Easy Analog Data Compression

Listing 2: Partial listing of decoder.cpp

class TDecoder
{
    private: TDecoder(); 
    // no default constructor wanted
    
    public: TDecoder(vector<int> &tiDestination);
    // init the TDecoder and pass a vector whereto decoded data shall
    // be written;

    public: void Decode(WORD wCode);
    // decode the word wCode and push_back the result into tiDestination

    private: int DeltaDecode(int iDelta); 

    int miXsample;      
    // the untouched fraction of the last 16-bit code passed to Decode()

    int miXbitsTaken; 
    // denotes what number of bits are valid in miXsample

    int miBitWidthOfCode; 
    // holds the bit width of the code

    int miAbsValueCoded;
    // this holds the number of WORDs left that code an absolute value

    int mpiLastSamplesDiv3[3];
    // usually holds the last 3 decoded samples

    int miSamples2MSBidentical;
// this counter is used for the reduction of the bit width (in
    // the adaption)

    vector<int> *mptiDestination;
    // pointer to the result vector

    static const int mpiABSCODE[17];        
    static const int mpiOVFLCODE[16];       
    static const int mpiALIGNCODE[16];      
    static const int mpiSIGNEXTENSION[16];
};


const int TDecoder::mpiABSCODE[17] = { ... }; // as in listing 1
const int TDecoder::mpiOVFLCODE[16] = { ... }; // as in listing 1
const int TDecoder::mpiALIGNCODE[16] = { ... }; // as in listing 1

const int TDecoder::mpiSIGNEXTENSION[16] =
{
    // the value at index i denotes a bitmask for 32-bits sign extension 
    // for an i-bits value
    0xFFFFFFFE, 0xFFFFFFFC, 0xFFFFFFF8, 0xFFFFFFF0,
    0xFFFFFFE0, 0xFFFFFFC0, 0xFFFFFF80, 0xFFFFFF00,
    0xFFFFFE00, 0xFFFFFC00, 0xFFFFF800, 0xFFFFF000,
    0xFFFFE000, 0xFFFFC000, 0xFFFF8000, 0xFFFF0000
};


TDecoder::TDecoder(vector<int> &tiDestination):
    miXsample(0), 
    miXbitsTaken(0), 
    miBitWidthOfCode(6), 
    miAbsValueCoded(0),
    miSamples2MSBidentical(0),
    mptiDestination(&tiDestination) 
{
    assert( sizeof(int) >= 4 ); // TDecoder does not run on 16-bit
                                // platforms
    for(int i = 0; i < 3; i++) mpiLastSamplesDiv3[i] = 0;
}


int TDecoder::DeltaDecode(int iDelta)
{
    int iPrediction = 4 * mpiLastSamplesDiv3[2] + mpiLastSamplesDiv3[1] - 
                        2 * mpiLastSamplesDiv3[0];

    int iDecodedSample = iPrediction + iDelta;
    mpiLastSamplesDiv3[0] = mpiLastSamplesDiv3[1];
    mpiLastSamplesDiv3[1] = mpiLastSamplesDiv3[2];
    mpiLastSamplesDiv3[2] = iDecodedSample / 3;
    return iDecodedSample;
}


void TDecoder::Decode(WORD wCode)
{
    unsigned uiCode = wCode; // use full 32-bit register width

    if( miAbsValueCoded == 1 )
    {
        miXsample = uiCode;
        if (uiCode & 0x8000) miXsample -=0x10000;  // unsigned -> int16
                                                   // conversion
        (*mptiDestination).push_back(miXsample);
        mpiLastSamplesDiv3[0] = miXsample/3;       // update for
                                                   // delta encoding
        mpiLastSamplesDiv3[1] = miXsample/3;
        mpiLastSamplesDiv3[2] = miXsample/3;
        miAbsValueCoded--;                   // switch back to delta
                                             // encoding
        miXsample = 0x0000;                  // reset buffered sub-word
    }
    else 
    {
        // a delta encoded value, not necessarily aligned with a word
        // boundary has been received
        uiCode <<= miXbitsTaken;  // shift it left to and combine it with
        miXsample |= uiCode;      // a buffered rest of preceding iCode:s
        miXbitsTaken += 16;       // 16 new bits to decode were passed

        while( miXbitsTaken >= miBitWidthOfCode )
        {
            // cut out a complete code; reduce contents of the buffer by
            // the code cut out; reduce number of valid bits accordingly
            int iSubword = miXsample & mpiABSCODE[miBitWidthOfCode]; 
            int iSignBitMask = 1 << (miBitWidthOfCode - 1);
            miXsample >>= miBitWidthOfCode;
            miXbitsTaken -= miBitWidthOfCode;

            int iMaxBitMask = 3 << (miBitWidthOfCode - 2);

            if( iSignBitMask & iSubword)
            {
                // a negative number was coded; sign extension is needed
                // as well for the comparison to mpiALIGNCODE[] as for
                // delta decoding
                iSubword |= mpiSIGNEXTENSION[miBitWidthOfCode - 1];
            }                
        
            if( iSubword == mpiOVFLCODE[miBitWidthOfCode - 1] )
            {
                miBitWidthOfCode++;  // increase bit width of code
                miSamples2MSBidentical = 0;
            }
            else if( iSubword == mpiABSCODE[miBitWidthOfCode - 1] )
            {
                miAbsValueCoded = 1;
                miXbitsTaken = 0;
                miXsample = 0;
            }
            else if( iSubword == mpiALIGNCODE[miBitWidthOfCode - 1] )
            {
                miXbitsTaken = 0;
                miXsample = 0;
            }
            else
            {
                if( (iSubword & iMaxBitMask) == 0x0000 || 
                    (iSubword & iMaxBitMask) == iMaxBitMask)
                {
                    miSamples2MSBidentical++;
                    if ((miSamples2MSBidentical == BIT_REDUCTION_LENGTH) &&
                        (miBitWidthOfCode > MIN_BIT_WIDTH))  
                    {
                        miBitWidthOfCode--;
                        miSamples2MSBidentical = 0;
                    }
                }
                else
                {
                    miSamples2MSBidentical = 0;
                }

                int iNewSample = DeltaDecode( iSubword );
                (*mptiDestination).push_back( iNewSample );
             }
        } // while
    }
} // TDecoder::Decode()