// Adler32.cs - Computes Adler32 data checksum of a data stream
// Copyright (C) 2001 Mike Krueger
//
// This file was translated from java, it was part of the GNU Classpath
// Copyright (C) 1999, 2000, 2001 Free Software Foundation, Inc.
//
// This program is free software; you can redistribute it and/or
// modify it under the terms of the GNU General Public License
// as published by the Free Software Foundation; either version 2
// of the License, or (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program; if not, write to the Free Software
// Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
//
// Linking this library statically or dynamically with other modules is
// making a combined work based on this library. Thus, the terms and
// conditions of the GNU General Public License cover the whole
// combination.
//
// As a special exception, the copyright holders of this library give you
// permission to link this library with independent modules to produce an
// executable, regardless of the license terms of these independent
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// independent module, the terms and conditions of the license of that
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// or based on this library. If you modify this library, you may extend
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// exception statement from your version.
using System;
namespace ICSharpCode.SharpZipLib.Checksums
{
///
/// Computes Adler32 checksum for a stream of data. An Adler32
/// checksum is not as reliable as a CRC32 checksum, but a lot faster to
/// compute.
///
/// The specification for Adler32 may be found in RFC 1950.
/// ZLIB Compressed Data Format Specification version 3.3)
///
///
/// From that document:
///
/// "ADLER32 (Adler-32 checksum)
/// This contains a checksum value of the uncompressed data
/// (excluding any dictionary data) computed according to Adler-32
/// algorithm. This algorithm is a 32-bit extension and improvement
/// of the Fletcher algorithm, used in the ITU-T X.224 / ISO 8073
/// standard.
///
/// Adler-32 is composed of two sums accumulated per byte: s1 is
/// the sum of all bytes, s2 is the sum of all s1 values. Both sums
/// are done modulo 65521. s1 is initialized to 1, s2 to zero. The
/// Adler-32 checksum is stored as s2*65536 + s1 in most-
/// significant-byte first (network) order."
///
/// "8.2. The Adler-32 algorithm
///
/// The Adler-32 algorithm is much faster than the CRC32 algorithm yet
/// still provides an extremely low probability of undetected errors.
///
/// The modulo on unsigned long accumulators can be delayed for 5552
/// bytes, so the modulo operation time is negligible. If the bytes
/// are a, b, c, the second sum is 3a + 2b + c + 3, and so is position
/// and order sensitive, unlike the first sum, which is just a
/// checksum. That 65521 is prime is important to avoid a possible
/// large class of two-byte errors that leave the check unchanged.
/// (The Fletcher checksum uses 255, which is not prime and which also
/// makes the Fletcher check insensitive to single byte changes 0 -
/// 255.)
///
/// The sum s1 is initialized to 1 instead of zero to make the length
/// of the sequence part of s2, so that the length does not have to be
/// checked separately. (Any sequence of zeroes has a Fletcher
/// checksum of zero.)"
///
///
///
public sealed class Adler32 : IChecksum
{
///
/// largest prime smaller than 65536
///
readonly static uint BASE = 65521;
uint checksum;
///
/// Returns the Adler32 data checksum computed so far.
///
public long Value {
get {
return checksum;
}
}
///
/// Creates a new instance of the Adler32 class.
/// The checksum starts off with a value of 1.
///
public Adler32()
{
Reset();
}
///
/// Resets the Adler32 checksum to the initial value.
///
public void Reset()
{
checksum = 1; //Initialize to 1
}
///
/// Updates the checksum with the byte b.
///
///
/// the data value to add. The high byte of the int is ignored.
///
public void Update(int bval)
{
//We could make a length 1 byte array and call update again, but I
//would rather not have that overhead
uint s1 = checksum & 0xFFFF;
uint s2 = checksum >> 16;
s1 = (s1 + ((uint)bval & 0xFF)) % BASE;
s2 = (s1 + s2) % BASE;
checksum = (s2 << 16) + s1;
}
///
/// Updates the checksum with the bytes taken from the array.
///
///
/// buffer an array of bytes
///
public void Update(byte[] buffer)
{
Update(buffer, 0, buffer.Length);
}
///
/// Updates the checksum with the bytes taken from the array.
///
///
/// an array of bytes
///
///
/// the start of the data used for this update
///
///
/// the number of bytes to use for this update
///
public void Update(byte[] buf, int off, int len)
{
if (buf == null) {
throw new ArgumentNullException("buf");
}
if (off < 0 || len < 0 || off + len > buf.Length) {
throw new ArgumentOutOfRangeException();
}
//(By Per Bothner)
uint s1 = checksum & 0xFFFF;
uint s2 = checksum >> 16;
while (len > 0) {
// We can defer the modulo operation:
// s1 maximally grows from 65521 to 65521 + 255 * 3800
// s2 maximally grows by 3800 * median(s1) = 2090079800 < 2^31
int n = 3800;
if (n > len) {
n = len;
}
len -= n;
while (--n >= 0) {
s1 = s1 + (uint)(buf[off++] & 0xFF);
s2 = s2 + s1;
}
s1 %= BASE;
s2 %= BASE;
}
checksum = (s2 << 16) | s1;
}
}
}