qam.h

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#ifndef QAM_H
#define QAM_H

#include <climits>

namespace fun
{

    template<int NumBits>
    class QAM
    {
        int d_gain;
        double d_scale_e;
        double d_scale_d;
    public:
        QAM (double power, int gain = 0)
        {
            d_gain = gain + CHAR_BIT - NumBits;
            const int nn = (1<<(NumBits-1));
            // sum((2k+1)^2,k=0..n-1)
            #if 0
            int sum2 = 0;
            for (int i = 0; i < nn; ++i) {
            sum2+= (2*i + 1)*(2*i + 1);
            }
            #else
            int sum2 = (4*nn*nn*nn-nn)/3;
            #endif
            double sf = sqrt(power * double(nn) / double(sum2));
            d_scale_e = sf;
            d_scale_d = (1 << d_gain) / sf;
        }

        static inline int sign(int v)
        {
            return +1 | (v >> (sizeof(int) * CHAR_BIT - 1));
        }

        static inline int clamp(int i)
        {
            return i < 0 ? 0 : (i > 255 ? 255 : i);
        }

        inline void encode (const char* bits, double *sym)
        {
            int pt = 0; // constellation point
            int flip = 1; // +1 or -1 -- for gray coding
            // unrolled with -O3
            for (int i = 0; i < NumBits; ++i)
                {
                int bit = *bits * 2 - 1; // +1 or -1
                pt = bit * flip + pt * 2;
                flip *= -bit;
                ++bits;
                }
            *sym = pt * d_scale_e;
        }

        inline void decode (double sym, unsigned char *bits)
        {
            int pt = sym * d_scale_d;
            int flip = 1; // +1 or -1 -- for gray coding
            int amp = (1 << (NumBits-1)) << d_gain;
            // unrolled with -O3
            for (int i = 0; i < NumBits; ++i)
            {
                *bits = clamp(flip * pt + 128);
                int bit = sign(pt);
                pt -= bit * amp;
                flip = -bit;
                amp /= 2;
                ++bits;
            }
        }
    };
}

#endif // QAM_H