double nilsegments::getMatch(double true_wind_direction, double true_track_heading, double true_wind_speed,double true_track_speed)
{
    //formula from https://en.wikipedia.org/wiki/Apparent_wind
    //V =      velocity (boat speed over ground, always => 0)
    //W =      true wind velocity (always => 0)
    //alpha  = true pointing angle in degrees (0 = upwind, 180 = downwind)
    //A =      apparent wind velocity (always => 0)
    //A=       sqrt ((W^2+V^2+2WV)\cos (alpha))

    double true_wind_heading=fmod((true_wind_direction + 180),360);
    double a = true_wind_heading;
    double b = true_track_heading;
    double diff = (a > b ? a - b : b - a);
    double mod_diff = fmod(diff, 360);
    double alpha = (mod_diff < 180 ? mod_diff : 360 - mod_diff);
    double W=true_wind_speed;
    double V=true_track_speed;
    double W_pow=qPow(W,2);
    double V_pow=qPow(V,2);
    double alpha_cos=qCos(qDegreesToRadians(alpha));
    double A_speed=qSqrt(W_pow+V_pow+(2*W*V*alpha_cos));
    //double A_angle = qRadiansToDegrees(qAcos((W*alpha_cos+V)/A_speed));

    //H=headwind
    double H=A_speed-W;
    return qPow(H,2)/qPow(true_track_speed,2);
}