lop1pexp_stable.cpp

int main () {
    auto dp = [](double x) {
        if (x <= -37)
            return std::exp(x);
        else if (x <= 18)
            return std::log1p(std::exp(x));
        else if (x <= 33.3)
            return x + std::exp(-x);
        else
            return x;
    };

    auto sp = [](float x) {
        if (x <= -20)
            return std::expf(x);
        else if (-20 < x && x <= 9.0)
            return std::log1pf(std::expf(x));
        else if(9.0 < x && x <= 14.6)
            return x + std::exp(-x);
        else
            return x;
    };

    std::cout << std::setprecision(100);

    for (float i = -60; i < 60; i += 0.001f) {
        float actual = float(sp(i));
        float expected = float(std::log1p(std::exp(double(i))));
        float delta = actual - expected;
        std::cout << i << "\t" << delta << std::endl;

        if (delta != 0.0) {
            float prev_error = std::nextafter(float(sp(i)), -FLT_MAX) - float(std::log1p(std::exp(double(i))));
            float next_error = std::nextafter(float(sp(i)), FLT_MAX) - float(std::log1p(std::exp(double(i))));

            std::cout << "prev: " << prev_error << std::endl;
            std::cout << "next: " << next_error << std::endl;

            if (prev_error != 0.0 && next_error != 0.0)
                std::cout << "ERROR MORE THAN CLOSEST REPRESENTABLE NUMBER\n";
        }
    }
   return 0; 
}