Description:

This class provides several algorithms to compress arrays of integral data and encode them in BitBuffer objects. Besides a standard Huffman compression , different simple approaches are "tested" and the best compression algorithm is then chosen. The general assumption of the approaches (besides the Huffman coding) is that the data contains "signal data", which is either

sparsely filled,
has incremental values, or
has just values of a certain smaller range. Also, combinations of these attributes are matched. Such data is often found in real-world applications and may be compressed much better than the generic Huffman approach may achieve.

Definition at line 51 of file ac.hpp.

#include <ac.hpp>

Inner Type Index:
class	Array

struct	Statistics

Public Type Index:
enum class	Algorithm { NONE = 0 , ALL = (1<< NumberOfAlgorithms) - 1 , Uncompressed = 1 , MinMax = 2 , Sparse = 4 , VerySparse = 8 , Incremental = 16 , Huffman = 32 , END_OF_ENUM = 64 }

Public Static Field Index:
static constexpr int	NumberOfAlgorithms = 6
	The number of algorithms implemented.

Public Static Method Index:
template<typename TValue >
static std::pair< size_t, Algorithm >	Compress (BitWriter &bitWriter, Array< TValue > &data, Algorithm algorithmsToTry=Algorithm::ALL, Statistics *statistics=nullptr)

template<typename TValue >
static void	Decompress (BitReader &bitReader, Array< TValue > &data, Algorithm algorithm=Algorithm::ALL, Statistics *statistics=nullptr)

Public Method Index:
	ArrayCompressor ()=delete
	Deleted default constructor (this class cannot be created)

Enumeration Details:

◆ Algorithm

enum class Algorithm

strong

This enumeration denotes the different algorithms provided for compression. This enum is defined to be bitwise.

Note: With the inclusion of at least one "ALib Camp" in the ALib Distribution, this enum is furthermore resourced and values are appendable to class AString and logable with ALox.

Enumerator
NONE	No compression method selected.
ALL	All compression methods selected.
Uncompressed	Stores the data as integer values, which includes a simple sort of possible compression as documented with BitWriter::Write<TIntegral>(TIntegral).
MinMax	Stores the differences between the minimum and maximum value found.
Sparse	Writes '1' if next value is equal to previous, '0' plus next value otherwise.
VerySparse	Writes the number of following equal or non equal values.
Incremental	Only distances of the values are written.
Huffman	Huffman encoding (byte based).

Definition at line 266 of file ac.hpp.

Field Details:

◆ NumberOfAlgorithms

int NumberOfAlgorithms = 6

staticconstexpr

The number of algorithms implemented.

Definition at line 54 of file ac.hpp.

Method Details:

◆ Compress()

template<typename TValue >

static std::pair< size_t, Algorithm > Compress	(	BitWriter &	bitWriter,
		Array< TValue > &	data,
		Algorithm	algorithmsToTry = Algorithm::ALL,
		Statistics *	statistics = nullptr )

static

Compresses the given array and writes the data into the given bit writer. Each algorithm included in parameter algorithmsToTry are executed and finally that one with the best compression result is chosen. Before the usage data, some bits that determine the chosen algorithm are written, to enable method Decompress to deserialize the data.

To gain efficiency, the number of probed algorithms can be narrowed by setting a corresponding mask in algorithmsToTry. However, in many use case scenarios, the execution time is a less critical design factor than the compression factor reached. The decompression speed is solely dependent on the algorithm finally chosen, not on the number of algorithms tested on compression.

Attention: If only one algorithm is specified in parameter algorithmsToTry, then no meta-information about the algorithm chosen is written. Consequently, when reading back the data using Decompress, the same single algorithm has to be provided.

Template Parameters

TValue The integral type of array data to compress.

Parameters

bitWriter	A bit writer to compress the data to.
data	The array to compress.
algorithmsToTry	The set of algorithms to be tried on compression for best efficiency. Defaults to Algorithm::ALL.
statistics	Pointer a struct to collect statistics for the efficiency of array compression related to given user data. If set, methods Compress and Decompress will measure execution performance and compression rates for each algorithm. With that, a software may collect information about which algorithm is most efficient for typical datasets found and a programmer may, based on such heuristics decide to exclude certain algorithms not efficient in a use case.

Returns: A pair of value containing the resulting size in bits and the algorithm chosen.

◆ Decompress()

template<typename TValue >

static void Decompress	(	BitReader &	bitReader,
		Array< TValue > &	data,
		Algorithm	algorithm = Algorithm::ALL,
		Statistics *	statistics = nullptr )

static

Decompresses an integral array from the given bit reader, which previously was encoded with methods Compress. The integral data type has to be the same as with encoding.

Attention: If compression was performed with specifying only one algorithm in parameter algorithmsToTry, then the same algorithm has to be exclusively set on decompression, because in this case no meta-information about the compression algorithm is stored in the bit stream.

Template Parameters

TValue The integral type of array data to decompress.

Parameters

bitReader	A bit reader to read the data from.
data	The array to decompress data to.
algorithm	The algorithm to use for read back. Must only be given, in case that compression was performed using a single algorithm of choice. Defaults to Algorithm::ALL.
statistics	An optional statistics record to store the measured de-compression time. See method Compress for more information.

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

ac.hpp

Description:

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