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1 /*
2  * The Apache Software License, Version 1.1
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4  *
5  * Copyright (c) 1999 The Apache Software Foundation. All rights
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57
58 package org.xquark.xpath.datamodel.xerces.utils;
59
60 /**
61  * Algorithm used to hash char arrays (strings).
62  *
63  * This class was created after it was discovered that parsing some
64  * documents was unexpectedly slow due to many different strings
65  * hashing to the same 32-bit value using the java.lang.String hash
66  * algorithm.
67  *
68  * The trick seems to be the shift of the top eight bits of the hashcode
69  * back down to the bottom to keep them from being rolled out.
70  *
71  * @version
72  */
73 public final class StringHasher {
74     /**
75      * generate a hashcode for a String
76      *
77      * @param str the String to hash
78      * @param strLength the length of the String to hash
79      * @return hashcode for the String
80      */
81     public static int hashString(String JavaDoc str, int strLength) {
82         int hashcode = 0;
83         for (int i = 0; i < strLength; i++) {
84             int top = hashcode >> 24;
85             hashcode += ((hashcode * 37) + top + ((int)str.charAt(i)));
86         }
87         hashcode = (hashcode & 0x7fffffff);
88         return (hashcode == 0) ? 1 : hashcode;
89     }
90     /**
91      * generate a hashcode for a character array
92      *
93      * @param chars the array to hash
94      * @param offset the offset to start hashing
95      * @param length the length of characters to hash
96      * @return hashcode for the character array
97      */
98     public static int hashChars(char[] chars, int offset, int length) {
99         int hashcode = 0;
100         for (int i = 0; i < length; i++) {
101             int top = hashcode >> 24;
102             hashcode += ((hashcode * 37) + top + ((int)(chars[offset++] & 0xFFFF)));
103         }
104         hashcode = (hashcode & 0x7fffffff);
105         return (hashcode == 0) ? 1 : hashcode;
106     }
107     /**
108      * generate partially completed character hashcode.
109      * this is mean to be iterated over individual characters in order to generate
110      * a full hash value
111      * @see #finishHash(int)
112      *
113      * @param hashcode a partially completed character hashcode
114      * @param ch the character to hash
115      * @return a partially completed character hashcode
116      */
117     public static int hashChar(int hashcode, int ch) {
118         int top = hashcode >> 24;
119         hashcode += ((hashcode * 37) + top + ch);
120         return hashcode;
121     }
122     /**
123      * finish hashing a partically completed character hashcode
124      * @see #hashChar(int,int)
125      *
126      * @param hashcode a partially completed character hashcode
127      * @return a character hashcode
128      */
129     public static int finishHash(int hashcode) {
130         hashcode = (hashcode & 0x7fffffff);
131         return (hashcode == 0) ? 1 : hashcode;
132     }
133 }
134
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