org.metastatic.rsync
Class Checksum32

java.lang.Object
  |
  +--org.metastatic.rsync.Checksum32

All Implemented Interfaces:

Cloneable, RollingChecksum, Serializable

public class Checksum32

extends Object

implements RollingChecksum, Cloneable, Serializable

A simple 32-bit "rolling" checksum. This checksum algorithm is based upon the algorithm outlined in the paper "The rsync algorithm" by Andrew Tridgell and Paul Mackerras. The algorithm works in such a way that if one knows the sum of a block X_k...X_l, then it is a simple matter to compute the sum for X_k+1...X_l+1.

See Also:

Serialized Form

Field Summary

protected int	a The first half of the checksum.
protected int	b The second half of the checksum.
protected byte[]	block The block from which the checksum is computed.
protected int	char_offset
protected int	k The place from whence the current checksum has been computed.
protected int	l The place to where the current checksum has been computed.
protected byte[]	new_block The block that is recieving new input.
protected int	new_index The index in new_block where the newest byte has been stored.

Constructor Summary

Checksum32()

Checksum32(int char_offset)
Creates a new rolling checksum.

Method Summary

void	check(byte[] buf, int off, int len) Update the checksum with an entirely different block, and potentially a different block length.
Object	clone() Copies this checksum instance into a new instance.
boolean	equals(Object o) Tests if a particular checksum is equal to this checksum.
int	getValue() Return the value of the currently computed checksum.
void	reset() Reset the checksum.
void	roll(byte bt) "Roll" the checksum.
void	trim() Update the checksum by trimming off a byte only, not adding anything.

Methods inherited from class java.lang.Object

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

Field Detail

char_offset

protected final int char_offset

a

protected int a

The first half of the checksum.

Since:

1.1

b

protected int b

The second half of the checksum.

Since:

1.1

k

protected int k

The place from whence the current checksum has been computed.

Since:

1.1

l

protected int l

The place to where the current checksum has been computed.

Since:

1.1

block

protected byte[] block

The block from which the checksum is computed.

Since:

1.1

new_index

protected int new_index

The index in new_block where the newest byte has been stored.

Since:

1.1

new_block

protected byte[] new_block

The block that is recieving new input.

Since:

1.1

Constructor Detail

Checksum32

public Checksum32(int char_offset)

Creates a new rolling checksum. The char_offset argument affects the output of this checksum; rsync uses a char offset of 0, librsync 31.

Checksum32

public Checksum32()

Method Detail

getValue

public int getValue()

Return the value of the currently computed checksum.

Specified by:

getValue in interface RollingChecksum

Returns:

The currently computed checksum.

Since:

1.1

reset

public void reset()

Reset the checksum.

Specified by:

reset in interface RollingChecksum

Since:

1.1

roll

public void roll(byte bt)

"Roll" the checksum. This method takes a single byte as byte X_l+1, and recomputes the checksum for X_k+1...X_l+1. This is the preferred method for updating the checksum.

Specified by:

roll in interface RollingChecksum

Parameters:

bt - The next byte.

Since:

1.1

trim

public void trim()

Update the checksum by trimming off a byte only, not adding anything.

Specified by:

trim in interface RollingChecksum

check

public void check(byte[] buf,
                  int off,
                  int len)

Update the checksum with an entirely different block, and potentially a different block length.

Specified by:

check in interface RollingChecksum

Parameters:

buf - The byte array that holds the new block.

off - From whence to begin reading.

len - The length of the block to read.

Since:

1.1

clone

public Object clone()

Description copied from interface: RollingChecksum

Copies this checksum instance into a new instance. This method should be optional, and only implemented if the class implements the Cloneable interface.

Specified by:

clone in interface RollingChecksum

Overrides:

clone in class Object

Returns:

A clone of this instance.

equals

public boolean equals(Object o)

Description copied from interface: RollingChecksum

Tests if a particular checksum is equal to this checksum. This means that the other object is an instance of this class, and its internal state equals this checksum's internal state.

Specified by:

equals in interface RollingChecksum

Overrides:

equals in class Object

Parameters:

o - The object to test.

Returns:

true if this checksum equals the other checksum.