    // Copyright (C) 2010  Davis E. King (davis@dlib.net)
    // License: Boost Software License   See LICENSE.txt for the full license.
    #ifndef DLIB_CROSS_VALIDATE_REGRESSION_TRaINER_H__
    #define DLIB_CROSS_VALIDATE_REGRESSION_TRaINER_H__

    #include <vector>
    #include "../matrix.h"
    #include "../statistics.h"
    #include "cross_validate_regression_trainer_abstract.h"

    namespace dlib
    {

    // ----------------------------------------------------------------------------------------

        template <
            typename reg_funct_type,
            typename sample_type,
            typename label_type
            >
        matrix<double,1,2>
        test_regression_function (
            const reg_funct_type& reg_funct,
            const std::vector<sample_type>& x_test,
            const std::vector<label_type>& y_test
        )
        {

            // make sure requires clause is not broken
            DLIB_ASSERT( is_learning_problem(x_test,y_test) == true,
                        "\tmatrix test_regression_function()"
                        << "\n\t invalid inputs were given to this function"
                        << "\n\t is_learning_problem(x_test,y_test): "
                        << is_learning_problem(x_test,y_test));

            running_stats<double> rs;
            running_scalar_covariance<double> rc;

            for (unsigned long i = 0; i < x_test.size(); ++i)
            {
                // compute error
                const double output = reg_funct(x_test[i]);
                const double temp = output - y_test[i];

                rs.add(temp*temp);
                rc.add(output, y_test[i]);
            }

            matrix<double,1,2> result;
            result = rs.mean(), std::pow(rc.correlation(),2);
            return result;
        }

    // ----------------------------------------------------------------------------------------

        template <
            typename trainer_type,
            typename sample_type,
            typename label_type
            >
        matrix<double,1,2>
        cross_validate_regression_trainer (
            const trainer_type& trainer,
            const std::vector<sample_type>& x,
            const std::vector<label_type>& y,
            const long folds
        )
        {

            // make sure requires clause is not broken
            DLIB_ASSERT(is_learning_problem(x,y) == true &&
                        1 < folds && folds <= static_cast<long>(x.size()),
                "\tmatrix cross_validate_regression_trainer()"
                << "\n\t invalid inputs were given to this function"
                << "\n\t x.size(): " << x.size()
                << "\n\t folds:  " << folds
                << "\n\t is_learning_problem(x,y): " << is_learning_problem(x,y)
                );

            const long num_in_test = x.size()/folds;
            const long num_in_train = x.size() - num_in_test;

            running_stats<double> rs;
            running_scalar_covariance<double> rc;

            std::vector<sample_type> x_test, x_train;
            std::vector<label_type> y_test, y_train;

            long next_test_idx = 0;

            for (long i = 0; i < folds; ++i)
            {
                x_test.clear();
                y_test.clear();
                x_train.clear();
                y_train.clear();

                // load up the test samples
                for (long cnt = 0; cnt < num_in_test; ++cnt)
                {
                    x_test.push_back(x[next_test_idx]);
                    y_test.push_back(y[next_test_idx]);
                    next_test_idx = (next_test_idx + 1)%x.size();
                }

                // load up the training samples
                long next = next_test_idx;
                for (long cnt = 0; cnt < num_in_train; ++cnt)
                {
                    x_train.push_back(x[next]);
                    y_train.push_back(y[next]);
                    next = (next + 1)%x.size();
                }

                try
                {
                    const typename trainer_type::trained_function_type& df = trainer.train(x_train,y_train);

                    // do the training and testing
                    for (unsigned long j = 0; j < x_test.size(); ++j)
                    {
                        // compute error
                        const double output = df(x_test[j]);
                        const double temp = output - y_test[j];

                        rs.add(temp*temp);
                        rc.add(output, y_test[j]);
                    }
                }
                catch (invalid_nu_error&)
                {
                    // just ignore cases which result in an invalid nu
                }

            } // for (long i = 0; i < folds; ++i)

            matrix<double,1,2> result;
            result = rs.mean(), std::pow(rc.correlation(),2);
            return result;
        }

    }

    // ----------------------------------------------------------------------------------------

    #endif // DLIB_CROSS_VALIDATE_REGRESSION_TRaINER_H__