CoolProp/include/finite_diff.h

#include <Eigen/Dense>

template <typename Callable>
auto romberg_diff(Callable& func, double x, std::size_t order = 2, double h = 0.1) {

    // Initialize the table
    auto r = Eigen::ArrayXd(order, order);

    // Compute the first column using the central difference formula
    for (auto i = 0; i < order; ++i) {
        r(i, 0) = (func(x + h) - func(x - h)) / (2 * h);
        h /= 2.0;
    }

    // Apply Richardson extrapolation
    for (auto i = 1; i < order; ++i) {
        for (auto j = i; j < order; ++j) {
            double fouri = pow(4, i);
            r(j, i) = (fouri * r(j, i - 1) - r(j - 1, i - 1)) / (fouri - 1);
        }
    }

    return r(order - 1, order - 1);
}