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 }