net.sf.zipme
Class Adler32

java.lang.Object
  net.sf.zipme.Adler32

All Implemented Interfaces:

Checksum

public class Adler32

extends java.lang.Object

implements Checksum

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.)"

Since:

JDK 1.1

See Also:

InflaterInputStream, DeflaterOutputStream

Constructor Summary

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

Method Summary

long	getValue() Returns the Adler32 data checksum computed so far.
void	reset() Resets the Adler32 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 b.

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 Adler32 checksum to the initial value.

Specified by:

reset in interface Checksum

update

public void update(int bval)

Updates the checksum with the byte b.

Specified by:

update in interface Checksum

Parameters:

bval - the data value to add. The high byte of the int 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.

Specified by:

update in interface Checksum

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 Adler32 data checksum computed so far.

Specified by:

getValue in interface Checksum