calculate number of bits set in byte

Question

I am interested, which is the optimal way of calculating the number of bits set in byte by this way

template< unsigned char byte > class BITS_SET
{
pub         


        

Answer 1

#include 
#include  // for CHAR_BIT (most likely to be 8)
#include  // for memset
#include  

static const int DUMMY = -1;

// first approch : activate the O(8) function in first get try... after that its O(1);
class bitsInByteflyLUT
{
    typedef unsigned char byte;

    public:
        bitsInByteflyLUT();     //CTOR - throws std::bad_alloc
        ~bitsInByteflyLUT();    //DTOR


        int Get_bitsInByte(byte _byte);     


    private:
        // CLASS DATA
        int*    flyLUT;

        // PRIVATE FUNCTIONS
        int bitsInByte(byte _byte);
        // O(8) for finding how many bits are ON in a byte.
        // answer can be between 0 to CHAR_BIT.

        bitsInByteflyLUT(const bitsInByteflyLUT & _class); // COPY CTOR - forbidden
        const bitsInByteflyLUT & operator= (const bitsInByteflyLUT& _class);
        // ASSIGN OPERATOR - forbidden

};

bitsInByteflyLUT::bitsInByteflyLUT()
{
    size_t nIndexes = 1 << CHAR_BIT;
    try
    {
        flyLUT =  new int[nIndexes];
    }
    catch (std::bad_alloc& ba)
    {
        throw;
    }
    memset(flyLUT, DUMMY, sizeof(int)*nIndexes);
}


bitsInByteflyLUT::~bitsInByteflyLUT()
{
    delete[] flyLUT;
}


int bitsInByteflyLUT::Get_bitsInByte(byte _byte)
{
    if (flyLUT[_byte] == DUMMY) // if its first time we try to get answer for this char.
    {
        flyLUT[_byte] = bitsInByte(_byte); // O(8)
    }
    return flyLUT[_byte]; // O(1) 
}

int bitsInByteflyLUT::bitsInByte(byte _byte)
{   
    byte nBits = CHAR_BIT;
    byte counter = 0;
    byte mask = 1;
    while(nBits--)
    {
        if(mask & _byte)
        {
            ++counter;
        }
        mask <<= 1;
    }
    return counter;
}





int main ()
{
    using std::cout;
    using std::endl;

    bitsInByteflyLUT flut;

    for (unsigned int i = 0; i < (1 << CHAR_BIT); i += 1)
    {   
        cout << i << " " << flut.Get_bitsInByte(i) << endl;
    }

    return 0;
}