interpolation_filter.h

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// SPDX-License-Identifier: GPL-2.0-only
//
// Copyright (c) 2006-2008 Marc Brooker and Michael Inggs
// Copyright (c) 2008-present FERS Contributors (see AUTHORS.md).
//
// See the GNU GPLv2 LICENSE file in the FERS project root for more information.
 
/**
 * @file interpolation_filter.h
 * @brief Interpolation filter implementation using Kaiser windowing.
 */
 
#pragma once
 
#include <cstddef>
#include <expected>
#include <span>
#include <vector>
 
#include "core/config.h"
 
namespace interp
{
    /**
     * @class InterpFilter
     * @brief Provides methods to generate interpolation filters using Kaiser windows.
     */
    class InterpFilter
    {
    public:
        InterpFilter(const InterpFilter&) = delete;
        InterpFilter(InterpFilter&&) = delete;
        InterpFilter& operator=(const InterpFilter&) = delete;
        InterpFilter& operator=(InterpFilter&&) = delete;
        ~InterpFilter() = default;
 
        /**
         * @brief Computes the sinc function for a given input.
         *
         * @param x The input value for which the sinc function is computed.
         * @return The computed sinc value.
         */
        static constexpr RealType sinc(const RealType x) noexcept
        {
            return x == 0.0 ? 1.0 : std::sin(x * PI) / (x * PI);
        }
 
        /**
         * @brief Computes the Kaiser window function for a given input.
         *
         * @param x The input value for the Kaiser window calculation.
         * @return The computed window value, or an error message if computation fails.
         */
        [[nodiscard]] std::expected<RealType, std::string> kaiserWinCompute(RealType x) const noexcept;
 
        /**
         * @brief Computes the interpolation filter value for a given input.
         *
         * @param x The input value for which the interpolation filter is computed.
         * @return The computed filter value, or an error message if computation fails.
         */
        [[nodiscard]] std::expected<RealType, std::string> interpFilter(RealType x) const noexcept;
 
        /**
         * @brief Retrieves a span of precomputed filter values for a given delay.
         *
         * @param delay The delay value for which the filter is retrieved.
         * @return A span of precomputed filter values.
         * @throws std::runtime_error If the delay value is out of range.
         */
        [[nodiscard]] std::span<const RealType> getFilter(RealType delay) const;
 
        /**
         * @brief Retrieves the singleton instance of the `InterpFilter` class.
         *
         * @return The singleton instance of the `InterpFilter` class.
         */
        static InterpFilter& getInstance() noexcept;
 
    private:
        InterpFilter(); ///< Private constructor to prevent instantiation.
 
        RealType _alpha; ///< The alpha value for the Kaiser window.
        RealType _beta = 5; ///< The beta value for the Kaiser window.
        RealType _bessel_beta; ///< The Bessel function value for the Kaiser window.
        std::size_t _length; ///< The length of the filter.
        std::size_t _table_filters; ///< The number of filters in the table.
        std::vector<RealType> _filter_table; ///< The table of precomputed filters.
    };
}