks
    112         len = ((len / ONE_MB_INTS)+1)*ONE_MB_INTS;
    113         this.m = len*32L;
    114         if ( m >= 1L<<54 ) {
    115         	throw new IllegalArgumentException( "This filter would require " + m + " bits" );
    116         }
    117 //        bits = new int[ len ];
    118         bits = new int[ len/ONE_MB_INTS ][ONE_MB_INTS];
    119
    120         if ( DEBUG ) System.err.println( "Number of bits: " + m );
    121
    122         // seeded for reproduceable behavior in repeated runs; BUT:
    123         // SecureRandom's default implementation (as of 1.5)
    124         // seems to mix in its own seeding.
    125         final SecureRandom random = new SecureRandom(new byte[] {19,96});
    126         weight = new int[ d ][];
    127         for( int i = 0; i < d; i++ ) {
    128             weight[ i ] = new int[ NUMBER_OF_WEIGHTS ];
    129             for( int j = 0; j < NUMBER_OF_WEIGHTS; j++ )
    130                  weight[ i ][ j ] = random.nextInt();
    131         }
    132     }
    133
    134     /*** The number of character sequences in the filter.
    135      *
    136      * @return the number of character sequences in the filter (but see {@link #contains(CharSequence)}).
    137      */
    138
    139     public int size() {
    140         return size;
    141     }
    142
    143     /*** Hashes the given sequence with the given hash function.
    144      *
    145      * @param s a character sequence.
    146      * @param l the length of <code>s</code>.
    147      * @param k a hash function index (smaller than {@link #d}).
    148      * @return the position in the filter corresponding to <code>s</code> for the hash function <code>k</code>.
    149      */
    150 	private long hash( final CharSequence s, final int l, final int k ) {
    151 		final int[] w = weight[ k ];
    152 		int h = 0, i = l;
    153 		while( i-- != 0 ) h ^= s.charAt( i ) * w[ i % NUMBER_OF_WEIGHTS ];
    154 		return ((long)h-Integer.MIN_VALUE) % m;
    155 	}
    156
    157     /*** Checks whether the given character sequence is in this filter.
    158      *
    159      * <P>Note that this method may return true on a character sequence that is has
    160      * not been added to the filter. This will happen with probability 2<sub>-<var>d</var></sub>,
    161      * where <var>d</var> is the number of hash functions specified at creation time, if
    162      * the number of the elements in the filter is less than <var>n</var>, the number
    163      * of expected elements specified at creation time.
    164      *
    165      * @param s a character sequence.
    166      * @return true if the sequence is in the filter (or if a sequence with the
    167      * same hash sequence is in the filter).
    168      */
    169
    170     public boolean contains( final CharSequence s ) {
    171         int i = d, l = s.length();
    172         while( i-- != 0 ) if ( ! getBit( hash( s, l, i ) ) ) return false;
    173         return true;
    174     }
    175
    176     /*** Adds a character sequence to the filter.
    177      *
    178      * @param s a character sequence.
    179      * @return true if the character sequence was not in the filter (but see {@link #contains(CharSequence)}).
    180      */
    181
    182     public boolean add( final CharSequence s ) {
    183         boolean result = false;
    184         int i = d, l = s.length();
    185         long h;
    186         while( i-- != 0 ) {
    187             h = hash( s, l, i );
    188             if ( ! setGetBit( h ) ) result = true;
    189         }
    190         if ( result ) size++;
    191         return result;
    192     }
    193
    194     protected final static long ADDRESS_BITS_PER_UNIT = 5; // 32=2^5
    195     protected final static long BIT_INDEX_MASK = 31; // = BITS_PER_UNIT - 1;
    196
    197     /***
    198      * Returns from the local bitvector the value of the bit with
    199      * the specified index. The value is <tt>true</tt> if the bit
    200      * with the index <tt>bitIndex</tt> is currently set; otherwise,
    201      * returns <tt>false</tt>.
    202      *
    203      * (adapted from cern.colt.bitvector.QuickBitVector)
    204      *
    205      * @param     bitIndex   the bit index.
    206      * @return    the value of the bit with the specified index.
    207      */
    208     protected boolean getBit(long bitIndex) {
    209         long intIndex = (bitIndex >>> ADDRESS_BITS_PER_UNIT);
    210         return ((bits[(int)(intIndex / ONE_MB_INTS)][(int)(intIndex % ONE_MB_INTS)]
    211             & (1 << (bitIndex & BIT_INDEX_MASK))) != 0);
    212     }
    213
    214     /***
    215      * Changes the bit with index <tt>bitIndex</tt> in local bitvector.
    216      *
    217      * (adapted from cern.colt.bitvector.QuickBitVector)
    218      *
    219      * @param     bitIndex   the index of the bit to be set.
    220      */
    221     protected void setBit(long bitIndex) {
    222         long intIndex = (bitIndex >>> ADDRESS_BITS_PER_UNIT);
    223         bits[(int)(intIndex / ONE_MB_INTS)][(int)(intIndex % ONE_MB_INTS)]
    224             |= 1 << (bitIndex & BIT_INDEX_MASK);
    225     }
    226
    227     /***
    228      * Sets the bit with index <tt>bitIndex</tt> in local bitvector --
    229      * returning the old value.
    230      *
    231      * (adapted from cern.colt.bitvector.QuickBitVector)
    232      *
    233      * @param     bitIndex   the index of the bit to be set.
    234      */
    235     protected boolean setGetBit(long bitIndex) {
    236         long intIndex = (int) (bitIndex >>> ADDRESS_BITS_PER_UNIT);
    237         int a = (int)(intIndex / ONE_MB_INTS);
    238         int b = (int)(intIndex % ONE_MB_INTS);
    239         int mask = 1 << (bitIndex & BIT_INDEX_MASK);
    240         boolean ret = ((bits[a][b] & (mask)) != 0);
    241         bits[a][b] |= mask;
    242         return ret;
    243     }
    244
    245 	/* (non-Javadoc)
    246 	 * @see org.archive.util.BloomFilter#getSizeBytes()
    247 	 */
    248 	public long getSizeBytes() {
    249 		return bits.length*bits[0].length*4;
    250 	}
    251 }