Class LZ77Compressor
- java.lang.Object
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- org.apache.commons.compress.compressors.lz77support.LZ77Compressor
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public class LZ77Compressor extends Object
Helper class for compression algorithms that use the ideas of LZ77.Most LZ77 derived algorithms split input data into blocks of uncompressed data (called literal blocks) and back-references (pairs of offsets and lengths) that state "add
length
bytes that are the same as those already written startingoffset
bytes before the current position. The details of how those blocks and back-references are encoded are quite different between the algorithms and some algorithms perform additional steps (Huffman encoding in the case of DEFLATE for example).This class attempts to extract the core logic - finding back-references - so it can be re-used. It follows the algorithm explained in section 4 of RFC 1951 (DEFLATE) and currently doesn't implement the "lazy match" optimization. The three-byte hash function used in this class is the same as the one used by zlib and InfoZIP's ZIP implementation of DEFLATE. The whole class is strongly inspired by InfoZIP's implementation.
LZ77 is used vaguely here (as well as many other places that talk about it :-), LZSS would likely be closer to the truth but LZ77 has become the synonym for a whole family of algorithms.
The API consists of a compressor that is fed
byte
s and emitsLZ77Compressor.Block
s to a registered callback where the blocks represent eitherliteral blocks
,back-references
orend of data markers
. In order to ensure the callback receives all information, the#finish
method must be used once all data has been fed into the compressor.Several parameters influence the outcome of the "compression":
windowSize
- the size of the sliding
window, must be a power of two - this determines the maximum
offset a back-reference can take. The compressor maintains a
buffer of twice of
windowSize
- real world values are in the area of 32k. minBackReferenceLength
- Minimal length of a back-reference found. A true minimum of 3 is hard-coded inside of this implementation but bigger lengths can be configured.
maxBackReferenceLength
- Maximal length of a back-reference found.
maxOffset
- Maximal offset of a back-reference.
maxLiteralLength
- Maximal length of a literal block.
- Since:
- 1.14
- See Also:
- "https://tools.ietf.org/html/rfc1951#section-4"
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Nested Class Summary
Nested Classes Modifier and Type Class Description static class
LZ77Compressor.BackReference
Represents a back-reference.static class
LZ77Compressor.Block
Base class representing blocks the compressor may emit.static interface
LZ77Compressor.Callback
Callback invoked while the compressor processes data.static class
LZ77Compressor.EOD
A simple "we are done" marker.static class
LZ77Compressor.LiteralBlock
Represents a literal block of data.
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Constructor Summary
Constructors Constructor Description LZ77Compressor(Parameters params, LZ77Compressor.Callback callback)
Initializes a compressor with parameters and a callback.
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Method Summary
All Methods Instance Methods Concrete Methods Modifier and Type Method Description void
compress(byte[] data)
Feeds bytes into the compressor which in turn may emit zero or more blocks to the callback during the execution of this method.void
compress(byte[] data, int off, int len)
Feeds bytes into the compressor which in turn may emit zero or more blocks to the callback during the execution of this method.void
finish()
Tells the compressor to process all remaining data and signal end of data to the callback.void
prefill(byte[] data)
Adds some initial data to fill the window with.
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Constructor Detail
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LZ77Compressor
public LZ77Compressor(Parameters params, LZ77Compressor.Callback callback)
Initializes a compressor with parameters and a callback.- Parameters:
params
- the parameterscallback
- the callback- Throws:
NullPointerException
- if either parameter isnull
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Method Detail
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compress
public void compress(byte[] data) throws IOException
Feeds bytes into the compressor which in turn may emit zero or more blocks to the callback during the execution of this method.- Parameters:
data
- the data to compress - must not be null- Throws:
IOException
- if the callback throws an exception
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compress
public void compress(byte[] data, int off, int len) throws IOException
Feeds bytes into the compressor which in turn may emit zero or more blocks to the callback during the execution of this method.- Parameters:
data
- the data to compress - must not be nulloff
- the start offset of the datalen
- the number of bytes to compress- Throws:
IOException
- if the callback throws an exception
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finish
public void finish() throws IOException
Tells the compressor to process all remaining data and signal end of data to the callback.The compressor will in turn emit at least one block (
LZ77Compressor.EOD
) but potentially multiple blocks to the callback during the execution of this method.- Throws:
IOException
- if the callback throws an exception
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prefill
public void prefill(byte[] data)
Adds some initial data to fill the window with.This is used if the stream has been cut into blocks and back-references of one block may refer to data of the previous block(s). One such example is the LZ4 frame format using block dependency.
- Parameters:
data
- the data to fill the window with.- Throws:
IllegalStateException
- if the compressor has already started to accept data
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