001    /* CRC32.java - Computes CRC32 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 CRC32 algorithm is taken from RFC 1952.
044     * Status:  Believed complete and correct.
045     */
046    
047    /**
048     * Computes CRC32 data checksum of a data stream.
049     * The actual CRC32 algorithm is described in RFC 1952
050     * (GZIP file format specification version 4.3).
051     * Can be used to get the CRC32 over a stream if used with checked input/output
052     * streams.
053     *
054     * @see InflaterInputStream
055     * @see DeflaterOutputStream
056     *
057     * @author Per Bothner
058     * @date April 1, 1999.
059     */
060    public class CRC32 implements Checksum
061    {
062      /** The crc data checksum so far. */
063      private int crc = 0;
064    
065      /** The fast CRC table. Computed once when the CRC32 class is loaded. */
066      private static int[] crc_table = make_crc_table();
067    
068      /** Make the table for a fast CRC. */
069      private static int[] make_crc_table ()
070      {
071        int[] crc_table = new int[256];
072        for (int n = 0; n < 256; n++)
073          {
074            int c = n;
075            for (int k = 8;  --k >= 0; )
076              {
077                if ((c & 1) != 0)
078                  c = 0xedb88320 ^ (c >>> 1);
079                else
080                  c = c >>> 1;
081              }
082            crc_table[n] = c;
083          }
084        return crc_table;
085      }
086    
087      /**
088       * Returns the CRC32 data checksum computed so far.
089       */
090      public long getValue ()
091      {
092        return (long) crc & 0xffffffffL;
093      }
094    
095      /**
096       * Resets the CRC32 data checksum as if no update was ever called.
097       */
098      public void reset () { crc = 0; }
099    
100      /**
101       * Updates the checksum with the int bval. 
102       *
103       * @param bval (the byte is taken as the lower 8 bits of bval)
104       */
105    
106      public void update (int bval)
107      {
108        int c = ~crc;
109        c = crc_table[(c ^ bval) & 0xff] ^ (c >>> 8);
110        crc = ~c;
111      }
112    
113      /**
114       * Adds the byte array to the data checksum.
115       *
116       * @param buf the buffer which contains the data
117       * @param off the offset in the buffer where the data starts
118       * @param len the length of the data
119       */
120      public void update (byte[] buf, int off, int len)
121      {
122        int c = ~crc;
123        while (--len >= 0)
124          c = crc_table[(c ^ buf[off++]) & 0xff] ^ (c >>> 8);
125        crc = ~c;
126      }
127    
128      /**
129       * Adds the complete byte array to the data checksum.
130       */
131      public void update (byte[] buf) { update(buf, 0, buf.length); }
132    }