/*

 * Copyright 2021 MusicScience37 (Kenta Kabashima)

 *

 * Licensed under the Apache License, Version 2.0 (the "License");

 * you may not use this file except in compliance with the License.

 * You may obtain a copy of the License at

 *

 *     http://www.apache.org/licenses/LICENSE-2.0

 *

 * Unless required by applicable law or agreed to in writing, software

 * distributed under the License is distributed on an "AS IS" BASIS,

 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

 * See the License for the specific language governing permissions and

 * limitations under the License.

 */

/*!

 * \file

 * \brief Definition of explicit_l_curve class.

 */

#pragma once

#include <cmath>

#include "num_collect/opt/function_object_wrapper.h"

#include "num_collect/opt/heuristic_global_optimizer.h"

#include "num_collect/regularization/concepts/explicit_regularized_solver.h"

#include "num_collect/regularization/explicit_param_searcher_base.h"

namespace num_collect::regularization {

/*!

 * \brief Class of objective function in L-curve.

 *

 * \warning This class is meant for use in optimizers.

 *

 * \tparam Solver Type of solvers.

 */

template <concepts::explicit_regularized_solver Solver>

class explicit_l_curve_objective_function {

public:

    //! Type of solvers.

    using solver_type = Solver;

    //! Type of scalars.

    using scalar_type = typename solver_type::scalar_type;

    /*!

     * \brief Constructor.

     *

     * \param[in] solver Solver.

     */

    explicit explicit_l_curve_objective_function(const solver_type& solver)

        : solver_(&solver) {}

    /*!

     * \brief Calculate the curvature of L-curve.

     *

     * \param[in] log_param Logarithm of a regularization parameter.

     * \return Negated value of the curvature.

     */

    [[nodiscard]] auto operator()(const scalar_type& log_param) const

        -> scalar_type {

        using std::pow;

        const scalar_type param = pow(static_cast<scalar_type>(10),  // NOLINT

            log_param);

        return -solver_->l_curve_curvature(param);

    }

private:

    //! Solver.

    const solver_type* solver_;

};

/*!

 * \brief Class to search optimal regularization parameter using l-curve.

 *

 * \tparam Solver Type of solvers.

 * \tparam Optimizer Type of optimizers.

 */

template <concepts::explicit_regularized_solver Solver,

    template <typename> typename Optimizer = opt::heuristic_global_optimizer>

class explicit_l_curve

    : public explicit_param_searcher_base<explicit_l_curve<Solver, Optimizer>,

          Solver> {

public:

    //! Type of base class.

    using base_type =

        explicit_param_searcher_base<explicit_l_curve<Solver, Optimizer>,

            Solver>;

    using typename base_type::data_type;

    using typename base_type::scalar_type;

    using typename base_type::solver_type;

    //! Type of optimizers.

    using optimizer_type =

        Optimizer<opt::function_object_wrapper<scalar_type(scalar_type),

            explicit_l_curve_objective_function<solver_type>>>;

    /*!

     * \brief Constructor.

     *

     * \param[in] solver Solver.

     */

    explicit explicit_l_curve(const solver_type& solver)

        : solver_(&solver),

          optimizer_(

              opt::make_function_object_wrapper<scalar_type(scalar_type)>(

                  explicit_l_curve_objective_function<solver_type>(solver))) {}

    //! \copydoc explicit_param_searcher_base::search

    void search() {

        using std::log10;

        using std::pow;

        const auto [min_param, max_param] = solver_->param_search_region();

        const scalar_type log_min_param = log10(min_param);

        const scalar_type log_max_param = log10(max_param);

        optimizer_.init(log_min_param, log_max_param);

        optimizer_.solve();

        opt_param_ = pow(static_cast<scalar_type>(10),  // NOLINT

            optimizer_.opt_variable());

    }

    //! \copydoc explicit_param_searcher_base::opt_param

    [[nodiscard]] auto opt_param() const -> scalar_type { return opt_param_; }

    //! \copydoc explicit_param_searcher_base::solve

    void solve(data_type& solution) const {

        solver_->solve(opt_param_, solution);

    }

private:

    //! Solver.

    const solver_type* solver_;

    //! Optimizer.

    optimizer_type optimizer_;

    //! Optimal regularization parameter.

    scalar_type opt_param_{};

};

}  // namespace num_collect::regularization