001/* Adler32.java - Computes Adler32 data checksum of a data stream
002   Copyright (C) 1999, 2000, 2001 Free Software Foundation, Inc.
003
004This file is part of GNU Classpath.
005
006GNU Classpath is free software; you can redistribute it and/or modify
007it under the terms of the GNU General Public License as published by
008the Free Software Foundation; either version 2, or (at your option)
009any later version.
010
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013MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
014General Public License for more details.
015
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017along with GNU Classpath; see the file COPYING.  If not, write to the
018Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
01902110-1301 USA.
020
021Linking this library statically or dynamically with other modules is
022making a combined work based on this library.  Thus, the terms and
023conditions of the GNU General Public License cover the whole
024combination.
025
026As a special exception, the copyright holders of this library give you
027permission to link this library with independent modules to produce an
028executable, regardless of the license terms of these independent
029modules, and to copy and distribute the resulting executable under
030terms of your choice, provided that you also meet, for each linked
031independent module, the terms and conditions of the license of that
032module.  An independent module is a module which is not derived from
033or based on this library.  If you modify this library, you may extend
034this exception to your version of the library, but you are not
035obligated to do so.  If you do not wish to do so, delete this
036exception statement from your version. */
037
038package java.util.zip;
039
040/*
041 * Written using on-line Java Platform 1.2 API Specification, as well
042 * as "The Java Class Libraries", 2nd edition (Addison-Wesley, 1998).
043 * The actual Adler32 algorithm is taken from RFC 1950.
044 * Status:  Believed complete and correct.
045 */
046
047/**
048 * Computes Adler32 checksum for a stream of data. An Adler32 
049 * checksum is not as reliable as a CRC32 checksum, but a lot faster to 
050 * compute.
051 *<p>
052 * The specification for Adler32 may be found in RFC 1950.
053 * (ZLIB Compressed Data Format Specification version 3.3)
054 *<p>
055 *<p>
056 * From that document:
057 *<p>
058 *      "ADLER32 (Adler-32 checksum)
059 *       This contains a checksum value of the uncompressed data
060 *       (excluding any dictionary data) computed according to Adler-32
061 *       algorithm. This algorithm is a 32-bit extension and improvement
062 *       of the Fletcher algorithm, used in the ITU-T X.224 / ISO 8073
063 *       standard. 
064 *<p>
065 *       Adler-32 is composed of two sums accumulated per byte: s1 is
066 *       the sum of all bytes, s2 is the sum of all s1 values. Both sums
067 *       are done modulo 65521. s1 is initialized to 1, s2 to zero.  The
068 *       Adler-32 checksum is stored as s2*65536 + s1 in most-
069 *       significant-byte first (network) order."
070 *<p>
071 * "8.2. The Adler-32 algorithm
072 *<p>
073 *    The Adler-32 algorithm is much faster than the CRC32 algorithm yet
074 *    still provides an extremely low probability of undetected errors.
075 *<p>
076 *    The modulo on unsigned long accumulators can be delayed for 5552
077 *    bytes, so the modulo operation time is negligible.  If the bytes
078 *    are a, b, c, the second sum is 3a + 2b + c + 3, and so is position
079 *    and order sensitive, unlike the first sum, which is just a
080 *    checksum.  That 65521 is prime is important to avoid a possible
081 *    large class of two-byte errors that leave the check unchanged.
082 *    (The Fletcher checksum uses 255, which is not prime and which also
083 *    makes the Fletcher check insensitive to single byte changes 0 <->
084 *    255.)
085 *<p>
086 *    The sum s1 is initialized to 1 instead of zero to make the length
087 *    of the sequence part of s2, so that the length does not have to be
088 *   checked separately. (Any sequence of zeroes has a Fletcher
089 *    checksum of zero.)"
090 *
091 * @author John Leuner, Per Bothner
092 * @since JDK 1.1
093 *
094 * @see InflaterInputStream
095 * @see DeflaterOutputStream
096 */
097public class Adler32 implements Checksum
098{
099
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}