Package totalcross.util.zip

Class Adler32

java.lang.Object

totalcross.util.zip.Checksum

totalcross.util.zip.Adler32

public class Adler32
extends Checksum

Computes an Adler-32 checksum for a stream of data. An Adler-32 checksum is not as reliable as a CRC-32 checksum, but a lot faster to compute.

The specification for Adler-32 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.)" Changes for TotalCross:
Extends the abstract class Checksum (which was originally an interface).

Since:

JDK 1.1

Constructor Summary

Constructors

Constructor	Description
Adler32()	Creates a new instance of the Adler32 class.

Method Summary

Modifier and Type	Method	Description
long	getValue()	Returns the Adler-32 data checksum computed so far.
void	reset()	Resets the Adler-32 checksum to the initial value.
void	update(byte[] buffer)	Updates the checksum with the bytes taken from the array.
void	update(byte[] buf, int off, int len)	Updates the checksum with the bytes taken from the array.
void	update(int bval)	Updates the checksum with the byte passed as a parameter.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

Adler32

public Adler32()

Creates a new instance of the Adler32 class. The checksum starts off with a value of 1.

Method Detail

reset

public void reset()

Resets the Adler-32 checksum to the initial value.

update

public void update(int bval)

Updates the checksum with the byte passed as a parameter.

Parameters:

bval - The data value to add. The higher byte of the integer is ignored.

update

public void update(byte[] buffer)

Updates the checksum with the bytes taken from the array.

Parameters:

buffer - An array of bytes.

update

public void update(byte[] buf,
                   int off,
                   int len)

Updates the checksum with the bytes taken from the array.

Parameters:

buf - An array of bytes.

off - The start of the data used for this update.

len - The number of bytes to use for this update.

getValue

public long getValue()

Returns the Adler-32 data checksum computed so far.

Returns:

A long with the data checksum computed so far.