001    /* Adler32.java - Computes Adler32 data checksum of a data stream
002       Copyright (C) 1999, 2000, 2001 Free Software Foundation, Inc.
003    
004    This file is part of GNU Classpath.
005    
006    GNU Classpath is free software; you can redistribute it and/or modify
007    it under the terms of the GNU General Public License as published by
008    the Free Software Foundation; either version 2, or (at your option)
009    any later version.
010    
011    GNU Classpath is distributed in the hope that it will be useful, but
012    WITHOUT ANY WARRANTY; without even the implied warranty of
013    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
014    General Public License for more details.
015    
016    You should have received a copy of the GNU General Public License
017    along with GNU Classpath; see the file COPYING.  If not, write to the
018    Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
019    02110-1301 USA.
020    
021    Linking this library statically or dynamically with other modules is
022    making a combined work based on this library.  Thus, the terms and
023    conditions of the GNU General Public License cover the whole
024    combination.
025    
026    As a special exception, the copyright holders of this library give you
027    permission to link this library with independent modules to produce an
028    executable, regardless of the license terms of these independent
029    modules, and to copy and distribute the resulting executable under
030    terms of your choice, provided that you also meet, for each linked
031    independent module, the terms and conditions of the license of that
032    module.  An independent module is a module which is not derived from
033    or based on this library.  If you modify this library, you may extend
034    this exception to your version of the library, but you are not
035    obligated to do so.  If you do not wish to do so, delete this
036    exception statement from your version. */
037    
038    package 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     */
097    public 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    }