Source Code Cross Referenced for StringHasher.java in  » Web-Server » Rimfaxe-Web-Server » org » apache » xerces » utils » Java Source Code / Java DocumentationJava Source Code and Java Documentation

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Java Source Code / Java Documentation » Web Server » Rimfaxe Web Server » org.apache.xerces.utils 
Source Cross Referenced  Class Diagram Java Document (Java Doc) 


001:        /*
002:         * The Apache Software License, Version 1.1
003:         *
004:         *
005:         * Copyright (c) 1999 The Apache Software Foundation.  All rights 
006:         * reserved.
007:         *
008:         * Redistribution and use in source and binary forms, with or without
009:         * modification, are permitted provided that the following conditions
010:         * are met:
011:         *
012:         * 1. Redistributions of source code must retain the above copyright
013:         *    notice, this list of conditions and the following disclaimer. 
014:         *
015:         * 2. Redistributions in binary form must reproduce the above copyright
016:         *    notice, this list of conditions and the following disclaimer in
017:         *    the documentation and/or other materials provided with the
018:         *    distribution.
019:         *
020:         * 3. The end-user documentation included with the redistribution,
021:         *    if any, must include the following acknowledgment:  
022:         *       "This product includes software developed by the
023:         *        Apache Software Foundation (http://www.apache.org/)."
024:         *    Alternately, this acknowledgment may appear in the software itself,
025:         *    if and wherever such third-party acknowledgments normally appear.
026:         *
027:         * 4. The names "Xerces" and "Apache Software Foundation" must
028:         *    not be used to endorse or promote products derived from this
029:         *    software without prior written permission. For written 
030:         *    permission, please contact apache@apache.org.
031:         *
032:         * 5. Products derived from this software may not be called "Apache",
033:         *    nor may "Apache" appear in their name, without prior written
034:         *    permission of the Apache Software Foundation.
035:         *
036:         * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESSED OR IMPLIED
037:         * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
038:         * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
039:         * DISCLAIMED.  IN NO EVENT SHALL THE APACHE SOFTWARE FOUNDATION OR
040:         * ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
041:         * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
042:         * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
043:         * USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
044:         * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
045:         * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
046:         * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
047:         * SUCH DAMAGE.
048:         * ====================================================================
049:         *
050:         * This software consists of voluntary contributions made by many
051:         * individuals on behalf of the Apache Software Foundation and was
052:         * originally based on software copyright (c) 1999, International
053:         * Business Machines, Inc., http://www.apache.org.  For more
054:         * information on the Apache Software Foundation, please see
055:         * <http://www.apache.org/>.
056:         */
057:
058:        package org.apache.xerces.utils;
059:
060:        /**
061:         * Algorithm used to hash char arrays (strings).
062:         *
063:         * This class was created after it was discovered that parsing some
064:         * documents was unexpectedly slow due to many different strings
065:         * hashing to the same 32-bit value using the java.lang.String hash
066:         * algorithm.
067:         *
068:         * The trick seems to be the shift of the top eight bits of the hashcode
069:         * back down to the bottom to keep them from being rolled out.
070:         *
071:         * @version
072:         */
073:        public final class StringHasher {
074:            /**
075:             * generate a hashcode for a String
076:             *
077:             * @param str the String to hash
078:             * @param strLength the length of the String to hash
079:             * @return hashcode for the String
080:             */
081:            public static int hashString(String str, int strLength) {
082:                int hashcode = 0;
083:                for (int i = 0; i < strLength; i++) {
084:                    int top = hashcode >> 24;
085:                    hashcode += ((hashcode * 37) + top + ((int) str.charAt(i)));
086:                }
087:                hashcode = (hashcode & 0x7fffffff);
088:                return (hashcode == 0) ? 1 : hashcode;
089:            }
090:
091:            /**
092:             * generate a hashcode for a character array
093:             *
094:             * @param chars the array to hash
095:             * @param offset the offset to start hashing
096:             * @param length the length of characters to hash
097:             * @return hashcode for the character array
098:             */
099:            public static int hashChars(char[] chars, int offset, int length) {
100:                int hashcode = 0;
101:                for (int i = 0; i < length; i++) {
102:                    int top = hashcode >> 24;
103:                    hashcode += ((hashcode * 37) + top + ((int) (chars[offset++] & 0xFFFF)));
104:                }
105:                hashcode = (hashcode & 0x7fffffff);
106:                return (hashcode == 0) ? 1 : hashcode;
107:            }
108:
109:            /**
110:             * generate partially completed character hashcode.
111:             * this is mean to be iterated over individual characters in order to generate
112:             * a full hash value
113:             * @see #finishHash(int)
114:             *
115:             * @param hashcode a partially completed character hashcode
116:             * @param ch the character to hash
117:             * @return a partially completed character hashcode
118:             */
119:            public static int hashChar(int hashcode, int ch) {
120:                int top = hashcode >> 24;
121:                hashcode += ((hashcode * 37) + top + ch);
122:                return hashcode;
123:            }
124:
125:            /**
126:             * finish hashing a partically completed character hashcode
127:             * @see #hashChar(int,int)
128:             * 
129:             * @param hashcode a partially completed character hashcode
130:             * @return a character hashcode
131:             */
132:            public static int finishHash(int hashcode) {
133:                hashcode = (hashcode & 0x7fffffff);
134:                return (hashcode == 0) ? 1 : hashcode;
135:            }
136:        }
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