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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> ≈ 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;
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
116
117
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;
189 protected final static long BIT_INDEX_MASK = 31;
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
218
219
220 public long getSizeBytes() {
221 return bits.length*4;
222 }
223 }