1 /* Adler32.java - Computes Adler32 data checksum of a data stream 2 Copyright (C) 1999, 2000, 2001 Free Software Foundation, Inc. 3 4 This file is part of GNU Classpath. 5 6 GNU Classpath is free software; you can redistribute it and/or modify 7 it under the terms of the GNU General Public License as published by 8 the Free Software Foundation; either version 2, or (at your option) 9 any later version. 10 11 GNU Classpath is distributed in the hope that it will be useful, but 12 WITHOUT ANY WARRANTY; without even the implied warranty of 13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU 14 General Public License for more details. 15 16 You should have received a copy of the GNU General Public License 17 along with GNU Classpath; see the file COPYING. If not, write to the 18 Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 19 02111-1307 USA. 20 21 Linking this library statically or dynamically with other modules is 22 making a combined work based on this library. Thus, the terms and 23 conditions of the GNU General Public License cover the whole 24 combination. 25 26 As a special exception, the copyright holders of this library give you 27 permission to link this library with independent modules to produce an 28 executable, regardless of the license terms of these independent 29 modules, and to copy and distribute the resulting executable under 30 terms of your choice, provided that you also meet, for each linked 31 independent module, the terms and conditions of the license of that 32 module. An independent module is a module which is not derived from 33 or based on this library. If you modify this library, you may extend 34 this exception to your version of the library, but you are not 35 obligated to do so. If you do not wish to do so, delete this 36 exception statement from your version. */ 37 38 package java.util.zip; 39 40 /* 41 * Written using on-line Java Platform 1.2 API Specification, as well 42 * as "The Java Class Libraries", 2nd edition (Addison-Wesley, 1998). 43 * The actual Adler32 algorithm is taken from RFC 1950. 44 * Status: Believed complete and correct. 45 */ 46 47 /** 48 * Computes Adler32 checksum for a stream of data. An Adler32 49 * checksum is not as reliable as a CRC32 checksum, but a lot faster to 50 * compute. 51 *<p> 52 * The specification for Adler32 may be found in RFC 1950. 53 * (ZLIB Compressed Data Format Specification version 3.3) 54 *<p> 55 *<p> 56 * From that document: 57 *<p> 58 * "ADLER32 (Adler-32 checksum) 59 * This contains a checksum value of the uncompressed data 60 * (excluding any dictionary data) computed according to Adler-32 61 * algorithm. This algorithm is a 32-bit extension and improvement 62 * of the Fletcher algorithm, used in the ITU-T X.224 / ISO 8073 63 * standard. 64 *<p> 65 * Adler-32 is composed of two sums accumulated per byte: s1 is 66 * the sum of all bytes, s2 is the sum of all s1 values. Both sums 67 * are done modulo 65521. s1 is initialized to 1, s2 to zero. The 68 * Adler-32 checksum is stored as s2*65536 + s1 in most- 69 * significant-byte first (network) order." 70 *<p> 71 * "8.2. The Adler-32 algorithm 72 *<p> 73 * The Adler-32 algorithm is much faster than the CRC32 algorithm yet 74 * still provides an extremely low probability of undetected errors. 75 *<p> 76 * The modulo on unsigned long accumulators can be delayed for 5552 77 * bytes, so the modulo operation time is negligible. If the bytes 78 * are a, b, c, the second sum is 3a + 2b + c + 3, and so is position 79 * and order sensitive, unlike the first sum, which is just a 80 * checksum. That 65521 is prime is important to avoid a possible 81 * large class of two-byte errors that leave the check unchanged. 82 * (The Fletcher checksum uses 255, which is not prime and which also 83 * makes the Fletcher check insensitive to single byte changes 0 <-> 84 * 255.) 85 *<p> 86 * The sum s1 is initialized to 1 instead of zero to make the length 87 * of the sequence part of s2, so that the length does not have to be 88 * checked separately. (Any sequence of zeroes has a Fletcher 89 * checksum of zero.)" 90 * 91 * @author John Leuner, Per Bothner 92 * @since JDK 1.1 93 * 94 * @see InflaterInputStream 95 * @see DeflaterOutputStream 96 */ 97 public class Adler32 implements Checksum 98 { 99 100 /** largest prime smaller than 65536 */ 101 private static final int BASE = 65521; 102 103 private int checksum; //we do all in int. 104 105 //Note that java doesn't have unsigned integers, 106 //so we have to be careful with what arithmetic 107 //we do. We return the checksum as a long to 108 //avoid sign confusion. 109 110 /** 111 * Creates a new instance of the <code>Adler32</code> class. 112 * The checksum starts off with a value of 1. 113 */ Adler32()114 public Adler32 () 115 { 116 reset(); 117 } 118 119 /** 120 * Resets the Adler32 checksum to the initial value. 121 */ reset()122 public void reset () 123 { 124 checksum = 1; //Initialize to 1 125 } 126 127 /** 128 * Updates the checksum with the byte b. 129 * 130 * @param bval the data value to add. The high byte of the int is ignored. 131 */ update(int bval)132 public void update (int bval) 133 { 134 //We could make a length 1 byte array and call update again, but I 135 //would rather not have that overhead 136 int s1 = checksum & 0xffff; 137 int s2 = checksum >>> 16; 138 139 s1 = (s1 + (bval & 0xFF)) % BASE; 140 s2 = (s1 + s2) % BASE; 141 142 checksum = (s2 << 16) + s1; 143 } 144 145 /** 146 * Updates the checksum with the bytes taken from the array. 147 * 148 * @param buffer an array of bytes 149 */ update(byte[] buffer)150 public void update (byte[] buffer) 151 { 152 update(buffer, 0, buffer.length); 153 } 154 155 /** 156 * Updates the checksum with the bytes taken from the array. 157 * 158 * @param buf an array of bytes 159 * @param off the start of the data used for this update 160 * @param len the number of bytes to use for this update 161 */ update(byte[] buf, int off, int len)162 public void update (byte[] buf, int off, int len) 163 { 164 //(By Per Bothner) 165 int s1 = checksum & 0xffff; 166 int s2 = checksum >>> 16; 167 168 while (len > 0) 169 { 170 // We can defer the modulo operation: 171 // s1 maximally grows from 65521 to 65521 + 255 * 3800 172 // s2 maximally grows by 3800 * median(s1) = 2090079800 < 2^31 173 int n = 3800; 174 if (n > len) 175 n = len; 176 len -= n; 177 while (--n >= 0) 178 { 179 s1 = s1 + (buf[off++] & 0xFF); 180 s2 = s2 + s1; 181 } 182 s1 %= BASE; 183 s2 %= BASE; 184 } 185 186 /*Old implementation, borrowed from somewhere: 187 int n; 188 189 while (len-- > 0) { 190 191 s1 = (s1 + (bs[offset++] & 0xff)) % BASE; 192 s2 = (s2 + s1) % BASE; 193 }*/ 194 195 checksum = (s2 << 16) | s1; 196 } 197 198 /** 199 * Returns the Adler32 data checksum computed so far. 200 */ getValue()201 public long getValue() 202 { 203 return (long) checksum & 0xffffffffL; 204 } 205 } 206