206 lines
6.4 KiB
Java
206 lines
6.4 KiB
Java
/* Adler32.java - Computes Adler32 data checksum of a data stream
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Copyright (C) 1999, 2000, 2001 Free Software Foundation, Inc.
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This file is part of GNU Classpath.
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GNU Classpath is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 2, or (at your option)
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any later version.
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GNU Classpath is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with GNU Classpath; see the file COPYING. If not, write to the
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Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
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02111-1307 USA.
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Linking this library statically or dynamically with other modules is
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making a combined work based on this library. Thus, the terms and
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conditions of the GNU General Public License cover the whole
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combination.
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As a special exception, the copyright holders of this library give you
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permission to link this library with independent modules to produce an
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executable, regardless of the license terms of these independent
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modules, and to copy and distribute the resulting executable under
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terms of your choice, provided that you also meet, for each linked
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independent module, the terms and conditions of the license of that
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module. An independent module is a module which is not derived from
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or based on this library. If you modify this library, you may extend
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this exception to your version of the library, but you are not
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obligated to do so. If you do not wish to do so, delete this
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exception statement from your version. */
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package java.util.zip;
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/*
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* Written using on-line Java Platform 1.2 API Specification, as well
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* as "The Java Class Libraries", 2nd edition (Addison-Wesley, 1998).
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* The actual Adler32 algorithm is taken from RFC 1950.
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* Status: Believed complete and correct.
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*/
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/**
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* Computes Adler32 checksum for a stream of data. An Adler32
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* checksum is not as reliable as a CRC32 checksum, but a lot faster to
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* compute.
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*<p>
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* The specification for Adler32 may be found in RFC 1950.
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* (ZLIB Compressed Data Format Specification version 3.3)
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*<p>
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*<p>
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* From that document:
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*<p>
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* "ADLER32 (Adler-32 checksum)
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* This contains a checksum value of the uncompressed data
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* (excluding any dictionary data) computed according to Adler-32
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* algorithm. This algorithm is a 32-bit extension and improvement
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* of the Fletcher algorithm, used in the ITU-T X.224 / ISO 8073
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* standard.
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*<p>
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* Adler-32 is composed of two sums accumulated per byte: s1 is
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* the sum of all bytes, s2 is the sum of all s1 values. Both sums
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* are done modulo 65521. s1 is initialized to 1, s2 to zero. The
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* Adler-32 checksum is stored as s2*65536 + s1 in most-
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* significant-byte first (network) order."
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*<p>
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* "8.2. The Adler-32 algorithm
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*<p>
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* The Adler-32 algorithm is much faster than the CRC32 algorithm yet
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* still provides an extremely low probability of undetected errors.
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*<p>
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* The modulo on unsigned long accumulators can be delayed for 5552
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* bytes, so the modulo operation time is negligible. If the bytes
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* are a, b, c, the second sum is 3a + 2b + c + 3, and so is position
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* and order sensitive, unlike the first sum, which is just a
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* checksum. That 65521 is prime is important to avoid a possible
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* large class of two-byte errors that leave the check unchanged.
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* (The Fletcher checksum uses 255, which is not prime and which also
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* makes the Fletcher check insensitive to single byte changes 0 <->
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* 255.)
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*<p>
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* The sum s1 is initialized to 1 instead of zero to make the length
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* of the sequence part of s2, so that the length does not have to be
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* checked separately. (Any sequence of zeroes has a Fletcher
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* checksum of zero.)"
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*
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* @author John Leuner, Per Bothner
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* @since JDK 1.1
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*
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* @see InflaterInputStream
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* @see DeflaterOutputStream
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*/
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public class Adler32 implements Checksum
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{
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/** largest prime smaller than 65536 */
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private static final int BASE = 65521;
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private int checksum; //we do all in int.
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//Note that java doesn't have unsigned integers,
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//so we have to be careful with what arithmetic
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//we do. We return the checksum as a long to
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//avoid sign confusion.
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/**
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* Creates a new instance of the <code>Adler32</code> class.
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* The checksum starts off with a value of 1.
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*/
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public Adler32 ()
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{
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reset();
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}
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/**
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* Resets the Adler32 checksum to the initial value.
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*/
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public void reset ()
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{
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checksum = 1; //Initialize to 1
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}
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/**
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* Updates the checksum with the byte b.
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*
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* @param bval the data value to add. The high byte of the int is ignored.
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*/
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public void update (int bval)
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{
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//We could make a length 1 byte array and call update again, but I
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//would rather not have that overhead
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int s1 = checksum & 0xffff;
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int s2 = checksum >>> 16;
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s1 = (s1 + (bval & 0xFF)) % BASE;
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s2 = (s1 + s2) % BASE;
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checksum = (s2 << 16) + s1;
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}
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/**
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* Updates the checksum with the bytes taken from the array.
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*
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* @param buffer an array of bytes
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*/
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public void update (byte[] buffer)
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{
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update(buffer, 0, buffer.length);
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}
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/**
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* Updates the checksum with the bytes taken from the array.
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*
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* @param buf an array of bytes
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* @param off the start of the data used for this update
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* @param len the number of bytes to use for this update
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*/
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public void update (byte[] buf, int off, int len)
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{
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//(By Per Bothner)
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int s1 = checksum & 0xffff;
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int s2 = checksum >>> 16;
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while (len > 0)
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{
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// We can defer the modulo operation:
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// s1 maximally grows from 65521 to 65521 + 255 * 3800
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// s2 maximally grows by 3800 * median(s1) = 2090079800 < 2^31
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int n = 3800;
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if (n > len)
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n = len;
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len -= n;
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while (--n >= 0)
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{
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s1 = s1 + (buf[off++] & 0xFF);
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s2 = s2 + s1;
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}
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s1 %= BASE;
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s2 %= BASE;
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}
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/*Old implementation, borrowed from somewhere:
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int n;
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while (len-- > 0) {
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s1 = (s1 + (bs[offset++] & 0xff)) % BASE;
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s2 = (s2 + s1) % BASE;
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}*/
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checksum = (s2 << 16) | s1;
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}
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/**
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* Returns the Adler32 data checksum computed so far.
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*/
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public long getValue()
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{
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return (long) checksum & 0xffffffffL;
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}
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}
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