lz

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.

Parsers may use different matchers to provide their functionality. One Example is [greedyParser] which can use multiple Matcher implementations.

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

Index

• Variables
• type Block
  • func (b *Block) Len() int64
  • func (b *Block) LenCheck() (n int64, err error)
• type Buffer
  • func (b *Buffer) ByteAt(off int64) (c byte, err error)
  • func (b *Buffer) Init(size int) error
  • func (b *Buffer) Prune(n int) int
  • func (b *Buffer) ReadAt(p []byte, off int64) (n int, err error)
  • func (b *Buffer) ReadFrom(r io.Reader) (n int64, err error)
  • func (b *Buffer) Reset(data []byte) error
  • func (b *Buffer) Write(p []byte) (n int, err error)
• type Decoder
  • func NewDecoder(opts *DecoderOptions) (b *Decoder, err error)
  • func (b *Decoder) ByteAtEnd(off int) byte
  • func (b *Decoder) Init(opts DecoderOptions) error
  • func (b *Decoder) Read(p []byte) (n int, err error)
  • func (b *Decoder) Reset()
  • func (b *Decoder) Write(p []byte) (n int, err error)
  • func (b *Decoder) WriteBlock(blk *Block) (n int, err error)
  • func (b *Decoder) WriteByte(c byte) error
  • func (b *Decoder) WriteMatch(mu, ou uint32) (n int, err error)
  • func (b *Decoder) WriteTo(w io.Writer) (n int64, err error)
• type DecoderOptions
• type MapperType
  • func (mt MapperType) MarshalText() ([]byte, error)
  • func (mt *MapperType) UnmarshalText(text []byte) error
• type Parser
  • func NewParser(opts *ParserOptions) (Parser, error)
• type ParserFlags
• type ParserOptions
• type ParserType
  • func (pt ParserType) MarshalText() ([]byte, error)
  • func (pt *ParserType) UnmarshalText(text []byte) error
• type Seq
  • func (s Seq) Len() int64

Variables

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

ErrEndOfBuffer is returned at the end of the buffer.

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

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

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

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

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

ErrStartOfBuffer is returned at the start of the buffer.

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

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

LenCheck computes the length of the block in bytes and checks that the sum of the literal lengths in the sequences matches the length of the Literals byte slice. If they do not match, an error is returned.

type Buffer

type Buffer struct {
	Data []byte
	// Window end index
	W int
	// maximum buffer size
	Size int
	// offset of Data
	Off int64
}

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

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 int) error

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

func (*Buffer) Prune

func (b *Buffer) Prune(n int) int

Prune cuts the first n bytes from the buffer. If n is larger than the window index W it will be set to W. The number of bytes actually pruned is returned.

func (*Buffer) ReadAt

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

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

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

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(opts *DecoderOptions) (b *Decoder, err error)

NewDecoder creates and initializes a new Decoder.

func (*Decoder) ByteAtEnd

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

ByteAtEnd returns the byte at the end of the buffer.

func (*Decoder) Init

func (b *Decoder) Init(opts DecoderOptions) error

Init initializes the DecoderBuffer.

func (*Decoder) Read

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

Read reads decoded data from the buffer.

func (*Decoder) Reset

func (b *Decoder) Reset()

Reset returns the DecoderBuffer to its initialized state.

func (*Decoder) Write

func (b *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 (b *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

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

WriteByte writes a single byte into the buffer.

func (*Decoder) WriteMatch

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

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

func (*Decoder) WriteTo

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

WriteTo writes the decoded data to the writer.

type DecoderOptions

type DecoderOptions struct {
	// Size of the sliding dictionary window in bytes.
	WindowSize int
	// Maximum size of the buffer in bytes.
	BufferSize int
}

DecoderOptions 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 MapperType

type MapperType int

const (
	Hash MapperType = 1 + iota
)

func (MapperType) MarshalText

func (mt MapperType) MarshalText() ([]byte, error)

func (*MapperType) UnmarshalText

func (mt *MapperType) UnmarshalText(text []byte) error

type Parser

type Parser interface {
	Parse(blk *Block, n int, flags ParserFlags) (parsed int, err error)

	Prune(n int) int
	Write(p []byte) (n int, err error)
	ReadFrom(r io.Reader) (n int64, err error)

	ReadAt(p []byte, off int64) (n int, err error)
	ByteAt(off int64) (c byte, err error)

	Reset(data []byte) error
	Buf() *Buffer

	Options() ParserOptions
}

Parser can parse the underlying byte stream into blocks of sequences.

func NewParser

func NewParser(opts *ParserOptions) (Parser, error)

type ParserFlags

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 ParserOptions

type ParserOptions struct {
	// generic options
	BlockSize      int  `json:",omitzero"`
	WindowSize     int  `json:",omitzero"`
	BufferSize     int  `json:",omitzero"`
	NoPruning      bool `json:",omitzero"`
	MaintainWindow bool `json:",omitzero"`
	MinMatchLen    int  `json:",omitzero"`
	MaxMatchLen    int  `json:",omitzero"`

	// specific parser
	// supported parsers: Greedy
	Parser ParserType `json:",omitzero"`

	// specific mapper
	// supported mappers: Hash
	Mapper MapperType `json:",omitzero"`

	// Options for the Hash mapper.
	InputLen int `json:",omitzero"`
	HashBits int `json:",omitzero"`
}

type ParserType

type ParserType int

const (
	Greedy ParserType = 1 + iota
)

func (ParserType) MarshalText

func (pt ParserType) MarshalText() ([]byte, error)

func (*ParserType) UnmarshalText

func (pt *ParserType) UnmarshalText(text []byte) error

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.

func (Seq) Len

func (s Seq) Len() int64

Len returns the complete length of the sequence in bytes.