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 }