代码来源：earcut test.cpp

template <typename Point>
double triangleArea(const Point &a, const Point &b, const Point &c) {
    using namespace mapbox::util;
    return double(std::abs((nth<0, Point>::get(a) - nth<0, Point>::get(c)) * (nth<1, Point>::get(b) - nth<1, Point>::get(a)) -
                           (nth<0, Point>::get(a) - nth<0, Point>::get(b)) * (nth<1, Point>::get(c) - nth<1, Point>::get(a)))) / 2;
}

template <typename Vertices, typename Indices>
double trianglesArea(const Vertices &vertices, const Indices &indices) {
    double area = 0;
    for (size_t i = 0; i < indices.size(); i += 3) {
        area += triangleArea(
            vertices[indices[i]],
            vertices[indices[i + 1]],
            vertices[indices[i + 2]]
        );
    }
    return area;
}

template <typename Ring>
double ringArea(const Ring &points) {
    using namespace mapbox::util;
    using Point = typename Ring::value_type;
    double sum = 0;
    for (size_t i = 0, len = points.size(), j = len - 1; i < len; j = i++) {
        sum += (nth<0, Point>::get(points[i]) - nth<0, Point>::get(points[j])) *
               (nth<1, Point>::get(points[i]) + nth<1, Point>::get(points[j]));
    }
    return std::abs(sum) / 2;
}

template <typename Polygon>
double polygonArea(const Polygon &rings) {
    if (rings.empty()) return .0;
    double sum = ringArea(rings[0]);
    for (size_t i = 1; i < rings.size(); i++) {
        sum -= ringArea(rings[i]);
    }
    return std::max(sum, .0);
}


std::string formatPercent(double num) {
    std::stringstream ss;
    ss.imbue(std::locale(""));
    ss << std::fixed << std::setprecision(6) << num * 100 << "%";
    return ss.str();
}