global_length_parameter_calculator.h

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/*
 * Copyright 2024 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.
 */
#pragma once
 
#include <cstddef>
#include <limits>
#include <vector>
 
#include "num_collect/base/exception.h"
#include "num_collect/base/index_type.h"
#include "num_collect/base/precondition.h"
#include "num_collect/constants/zero.h"  // IWYU pragma: keep
#include "num_collect/logging/logging_macros.h"
#include "num_collect/rbf/concepts/distance_function.h"
 
namespace num_collect::rbf::length_parameter_calculators {

template <concepts::distance_function DistanceFunction>
class global_length_parameter_calculator {
public:
    using distance_function_type = DistanceFunction;

    using variable_type = typename DistanceFunction::variable_type;

    using scalar_type = typename DistanceFunction::value_type;

    static constexpr bool uses_global_length_parameter = true;

    global_length_parameter_calculator() = default;

    void compute(const std::vector<variable_type>& variables,
        const distance_function_type& distance_function) {
        const std::size_t num_samples = variables.size();
        NUM_COLLECT_PRECONDITION(
            num_samples > 0, "Sample points must be given.");
 
        auto max_min_distance = constants::zero<scalar_type>;
        for (std::size_t i = 0; i < num_samples; ++i) {
            auto min_distance = std::numeric_limits<scalar_type>::max();
            for (std::size_t j = 0; j < num_samples; ++j) {
                if (i != j) {
                    const auto distance =
                        distance_function(variables[i], variables[j]);
                    if (distance < min_distance) {
                        min_distance = distance;
                    }
                }
            }
            if (min_distance > max_min_distance) {
                max_min_distance = min_distance;
            }
        }
 
        length_parameter_ = scale_ * max_min_distance;
    }

    [[nodiscard]] auto length_parameter_at(index_type i) const -> scalar_type {
        (void)i;
        return length_parameter_;
    }

    [[nodiscard]] auto scale() const noexcept -> scalar_type { return scale_; }

    void scale(scalar_type value) {
        NUM_COLLECT_PRECONDITION(value > constants::zero<scalar_type>,
            "Scale of length parameters must be a positive number.");
        scale_ = value;
    }
 
private:
    static constexpr auto default_scale = static_cast<scalar_type>(10);

    scalar_type scale_{default_scale};

    scalar_type length_parameter_{constants::zero<scalar_type>};
};
 
}  // namespace num_collect::rbf::length_parameter_calculators