#ifndef STAN_MCMC_COVAR_ADAPTATION_HPP
#define STAN_MCMC_COVAR_ADAPTATION_HPP

#include <stan/math/prim/mat.hpp>
#include <stan/mcmc/windowed_adaptation.hpp>
#include <vector>

namespace stan {

  namespace mcmc {

    class covar_adaptation: public windowed_adaptation {
    public:
      explicit covar_adaptation(int n)
        : windowed_adaptation("covariance"), estimator_(n) {}

      bool learn_covariance(Eigen::MatrixXd& covar, const Eigen::VectorXd& q) {
        if (adaptation_window())
          estimator_.add_sample(q);

        if (end_adaptation_window()) {
          compute_next_window();

          estimator_.sample_covariance(covar);

          double n = static_cast<double>(estimator_.num_samples());
          covar = (n / (n + 5.0)) * covar
            + 1e-3 * (5.0 / (n + 5.0))
            * Eigen::MatrixXd::Identity(covar.rows(), covar.cols());

          estimator_.restart();

          ++adapt_window_counter_;
          return true;
        }

        ++adapt_window_counter_;
        return false;
      }

    protected:
      stan::math::welford_covar_estimator estimator_;
    };

  }  // mcmc

}  // stan

#endif