public class Adler32 extends Checksum
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).
| Constructor and Description |
|---|
Adler32()
Creates a new instance of the
Adler32 class. |
| Modifier and Type | Method and 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.
|
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.