1 /*
2  * Copyright (C) 2008 The Guava Authors
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 com.google.common.collect;
18 
19 import com.google.common.annotations.GwtCompatible;
20 import com.google.common.primitives.Ints;
21 
22 import javax.annotation.Nullable;
23 
24 /**
25  * Static methods for implementing hash-based collections.
26  *
27  * @author Kevin Bourrillion
28  * @author Jesse Wilson
29  * @author Austin Appleby
30  */
31 @GwtCompatible
32 final class Hashing {
Hashing()33   private Hashing() {}
34 
35   private static final int C1 = 0xcc9e2d51;
36   private static final int C2 = 0x1b873593;
37 
38   /*
39    * This method was rewritten in Java from an intermediate step of the Murmur hash function in
40    * http://code.google.com/p/smhasher/source/browse/trunk/MurmurHash3.cpp, which contained the
41    * following header:
42    *
43    * MurmurHash3 was written by Austin Appleby, and is placed in the public domain. The author
44    * hereby disclaims copyright to this source code.
45    */
smear(int hashCode)46   static int smear(int hashCode) {
47     return C2 * Integer.rotateLeft(hashCode * C1, 15);
48   }
49 
smearedHash(@ullable Object o)50   static int smearedHash(@Nullable Object o) {
51     return smear((o == null) ? 0 : o.hashCode());
52   }
53 
54   private static int MAX_TABLE_SIZE = Ints.MAX_POWER_OF_TWO;
55 
closedTableSize(int expectedEntries, double loadFactor)56   static int closedTableSize(int expectedEntries, double loadFactor) {
57     // Get the recommended table size.
58     // Round down to the nearest power of 2.
59     expectedEntries = Math.max(expectedEntries, 2);
60     int tableSize = Integer.highestOneBit(expectedEntries);
61     // Check to make sure that we will not exceed the maximum load factor.
62     if (expectedEntries > (int) (loadFactor * tableSize)) {
63       tableSize <<= 1;
64       return (tableSize > 0) ? tableSize : MAX_TABLE_SIZE;
65     }
66     return tableSize;
67   }
68 
needsResizing(int size, int tableSize, double loadFactor)69   static boolean needsResizing(int size, int tableSize, double loadFactor) {
70     return size > loadFactor * tableSize && tableSize < MAX_TABLE_SIZE;
71   }
72 }
73