1 /*
2 * Licensed to the Apache Software Foundation (ASF) under one or more
3 * contributor license agreements. See the NOTICE file distributed with
4 * this work for additional information regarding copyright ownership.
5 * The ASF licenses this file to You under the Apache License, Version 2.0
6 * (the "License"); you may not use this file except in compliance with
7 * the License. You may obtain a copy of the License at
8 *
9 * http://www.apache.org/licenses/LICENSE-2.0
10 *
11 * Unless required by applicable law or agreed to in writing, software
12 * distributed under the License is distributed on an "AS IS" BASIS,
13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14 * See the License for the specific language governing permissions and
15 * limitations under the License.
16 */
17 package org.apache.commons.collections4.bloomfilter;
18
19 import java.util.Objects;
20 import java.util.function.IntPredicate;
21
22 /**
23 * A Hasher that implements simple combinatorial hashing as described by
24 * <a href="https://www.eecs.harvard.edu/~michaelm/postscripts/tr-02-05.pdf">Krisch and Mitzenmacher</a>.
25 *
26 * <p>To be used for testing only.</p>
27 */
28 public final class IncrementingHasher implements Hasher {
29
30 /**
31 * The initial hash value.
32 */
33 private final long initial;
34
35 /**
36 * The value to increment the hash value by.
37 */
38 private final long increment;
39
40 /**
41 * Constructs the IncrementingHasher from 2 longs. The long values will be interpreted as unsigned values.
42 * <p>
43 * The initial hash value will be the modulus of the initial value.
44 * Subsequent values will be calculated by repeatedly adding the increment to the last value and taking the modulus.
45 * </p>
46 * @param initial The initial value for the hasher.
47 * @param increment The value to increment the hash by on each iteration.
48 */
49 public IncrementingHasher(final long initial, final long increment) {
50 this.initial = initial;
51 this.increment = increment;
52 }
53
54 @Override
55 public IndexProducer indices(final Shape shape) {
56 Objects.requireNonNull(shape, "shape");
57
58 return new IndexProducer() {
59 @Override
60 public int[] asIndexArray() {
61 final int[] result = new int[shape.getNumberOfHashFunctions()];
62 final int[] idx = new int[1];
63
64 // This method needs to return duplicate indices
65
66 forEachIndex(i -> {
67 result[idx[0]++] = i;
68 return true;
69 });
70 return result;
71 }
72
73 @Override
74 public boolean forEachIndex(final IntPredicate consumer) {
75 Objects.requireNonNull(consumer, "consumer");
76 final int bits = shape.getNumberOfBits();
77
78 // Essentially this is computing a wrapped modulus from a start point and an
79 // increment. So actually you only need two modulus operations before the loop.
80 // This avoids any modulus operation inside the while loop. It uses a long index
81 // to avoid overflow.
82
83 long index = BitMap.mod(initial, bits);
84 final int inc = BitMap.mod(increment, bits);
85
86 for (int functionalCount = 0; functionalCount < shape.getNumberOfHashFunctions(); functionalCount++) {
87 if (!consumer.test((int) index)) {
88 return false;
89 }
90 index += inc;
91 index = index >= bits ? index - bits : index;
92 }
93 return true;
94 }
95 };
96 }
97 }