001/* CRC32.java - Computes CRC32 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
011GNU Classpath is distributed in the hope that it will be useful, but
012WITHOUT ANY WARRANTY; without even the implied warranty of
013MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
014General Public License for more details.
015
016You should have received a copy of the GNU General Public License
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 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 */
060public 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}