A dictionary for the encoding of a bytestream using the LZW algorithm. More...

#include <LSCLzwEncodingDict.h>

Classes
struct	LSN_LZW_DICT_ENTRY
Public Types
enum	LSN_LZW_INVALID_CODE { LSN_LZW_INVALID_CODE = ~0UL, LSN_LZW_FORCE_DWORD = 0x7FFFFFFFUL }
typedef struct lsc::CLzwEncodingDict::LSN_LZW_DICT_ENTRY *	LPLSN_LZW_DICT_ENTRY
typedef struct lsc::CLzwEncodingDict::LSN_LZW_DICT_ENTRY	LPCLSN_LZW_DICT_ENTRY
Public Member Functions
LSVOID LSE_CALL	Reset ()
LSVOID LSE_CALL	SetAllocator (CStdAllocator *_psaAllocator)
LSBOOL LSE_CALL	InitDict (LSUINT32 _ui32RootCodes, LSUINT32 _ui32MaxBits)
LSUINT32 LSE_CALL	GetEntryIndex (LSUINT32 _ui32Code, LSUINT32 _ui32Char) const
const LSN_LZW_DICT_ENTRY &LSE_CALL	GetEntry (LSUINT32 _ui32Index) const
LSVOID LSE_CALL	AddEntry (LSUINT32 _ui32Index, LSUINT32 _ui32BaseCode, LSUINT32 _ui32Char)
LSUINT32 LSE_CALL	TotalEntries () const
LSUINT32 LSE_CALL	GetMaxDictionarySize () const
LSUINT32 LSE_CALL	GetStartingBits () const
LSUINT32 LSE_CALL	GetResetCode () const
LSUINT32 LSE_CALL	GetEndStreamCode () const
LSUINT32 LSE_CALL	GetBits () const
Protected Attributes
CAllocator	m_caAllocator
LSUINT32	m_ui32AllocatedDictSize
LSUINT32	m_ui32MaxBits
LSUINT32	m_ui32DictSize
LSUINT16	m_ui16MinBits
LSUINT16	m_ui16StartingBits
LSUINT32	m_ui32RootCodes
LPLSN_LZW_DICT_ENTRY	m_lpldeDictionary

Detailed Description

A dictionary for the encoding of a bytestream using the LZW algorithm.

Class CLzwEncodingDict Description: When encoding bytes, the dictionary for the LZW algorithm is stored differently from when the bytes are being decoded. During encryption, codes are referenced via (code+char) combination which requires a hash look-up. For this reason, we cannot store the codes in such a way that index = code. During decoding, however, codes are referenced only by code, so we are able to store the codes linearly such that index = code. This saves space over the encoding algorithm and gives us cause to keep the two dictionaries separate. The dictionary is created with a maximum size at the beginning, which depends on the maximum number of bits a code may have. This class can output variable-sized codes or predefined-sized codes as per the desires of the user. Variable-sized codes simply means that the smallest number of bits needed to access any code in the dictionary are output instead of using a fixed size such as 15 bits for every code. This is the typical method for encoding.

Member Typedef Documentation

typedef struct lsc::CLzwEncodingDict::LSN_LZW_DICT_ENTRY * lsc::CLzwEncodingDict::LPLSN_LZW_DICT_ENTRY

A dictionary entry.

Member Enumeration Documentation

enum lsc::CLzwEncodingDict::LSN_LZW_INVALID_CODE

Flags we use.

Member Function Documentation

LSVOID LSE_CALL lsc::CLzwEncodingDict::AddEntry	(	LSUINT32	_ui32Index,
		LSUINT32	_ui32BaseCode,
		LSUINT32	_ui32Char
	)

Add an entry at a given index. The index must have been returned by a previous call to GetEntryIndex() and must be unused.

Parameters:

_ui32Index	Index, returned by a previous call to GetEntryIndex(), where the code is to be added.
_ui32BaseCode	The base code of the new code to be added. The base code is the code of an existing entry off which this entry is based. This entry extends the base code by one character, given in _ui32Char.
_ui32Char	The character by which the new code extends the base code.

LSUINT32 LSE_CALL lsc::CLzwEncodingDict::GetBits ( ) const

Get the bits needed to read any code in the dictionary.

Returns:: Returns the bits needed to read any code in the dictionary.

LSUINT32 LSE_CALL lsc::CLzwEncodingDict::GetEndStreamCode ( ) const

Get the end-stream code.

Returns:: Returns the end-stream code.

const LSN_LZW_DICT_ENTRY& LSE_CALL lsc::CLzwEncodingDict::GetEntry ( LSUINT32 _ui32Index ) const

Get an entry by index for read only. The entry is not guaranteed to contain a valid code.

Parameters:

_ui32Index Index of the entry to retrieve.

Returns:: Returns a constant reference to the entry at the given index.

LSUINT32 LSE_CALL lsc::CLzwEncodingDict::GetEntryIndex	(	LSUINT32	_ui32Code,
		LSUINT32	_ui32Char
	)		const

Get the index of an entry in the dictionary. If the entry does not exist, the returned index is where it should be added to the dictionary.

Parameters:

_ui32Code	The code of the entry to find.
_ui32Char	The allocator to use by this class.

Returns:: Returns false if allocation of the dictionary failed, or true otherwise.

LSUINT32 LSE_CALL lsc::CLzwEncodingDict::GetMaxDictionarySize ( ) const

Get the maximum dictionary size.

Returns:: Returns the maximum dictionary size.

LSUINT32 LSE_CALL lsc::CLzwEncodingDict::GetResetCode ( ) const

Get the reset-dictionary code.

Returns:: Returns the reset-dictionary code.

LSUINT32 LSE_CALL lsc::CLzwEncodingDict::GetStartingBits ( ) const

Get the number of bits needed at the beginning of the dictionary (when the dictionary is set to its root size).

Returns:: Returns the number of bits needed to access any code in the dictionary when the dictionary is in its default state.

LSBOOL LSE_CALL lsc::CLzwEncodingDict::InitDict	(	LSUINT32	_ui32RootCodes,
		LSUINT32	_ui32MaxBits
	)

Begin a compression. We need to know the maximum number of bits to create the dictionary. The dictionary can be cleared and recreated many times during compression.

Parameters: