public class Adler32 extends Checksum
The specification for Adler32 may be found in RFC 1950. (ZLIB Compressed Data Format Specification version 3.3).
From that document:
"ADLER32 (Adler32 checksum) This contains a checksum value of the uncompressed data (excluding any dictionary data) computed according to Adler32 algorithm. This algorithm is a 32bit extension and improvement of the Fletcher algorithm, used in the ITUT X.224 / ISO 8073 standard.
Adler32 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 Adler32 checksum is
stored as s2*65536 + s1
in most significantbyte first (network) order."
"8.2. The Adler32 algorithm
The Adler32 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
twobyte 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).
Constructor and Description 

Adler32()
Creates a new instance of the
Adler32 class. 
Modifier and Type  Method and Description 

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 passed as a parameter.

public Adler32()
Adler32
class. The checksum starts off with a value of 1.public void reset()
public void update(int bval)
bval
 The data value to add. The higher byte of the integer is ignored.public void update(byte[] buffer)
buffer
 An array of bytes.public void update(byte[] buf, int off, int len)
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.public long getValue()
long
with the data checksum computed so far.