Source for java.util.zip.CRC32

   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., 51 Franklin Street, Fifth Floor, Boston, MA
  19: 02110-1301 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: }