libjava/java/util/zip/Adler32.java

   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    */
 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 }