lz

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 Go to latest Published: Jun 6, 2026 License: BSD-3-Clause Imports: 9 Imported by: 2

README

Module LZ

The LZ module provides sequencers that convert byte streams into blocks of Lempel-Ziv 77 sequences. It is designed to support multiple compression methods that differ in the way they are encoding those LZ77 sequences.

Documentation

Overview

Package lz supports encoding and decoding of LZ77 sequences. A sequence, as described in the Zstandard specification, consists of a literal copy command followed by a match copy command. The literal copy command is described by the length in literal bytes to be copied, and the match command consists of the distance of the match to copy and the length of the match in bytes.

A Parser converts a byte stream into blocks of sequences. The Decoder converts the block of sequences into the original decompressed byte stream.

The module provides multiple parser implementations that offer different combinations of encoding speed and compression ratios. Usually, a slower parser will generate a better compression ratio.

The package has a generic parser that needs a path finder, which selects the sequences for data block and a mapper that finds potential matches in the byte stream.

For optimization we may provide custom implementations that integrate matcher or path finder into the parser and avoids the calling overhead through interfaces.

The library supports the implementation of parsers outside of this package.

Index

Constants

This section is empty.

Variables

View Source 
var ErrEndOfBuffer = errors.New("lz: end of buffer")

ErrEndOfBuffer is returned at the end of the buffer.

View Source 
var ErrFullBuffer = errors.New("lz: full buffer")

ErrFullBuffer is returned when the buffer is full and no more data can be written to it.

View Source 
var ErrOutOfBuffer = errors.New("lz: offset outside of buffer")

ErrOutOfBuffer is returned when the offset is outside of the buffer.

View Source 
var ErrStartOfBuffer = errors.New("lz: start of buffer")

ErrStartOfBuffer is returned at the start of the buffer.

Functions

This section is empty.

Types

type Block 

type Block struct {
	Sequences []Seq
	Literals  []byte
}

Block stores sequences and literals. Note that the sequences stored in the Sequences slice might not consume the entire Literals slice. The remaining literal bytes must be added to the decoded text after all sequences have been decoded.

func (*Block) Len 

func (b *Block) Len() int64

Len computes the length of the block in bytes. It assumes that the sum of the literal lengths in the sequences does not exceed the length of the Literals byte slice.

func (*Block) LenCheck added in v0.6.5 

func (b *Block) LenCheck() (n int64, err error)

LenCheck computes the length of the block in bytes and verifies that the sum of the literal lengths in the sequences is less than the bytes in the Literals field. If that is not the case an error is returned.

type Buffer 

type Buffer struct {
	Data []byte
	// Window end index
	W int
	// maximum buffer size
	Size int
	// RetentionSize is the number of bytes to keep when pruning the buffer.
	RetentionSize int
	// offset of Data
	Off int64

	// Shift will be called when the buffer is pruned to inform other
	// components about the number of bytes removed from the start of
	// the buffer.
	Shift ShiftFunc
}

Buffer is the Buffer used for LZ parsing.

The Off field describes the offset of Data[0] in the original stream. The W points to the end of sliding window used for copying matches.

Data is not fully allocated at the beginning. It grows with the usage. There must be always 7 extra bytes allocated at the end of Data to allow easy reads of data from the Buffer.

func (*Buffer) ByteAt added in v0.5.0 

func (b *Buffer) ByteAt(off int64) (c byte, err error)

ByteAt returns the byte at offset off. If off is outside of the buffer, ErrOutOfBuffer is returned.

func (*Buffer) Init 

func (b *Buffer) Init(size, retentionSize int, shift ShiftFunc) error

Init initializes the buffer. The old data slice is reused and the capacity might be larger than the new buffer size.

func (*Buffer) Parsable added in v0.6.11 

func (b *Buffer) Parsable() int

Parsable returns the number of bytes that can be parsed from the buffer.

func (*Buffer) ReadAt added in v0.5.0 

func (b *Buffer) ReadAt(p []byte, off int64) (n int, err error)

ReadAt reads len(p) bytes from the buffer starting at byte offset off. It returns the number of bytes read and any error encountered. If off is outside of the buffer, ErrOutOfBuffer is returned.

func (*Buffer) ReadFrom added in v0.5.0 

func (b *Buffer) ReadFrom(r io.Reader) (n int64, err error)

ReadFrom reads data from r until EOF or error. It returns the number of bytes read and any error encountered.

func (*Buffer) Reset 

func (b *Buffer) Reset(data []byte) error

Reset resets the buffer with the provided data slice. If the data slice is larger than the buffer size, the buffer size will be updated. Note that the data slice should have 7 extra bytes, len(data)+7 <= cap(data). Otherwise the old slice will be used or a new one need to be allocated.

func (*Buffer) Write 

func (b *Buffer) Write(p []byte) (n int, err error)

Write writes data to the buffer. If not all data can be written, ErrFullBuffer is returned.

type Decoder 

type Decoder struct {
	// Data is the actual buffer. The end of the slice is also the head of
	// the dictionary window.
	Data []byte
	// R tracks the position of the reads from the buffer and must be less
	// or equal to the length of the Data slice.
	R int
	// Off records the total offset and marks the end of the Data slice,
	// which is also the end of the dictionary window.
	Off int64

	// DecoderConfig provides the configuration parameters WindowSize and
	// BufferSize.
	DecoderConfig
}

Decoder provides a simple buffer for decoding LZ77 sequences. Data is the actual buffer. The end of the slice is also the head of the dictionary window. R tracks the read position in the buffer and must be less than or equal to the length of the Data slice. Off records the total offset and marks the end of the Data slice, which is also the end of the dictionary window. DecoderConfig provides the configuration parameters WindowSize and BufferSize.

func NewDecoder 

func NewDecoder(cfg DecoderConfig) (*Decoder, error)

NewDecoder creates a new Decoder with the given options. If no options are provided, the default values are used. The default WindowSize is 8 MiB and the default BufferSize is 16 MiB. The function returns an error if the options are invalid.

func (*Decoder) Available added in v0.7.6 

func (d *Decoder) Available() int

Available returns the number of bytes that can be written to the Decoder. The function returns 0 if the buffer is full.

func (*Decoder) ByteAtEnd added in v0.6.0 

func (d *Decoder) ByteAtEnd(off int) byte

ByteAtEnd returns the byte at the end of the buffer.

func (*Decoder) Init 

func (d *Decoder) Init(cfg DecoderConfig) error

Init initializes the DecoderBuffer using the provided options. If no options are provided, the default values are used. The default WindowSize is 8 MiB and the default BufferSize is 16 MiB. The function returns an error if the options are invalid.

func (*Decoder) Read added in v0.6.0 

func (d *Decoder) Read(p []byte) (n int, err error)

Read reads decoded data from the buffer.

func (*Decoder) Reset 

func (d *Decoder) Reset()

Reset returns the DecoderBuffer to its initialized state.

func (*Decoder) Write 

func (d *Decoder) Write(p []byte) (n int, err error)

Write inserts the slice into the buffer. The method will write the entire slice or return 0 and ErrFullBuffer.

func (*Decoder) WriteBlock 

func (d *Decoder) WriteBlock(blk *Block) (n int, err error)

WriteBlock writes sequences from the block into the buffer. Each sequence is written atomically, as the block value is not modified. If there is not enough space in the buffer, ErrFullBuffer will be returned. All written sequences and literals will be removed from the block.

The capacity of the block slices will not be maintained. You have to keep a copy of the block to achieve that.

The growth of the array is limited to BufferSize.

The function returns the number of bytes written.

func (*Decoder) WriteByte added in v0.2.1 

func (d *Decoder) WriteByte(c byte) error

WriteByte writes a single byte into the buffer.

func (*Decoder) WriteMatch 

func (d *Decoder) WriteMatch(mu, ou uint32) (n int, err error)

WriteMatch appends the match to the end of the buffer. The match will be written completely, or n=0 and ErrFullBuffer will be returned.

func (*Decoder) WriteTo added in v0.6.0 

func (d *Decoder) WriteTo(w io.Writer) (n int64, err error)

WriteTo writes the decoded data to the writer.

type DecoderConfig added in v0.3.0 

type DecoderConfig struct {
	// Size of the sliding dictionary window in bytes.
	WindowSize opt.Value[int]
	// Maximum size of the buffer in bytes.
	BufferSize int
}

DecoderConfig contains the parameters for the DecoderBuffer and decoder types. WindowSize must be smaller than BufferSize. It is recommended to set BufferSize to twice the WindowSize.

type Entry added in v0.6.3 

type Entry struct {
	// contains filtered or unexported fields
}

Entry is returned by a Mapper for a found match. It provides the position i of the match in the Data slice of the buffer and v contains the leading 4 bytes of the match to avoid a lookup in the buffer.

type Mapper added in v0.6.3 

type Mapper interface {
	// InputLen returns the length of the input data into the table. We are
	// supporting length from 2 to 8 bytes.
	InputLen() int

	// Reset resets the internal state of the mapper.
	Reset()

	// Shift is called by the number of bytes pruned from the buffer and
	// provide the new extended buffer to the mapper.
	Shift(delta int)

	// Put adds all values between a and w to the mapper. We assume that
	// cap(p) >= len(p) + 7.
	Put(p []byte, a, w int) int

	// Get returns all candidate entries for the provided hash value. The
	// entry value v contains the all 4 bytes stored a position i.
	Get(v uint64) []Entry
}

Mapper provides potential matches for a given position in the byte stream. Is it usually implemented by hash tables.

func NewMapper added in v0.7.1 

func NewMapper(name string) (Mapper, error)

NewMapper creates a new Mapper for the provided name of the algorithm. The mappers supported are described below.

hash_<inputLen>:<hashBits> A hash table with the provided input length and hash bits. The input length is between 2 and 8 bytes, and the hash bits can be 24 bits at maximum.

type Matcher added in v0.6.0 

type Matcher interface {
	// Edges returns the potential sequence at the current position.
	Edges(n int) []Seq

	// Skip is called to advance the current position by n bytes. An error
	// is only returned if the there are not enough bytes in the buffer.
	// Note that n can be negative, to allow to set the current position
	// backwards.
	Skip(n int) (skipped int, err error)

	// Parsable returns the number of bytes that are available in the buffer
	// for parsing.
	Parsable() int
}

Matcher is responsible to find matches or Literal bytes in the byte stream. It is only relevant for the PathFinder.

type Parser added in v0.3.0 

type Parser interface {
	// Parse up to block size bytes from the internal buffer and provides
	// the sequences in the block structure. While slices will be reused,
	// not old information will be maintained.
	Parse(blk *Block, n int, flags ParserFlags) (parsed int, err error)

	// Write writes data into the internal buffer.
	Write(p []byte) (n int, err error)

	// ReadFrom reads data from the provided reader into the internal
	// buffer.
	ReadFrom(r io.Reader) (n int64, err error)

	// ReadAt reads len(p) bytes from the internal buffer at offset off.
	ReadAt(p []byte, off int64) (n int, err error)

	// ByteAt returns the byte at offset off in the internal buffer.
	ByteAt(off int64) (c byte, err error)

	// Reset resets the internal buffer to the provided data.
	Reset(data []byte) error

	// Config returns the parser configuration.
	Config() ParserConfig
}

Parser provides the possibility to parse a byte stream into LZ77 sequences.

func NewParser added in v0.6.4 

func NewParser(cfg ParserConfig) (Parser, error)

type ParserConfig added in v0.3.0 

type ParserConfig struct {
	// PathFinder describes the algorithm to find a path through the
	// possible matches at each position of the encoding process.
	//
	// Support path finders are:
	//   * greedy
	PathFinder string `json:",omitzero"`

	// Mapper describes the algorithm to find potential matches in the byte
	// stream. Support mappers are:
	//   * hash_<inputLen>:<hashBits> A hash table with the provided input
	//     length and hash bits. The input length is between 2 and 8 bytes,
	//     and the hash bits can be 24 bits at maximum.
	Mapper string `json:",omitzero"`

	// WindowSize is the maximum distance of a match to be copied. The
	// window size is allowed to be 0, for experimental purposes.
	WindowSize opt.Value[int] `json:",omitzero"`

	// RetentionSize is the number of bytes that are kept if the buffer is
	// full and the data needs to be shifted. The retention size can be 0.
	RetentionSize opt.Value[int] `json:",omitzero"`

	// BufferSize is the maximum size of the internal buffer for the parser.
	// It must at least support 1 byte.
	BufferSize int `json:",omitzero"`

	// MinMatchLen is the minimum match length for the parser. The minimum
	// match length must be less than or equal to the maximum match length
	// and must be at least 2 bytes.
	MinMatchLen int `json:",omitzero"`

	// MaxMatchLen is the maximum match length for the parser. The maximum
	// match length must be greater than or equal to the minimum match
	// length and must be at least 2 bytes.
	MaxMatchLen int `json:",omitzero"`
}

ParserConfig provides the configuration for a parser.

type ParserFlags added in v0.6.0 

type ParserFlags int

ParserFlags define optional parser behavior. 

const (
	// NoTrailingLiterals indicates that the parser should not generate
	// trailing literal bytes in the output.
	NoTrailingLiterals ParserFlags = 1 << iota
)

type PathFinder added in v0.7.0 

type PathFinder interface {
	Parse(block *Block, n int, flags ParserFlags) (parsed int, err error)
	Reset()
}

PathFinder implements the central Parse Function of the Parser.

func NewPathFinder added in v0.7.1 

func NewPathFinder(name string, m Matcher) (PathFinder, error)

NewPathFinder creates a new PathFinder for the provided name of the algorithm and the Matcher. The path finders supported are described below.

greedy The greedy path finder selects the longest match at each position.

type Seq 

type Seq struct {
	LitLen   uint32
	MatchLen uint32
	Offset   uint32
	Aux      uint32
}

Seq represents a single Lempel-Ziv 77 sequence describing a match, consisting of the offset, the length of the match, and the number of literals preceding the match. The Aux field can be used in upper layers to store additional information. One use case is to store up to 4 bytes of literals.

func (Seq) Len 

func (s Seq) Len() int64

Len returns the complete length of the sequence in bytes.

type ShiftFunc added in v0.6.9 

type ShiftFunc func(delta int)

ShiftFunc defines the type of the shift function, called when the buffer is pruned to provide more available space.

Source Files

View all Source files

Directories

Path Synopsis
Package lz supports encoding and decoding of LZ77 sequences.
 Package lz supports encoding and decoding of LZ77 sequences.
Package suffix provides a suffix sort algorithm.
 Package suffix provides a suffix sort algorithm.

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