Class Adler32

java.lang.Object
net.sf.jazzlib.Adler32
All Implemented Interfaces:
Checksum
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 invalid input: '<'-> 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:

  • Field Summary

    Fields
    Modifier and Type
    Field
    Description
    private static final int
    BASE
    largest prime smaller than 65536
    private int
    checksum
     

  • 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 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 Object

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

  • Field Details

    • BASE

      private static final int BASE
      largest prime smaller than 65536
      See Also:

    • checksum

      private int checksum

  • Constructor Details

    • Adler32

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

  • Method Details

    • 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