vrs::Compressor Class Reference

Helper class to compress data using lz4 or zstd presets. More...

#include <Compressor.h>

Classes
class	CompressorImpl
struct	uninitialized_byte

Public Member Functions
uint32_t	compress (const void *data, size_t dataSize, CompressionPreset preset, size_t headerSpace=0)
int	startFrame (size_t frameSize, CompressionPreset zstdPreset, uint32_t &outSize)
int	addFrameData (WriteFileHandler &file, const void *data, size_t dataSize, uint32_t &inOutCompressedSize, size_t maxCompressedSize=0)
int	endFrame (WriteFileHandler &file, uint32_t &inOutCompressedSize, size_t maxCompressedSize=0)
const void *	getData () const
template<class HeaderType >
HeaderType *	getHeader ()
	Get the space reserved for a header.
CompressionType	getCompressionType () const
void	clear ()
	Really deallocate the buffer's memory (clear() doesn't do that)

Static Public Member Functions
static bool	shouldTryToCompress (CompressionPreset preset, size_t size)

Static Public Attributes
static const size_t	kMinByteCountForCompression = 250

Detailed Description

Helper class to compress data using lz4 or zstd presets.

You can switch between presets at no cost. If presets don't work well enough with your data, you can easily experiment with new settings and add your own preset to CompressionPreset.

Member Function Documentation

addFrameData()

int vrs::Compressor::addFrameData	(	WriteFileHandler &	file,
		const void *	data,
		size_t	dataSize,
		uint32_t &	inOutCompressedSize,
		size_t	maxCompressedSize = 0
	)

Add data to a frame started before. The total amount of data may not exceed the declared size of the frame when it was started.

Parameters

file	A file open for writing.
data	A pointer to the data to add to the frame.
dataSize	Number of bytes to add to the frame.
inOutCompressedSize	Updated number of bytes written out to the file.
maxCompressedSize	Max number of bytes the compressed record may get. Fail if the compressed data is larger, while guarantying that fewer bytes have been written to disk.

Returns

0 for success, or an error code. The input buffer may be recycled immediately, but all the data may not have been written out to the file yet. You may add all the data of the frame in as many calls as you wish, one byte at a time, all at once, and anything in between.

compress()

uint32_t vrs::Compressor::compress	(	const void *	data,
		size_t	dataSize,
		CompressionPreset	preset,
		size_t	headerSpace = 0
	)

Compress some data using a specific preset. If the compression setting doesn't lead to a smaller payload, because the data can't be compressed, then the record won't be compressed at all.

Parameters

data	Pointer to the data to compress.
dataSize	Number of bytes in the buffer to compress.
preset	Compression preset to use.
headerSpace	Number of bytes to reserve at the beginning of the buffer for a header initialized manually later.

Returns

The number of bytes of compressed data, or 0 in case of failure.

endFrame()

int vrs::Compressor::endFrame	(	WriteFileHandler &	file,
		uint32_t &	inOutCompressedSize,
		size_t	maxCompressedSize = 0
	)

Write out all the data left in internal compression buffers to disk, and complete the frame. After this call has been made, a new frame maybe started.

Parameters

file	A file open for writing.
inOutCompressedSize	Updated number of bytes written out to the file.
maxCompressedSize	Max number of bytes the compressed record may get. Fail if the compressed data is larger, while guarantying that fewer bytes have been written to disk.

Returns

0 for success, or an error code. After this call, inOutSize counts the total number of bytes used for the compressed frame, including compression metadata. This number should be smaller than the frame size, but might be slightly larger, if the data couldn't be compressed.

getData()

const void * vrs::Compressor::getData ( ) const

inline

Get the compressed data after compression. The size to consider was returned by compress().

Returns

Pointer to the compressed data.

startFrame()

int vrs::Compressor::startFrame	(	size_t	frameSize,
		CompressionPreset	zstdPreset,
		uint32_t &	outSize
	)

Frame compression APIs, with streaming to a file. Write to a file a block of data (a "frame") to be compressed. That data will be logically self contained, and its size will be retrievable when decoding the first byte. The frame can be added in multiple calls, making it easy to write a lot of data without needing large intermediate buffers.

Start a new frame, declaring its full size upfront. This size must be respected.

Parameters

frameSize	exact number of bytes that will be added to the frame in total.
zstdPreset	compression preset to use. Only zstd presets are supported by this API.
outSize	total number of compressed bytes written.

Returns

0 for success, or an error code. This call initializes outSize to 0.

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The documentation for this class was generated from the following files:

• vrs/Compressor.h
• vrs/Compressor.cpp