double StandardNormalDistribution::cdf(const double& x) const {
  double k = 1.0/(1.0 + 0.2316419*x);
  double k_sum = k*(0.319381530 + k*(-0.356563782 + k*(1.781477937 + k*(-1.821255978 + 1.330274429*k))));

  if (x >= 0.0) {
    return (1.0 - (1.0/(pow(2*M_PI,0.5)))*exp(-0.5*x*x) * k_sum);
  } else {
    return 1.0 - cdf(-x);
  }
}