org.mapdb

Class CompressLZF

java.lang.Object

org.mapdb.CompressLZF

public final class CompressLZF
extends java.lang.Object

This class implements the LZF lossless data compression algorithm. LZF is a Lempel-Ziv variant with byte-aligned output, and optimized for speed.

Safety/Use Notes:

• Each instance should be used by a single thread only.
• The data buffers should be smaller than 1 GB.
• For performance reasons, safety checks on expansion are omitted.
• Invalid compressed data can cause an ArrayIndexOutOfBoundsException.

The LZF compressed format knows literal runs and back-references:

• Literal run: directly copy bytes from input to output.
• Back-reference: copy previous data to output stream, with specified offset from location and length. The length is at least 3 bytes.

The first byte of the compressed stream is the control byte. For literal runs, the highest three bits of the control byte are not set, the the lower bits are the literal run length, and the next bytes are data to copy directly into the output. For back-references, the highest three bits of the control byte are the back-reference length. If all three bits are set, then the back-reference length is stored in the next byte. The lower bits of the control byte combined with the next byte form the offset for the back-reference.

Field Summary

Fields

Modifier and Type	Field and Description
private int[]	cachedHashTable Hash table for matching byte sequences (reused for performance).
private static int	HASH_SIZE The number of entries in the hash table.
private static int	MAX_LITERAL The maximum number of literals in a chunk (32).
private static int	MAX_OFF The maximum offset allowed for a back-reference (8192).
private static int	MAX_REF The maximum back-reference length (264).

Constructor Summary

Constructors

Constructor and Description
CompressLZF()

Method Summary

Modifier and Type	Method and Description
int	compress(byte[] in, int inLen, byte[] out, int outPos)
void	expand(java.nio.ByteBuffer in, int inPos, byte[] out, int outPos, int outLen)
void	expand(java.io.DataInput in, byte[] out, int outPos, int outLen)
private static int	first(byte[] in, int inPos) Return byte with lower 2 bytes being byte at index, then index+1.
private static int	hash(int h) Compute the address in the hash table.
private static int	next(int v, byte[] in, int inPos) Shift v 1 byte left, add value at index inPos+2.

Methods inherited from class java.lang.Object

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

Field Detail

HASH_SIZE

private static final int HASH_SIZE

The number of entries in the hash table. The size is a trade-off between hash collisions (reduced compression) and speed (amount that fits in CPU cache).

See Also:

Constant Field Values

MAX_LITERAL

private static final int MAX_LITERAL

The maximum number of literals in a chunk (32).

See Also:

Constant Field Values

MAX_OFF

private static final int MAX_OFF

The maximum offset allowed for a back-reference (8192).

See Also:

Constant Field Values

MAX_REF

private static final int MAX_REF

The maximum back-reference length (264).

See Also:

Constant Field Values

cachedHashTable

private int[] cachedHashTable

Hash table for matching byte sequences (reused for performance).

Constructor Detail

CompressLZF

public CompressLZF()

Method Detail

first

private static int first(byte[] in,
                         int inPos)

Return byte with lower 2 bytes being byte at index, then index+1.

next

private static int next(int v,
                        byte[] in,
                        int inPos)

Shift v 1 byte left, add value at index inPos+2.

hash

private static int hash(int h)

Compute the address in the hash table.

compress

public int compress(byte[] in,
                    int inLen,
                    byte[] out,
                    int outPos)

expand

public void expand(java.io.DataInput in,
                   byte[] out,
                   int outPos,
                   int outLen)
            throws java.io.IOException

Throws:

java.io.IOException

expand

public void expand(java.nio.ByteBuffer in,
                   int inPos,
                   byte[] out,
                   int outPos,
                   int outLen)