libjava/java/util/zip/CRC32.java

   1 /* CRC32.java - Computes CRC32 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 CRC32 algorithm is taken from RFC 1952.
  44  * Status:  Believed complete and correct.
  45  */
  46
  47 /**
  48  * Computes CRC32 data checksum of a data stream.
  49  * The actual CRC32 algorithm is described in RFC 1952
  50  * (GZIP file format specification version 4.3).
  51  * Can be used to get the CRC32 over a stream if used with checked input/output
  52  * streams.
  53  *
  54  * @see InflaterInputStream
  55  * @see DeflaterOutputStream
  56  *
  57  * @author Per Bothner
  58  * @date April 1, 1999.
  59  */
  60 public class CRC32 implements Checksum
  61 {
  62   /** The crc data checksum so far. */
  63   private int crc = 0;
  64
  65   /** The fast CRC table. Computed once when the CRC32 class is loaded. */
  66   private static int[] crc_table = make_crc_table();
  67
  68   /** Make the table for a fast CRC. */
  69   private static int[] make_crc_table ()
  70   {
  71     int[] crc_table = new int[256];
  72     for (int n = 0; n < 256; n++)
  73       {
  74         int c = n;
  75         for (int k = 8;  --k >= 0; )
  76           {
  77             if ((c & 1) != 0)
  78               c = 0xedb88320 ^ (c >>> 1);
  79             else
  80               c = c >>> 1;
  81           }
  82         crc_table[n] = c;
  83       }
  84     return crc_table;
  85   }
  86
  87   /**
  88    * Returns the CRC32 data checksum computed so far.
  89    */
  90   public long getValue ()
  91   {
  92     return (long) crc & 0xffffffffL;
  93   }
  94
  95   /**
  96    * Resets the CRC32 data checksum as if no update was ever called.
  97    */
  98   public void reset () { crc = 0; }
  99
 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 }