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 import android.annotation.UnsupportedAppUsage; 20 21 /** 22 * Provide hashing functions using the Modified Bernstein hash 23 */ 24 public final class HashCodeHelpers { 25 26 /** 27 * Hash every element uniformly using the Modified Bernstein hash. 28 * 29 * <p>Useful to implement a {@link Object#hashCode} for uniformly distributed data.</p> 30 * 31 * @param array a non-{@code null} array of integers 32 * 33 * @return the numeric hash code 34 */ 35 @UnsupportedAppUsage hashCode(int... array)36 public static int hashCode(int... array) { 37 if (array == null) { 38 return 0; 39 } 40 41 /* 42 * Note that we use 31 here instead of 33 since it's preferred in Effective Java 43 * and used elsewhere in the runtime (e.g. Arrays#hashCode) 44 * 45 * That being said 33 and 31 are nearly identical in terms of their usefulness 46 * according to http://svn.apache.org/repos/asf/apr/apr/trunk/tables/apr_hash.c 47 */ 48 int h = 1; 49 for (int x : array) { 50 // Strength reduction; in case the compiler has illusions about divisions being faster 51 h = ((h << 5) - h) ^ x; // (h * 31) XOR x 52 } 53 54 return h; 55 } 56 57 /** 58 * Hash every element uniformly using the Modified Bernstein hash. 59 * 60 * <p>Useful to implement a {@link Object#hashCode} for uniformly distributed data.</p> 61 * 62 * @param array a non-{@code null} array of floats 63 * 64 * @return the numeric hash code 65 */ hashCode(float... array)66 public static int hashCode(float... array) { 67 if (array == null) { 68 return 0; 69 } 70 71 int h = 1; 72 for (float f : array) { 73 int x = Float.floatToIntBits(f); 74 h = ((h << 5) - h) ^ x; // (h * 31) XOR x 75 } 76 77 return h; 78 } 79 80 /** 81 * Hash every element uniformly using the Modified Bernstein hash. 82 * 83 * <p>Useful to implement a {@link Object#hashCode} for uniformly distributed data.</p> 84 * 85 * @param array a non-{@code null} array of objects 86 * 87 * @return the numeric hash code 88 */ hashCodeGeneric(T... array)89 public static <T> int hashCodeGeneric(T... array) { 90 if (array == null) { 91 return 0; 92 } 93 94 int h = 1; 95 for (T o : array) { 96 int x = (o == null) ? 0 : o.hashCode(); 97 h = ((h << 5) - h) ^ x; // (h * 31) XOR x 98 } 99 100 return h; 101 } 102 103 } 104