Adler32 (GNU Classpath 0.94-pre Documentation)

Class Adler32

public class Adler32 extends 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

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

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(int bval) Updates the checksum with the byte b.
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.

Method Summary

long

getValue()

Returns the Adler32 data checksum computed so far.

void

reset()

Resets the Adler32 checksum to the initial value.

void

update(int bval)

Updates the checksum with the byte b.

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.

Method Detail

public long getValue()

Returns the Adler32 data checksum computed so far.

public void reset()

Resets the Adler32 checksum to the initial value.

public void update(int bval)

Updates the checksum with the byte b.

Parameters: bval the data value to add. The high byte of the int is ignored.

public void update(byte[] buffer)

Updates the checksum with the bytes taken from the array.

Parameters: buffer an array of bytes

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