freq doubler - multiples +45 deg with -45 deg phase shift

hilbert-freq-doubler.cpp

class FrequencyDoubler
{
public:
    FrequencyDoubler() {
		coeffs = ReSampler::makeHilbert(1001);
        length = coeffs.size();
        history.resize(length);
        centerTap = length / 2;
        currentIndex = length - 1;
    }

    template<typename FloatType>
    double filter(FloatType input)
    {
        history[currentIndex] = input; // place input into history
        int d = currentIndex + centerTap;
        FloatType s0 = history[d >= length ? d - length : d]; // delay only
        FloatType s1 = 0.0;
        int p = currentIndex;
        for(int j = 0 ; j < length; j++) {
            FloatType v = history.at(p++);
            if(p == length) {
                p = 0;
            }
            s1 += coeffs.at(j) * v;
        }

        if(currentIndex == 0) {
            currentIndex = length - 1;
        } else {
            currentIndex--;
        }

        // multiply +45deg signal with -45deg
        return (s0 + s1) * (s0 - s1);
    }

private:
    std::vector<double> coeffs;
    std::vector<double> history;
    int length;
    int centerTap;
    int currentIndex;
};