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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., 51 Franklin Street, Fifth Floor, Boston, MA 19: 02110-1301 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: */ 114: public Adler32 () 115: { 116: reset(); 117: } 118: 119: /** 120: * Resets the Adler32 checksum to the initial value. 121: */ 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: */ 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: */ 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: */ 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: */ 201: public long getValue() 202: { 203: return (long) checksum & 0xffffffffL; 204: } 205: }

GNU Classpath (0.95) |