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      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      */
     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      */
     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      */
     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