Compressing and Decompressing Memory Streams

The Zip Compression Library, EZLib provides stream compression and decompression functionality for the Symbian platforms. This tutorial summarizes the procedure involved in the compression and decompression of memory streams by EZLib.

Context

The functions used for memory stream compression and decompression are provided by the CEZCompressor class.

The compression can be executed in single or multiple steps. This is depenedent on the buffer size of the memory stream. Small buffers are compressed in a single step. Larger buffers are compressed in small chunks repeatedly till the entire buffer is compressed. Clients must respond to callbacks from the library to provide information and resources required to complete the task.

Steps

Define a buffer manager that implements MEZBufferManager.

Create an instance of the classs CEZCompressor.

Invoke CEZCompressor::CompressL() for small buffers.
Invoke CEZCompressor::DeflateL() repeatedly on larger buffers until the entire buffer is compressed.

Example:

class CBufferManager : public CBase, public MEZBufferManager
    {
public:
  CBufferManager* CBufferManager::NewLC();
  CBufferManager* CBufferManager::NewL();
    ~CBufferManager();
    void InitializeL(CEZZStream &aZStream);
    void NeedInputL(CEZZStream &aZStream);
    void NeedOutputL(CEZZStream &aZStream);
    void FinalizeL(CEZZStream &aZStream);
    };
        
CBufferManager* CBufferManager::NewLC()
    {
    CBufferManager* bm = new (ELeave) CBufferManager();
    CleanupStack::PushL(bm);
    return bm;
    }

CBufferManager* CBufferManager::NewL()
    {
    CBufferManager* bm = new (ELeave) CBufferManager();
    CleanupStack::PushL(bm);
    CleanupStack::Pop();
    return bm;
    }
 
static void doExampleL()
    {
    
  CBufferManager* bm = CBufferManager::NewLC();
  //TInt aLevel = EDefaultCompression, TInt aWindowBits = EMaxWBits, TInt aMemLevel = EDefMemLevel, TStrategy aStrategy = EDefaultStrategy);
  CEZCompressor* iCEZCompressor = CEZCompressor::NewLC(*bm); // all other flag values are default in its constructer
   
  while(icompressor->DeflateL())
        {       
        }     
   

  CleanupStack::PopAndDestroy(2);

    }

Next actions

To decompress the memory streams:

Define a buffer manager that implements MEZBufferManager.
Create an instance of the classs CEZDecompressor.
Invoke CEZDecompressor::DecompressL() for small buffers.

Invoke CEZDecompressor::InflateL() repeatedly on larger buffers until the entire buffer is compressed.