1 /*
2  * Copyright (C) 2014 The Android Open Source Project
3  *
4  * Licensed under the Apache License, Version 2.0 (the "License");
5  * you may not use this file except in compliance with the License.
6  * You may obtain a copy of the License at
7  *
8  *      http://www.apache.org/licenses/LICENSE-2.0
9  *
10  * Unless required by applicable law or agreed to in writing, software
11  * distributed under the License is distributed on an "AS IS" BASIS,
12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13  * See the License for the specific language governing permissions and
14  * limitations under the License.
15  */
16 
17 package android.hardware.camera2.utils;
18 
19 /**
20  * Provide hashing functions using the Modified Bernstein hash
21  */
22 public final class HashCodeHelpers {
23 
24     /**
25      * Hash every element uniformly using the Modified Bernstein hash.
26      *
27      * <p>Useful to implement a {@link Object#hashCode} for uniformly distributed data.</p>
28      *
29      * @param array a non-{@code null} array of integers
30      *
31      * @return the numeric hash code
32      */
hashCode(int... array)33     public static int hashCode(int... array) {
34         if (array == null) {
35             return 0;
36         }
37 
38         /*
39          *  Note that we use 31 here instead of 33 since it's preferred in Effective Java
40          *  and used elsewhere in the runtime (e.g. Arrays#hashCode)
41          *
42          *  That being said 33 and 31 are nearly identical in terms of their usefulness
43          *  according to http://svn.apache.org/repos/asf/apr/apr/trunk/tables/apr_hash.c
44          */
45         int h = 1;
46         for (int x : array) {
47             // Strength reduction; in case the compiler has illusions about divisions being faster
48             h = ((h << 5) - h) ^ x; // (h * 31) XOR x
49         }
50 
51         return h;
52     }
53 
54     /**
55      * Hash every element uniformly using the Modified Bernstein hash.
56      *
57      * <p>Useful to implement a {@link Object#hashCode} for uniformly distributed data.</p>
58      *
59      * @param array a non-{@code null} array of floats
60      *
61      * @return the numeric hash code
62      */
hashCode(float... array)63     public static int hashCode(float... array) {
64         if (array == null) {
65             return 0;
66         }
67 
68         int h = 1;
69         for (float f : array) {
70             int x = Float.floatToIntBits(f);
71             h = ((h << 5) - h) ^ x; // (h * 31) XOR x
72         }
73 
74         return h;
75     }
76 
77     /**
78      * Hash every element uniformly using the Modified Bernstein hash.
79      *
80      * <p>Useful to implement a {@link Object#hashCode} for uniformly distributed data.</p>
81      *
82      * @param array a non-{@code null} array of objects
83      *
84      * @return the numeric hash code
85      */
hashCodeGeneric(T... array)86     public static <T> int hashCodeGeneric(T... array) {
87         if (array == null) {
88             return 0;
89         }
90 
91         int h = 1;
92         for (T o : array) {
93             int x = (o == null) ? 0 : o.hashCode();
94             h = ((h << 5) - h) ^ x; // (h * 31) XOR x
95         }
96 
97         return h;
98     }
99 
100 }
101