noise_generators.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 noise_generators.h
 * @brief Header file for noise generator classes.
 */
 
#pragma once
 
#include <cstddef>
#include <functional>
#include <memory>
#include <random>
#include <vector>
 
#include "core/config.h"
#include "falpha_branch.h"
 
namespace noise
{
    /**
     * @class NoiseGenerator
     * @brief Abstract base class for noise generators.
     */
    class NoiseGenerator
    {
    public:
        NoiseGenerator() = default;
 
        virtual ~NoiseGenerator() = default;
 
        NoiseGenerator(const NoiseGenerator&) = delete;
 
        NoiseGenerator& operator=(const NoiseGenerator&) = delete;
 
        NoiseGenerator(NoiseGenerator&&) = delete;
 
        NoiseGenerator& operator=(NoiseGenerator&&) = delete;
 
        /**
         * @brief Pure virtual method to generate a noise sample.
         *
         * @return A noise sample of type RealType.
         */
        virtual RealType getSample() = 0;
    };
 
    /**
     * @class WgnGenerator
     * @brief Generates white Gaussian noise.
     */
    class WgnGenerator final : public NoiseGenerator
    {
    public:
        /**
         * @brief Constructor to initialize the WGN generator with a given standard deviation.
         *
         * @param rngEngine The random number engine to use for generation.
         * @param stddev The standard deviation of the generated Gaussian noise. Default is 1.0.
         */
        explicit WgnGenerator(std::mt19937& rngEngine, const RealType stddev = 1.0) noexcept :
            _rng_engine(rngEngine), _dist(0.0, stddev), _stddev(stddev)
        {
        }
 
        /**
         * @brief Generates a sample of white Gaussian noise.
         *
         * @return A noise sample of type RealType.
         */
        RealType getSample() noexcept override { return _dist(_rng_engine.get()); }
 
    private:
        std::reference_wrapper<std::mt19937> _rng_engine; ///< Reference to the RNG engine.
        std::normal_distribution<> _dist; ///< Normal distribution for generating Gaussian noise.
        RealType _stddev; ///< Standard deviation of the generated noise.
    };
 
    /**
     * @class GammaGenerator
     * @brief Generates Gamma-distributed noise.
     */
    class GammaGenerator final : public NoiseGenerator
    {
    public:
        /**
         * @brief Constructor to initialize the Gamma generator with a shape parameter.
         *
         * @param rngEngine The random number engine to use for generation.
         * @param k The shape parameter of the Gamma distribution.
         */
        explicit GammaGenerator(std::mt19937& rngEngine, const RealType k) noexcept :
            _rng_engine(rngEngine), _dist(k, 1.0)
        {
        }
 
        /**
         * @brief Generates a sample of Gamma noise.
         *
         * @return A noise sample of type RealType.
         */
        RealType getSample() noexcept override { return _dist(_rng_engine.get()); }
 
    private:
        std::reference_wrapper<std::mt19937> _rng_engine; ///< Reference to the RNG engine.
        std::gamma_distribution<> _dist; ///< Gamma distribution for generating noise.
    };
 
    /**
     * @class MultirateGenerator
     * @brief Generates multirate noise using a hierarchical tree structure.
     */
    class MultirateGenerator final : public NoiseGenerator
    {
    public:
        /**
         * @brief Constructor to initialize the multirate generator.
         *
         * @param rngEngine The random number engine to use for generation.
         * @param alpha The scaling parameter that controls the noise properties.
         * @param branches The number of branches in the tree structure.
         */
        MultirateGenerator(std::mt19937& rngEngine, RealType alpha, unsigned branches);
 
        /**
         * @brief Generates a multirate noise sample.
         *
         * @return A noise sample of type RealType.
         */
        RealType getSample() override { return _topbranch->getSample() * _scale; }
 
        /**
         * @brief Skips a number of samples in the noise sequence.
         *
         * @param samples The number of samples to skip.
         */
        void skipSamples(std::size_t samples) noexcept;
 
        /**
         * @brief Resets the noise generator state.
         */
        void reset() noexcept;
 
    private:
        RealType _scale; ///< Scaling factor for the noise.
        std::unique_ptr<FAlphaBranch> _topbranch; ///< Pointer to the top branch in the tree structure.
        std::reference_wrapper<std::mt19937> _rng_engine; ///< Reference to the RNG engine.
 
        /**
         * @brief Helper method to create the hierarchical tree structure.
         *
         * @param fAlpha Fractional alpha value.
         * @param fInt Integer part of the scaling factor.
         * @param branches The number of branches in the tree.
         */
        void createTree(RealType fAlpha, int fInt, unsigned branches);
    };
 
    /**
     * @class ClockModelGenerator
     * @brief Generates noise using a clock model with multiple rates.
     *
     * This is useful for simulating clock jitter or other similar phenomena.
     */
    class ClockModelGenerator final : public NoiseGenerator
    {
    public:
        /**
         * @brief Constructor to initialize the clock model generator.
         *
         * @param rngEngine The random number engine to use for generation.
         * @param alpha Vector of scaling parameters for the noise.
         * @param inWeights Vector of weights for each rate process.
         * @param frequency The base frequency of the clock model.
         * @param phaseOffset The phase offset of the generated noise.
         * @param freqOffset The frequency offset of the generated noise.
         * @param branches The number of branches in each rate process.
         */
        ClockModelGenerator(std::mt19937& rngEngine, const std::vector<RealType>& alpha,
                            const std::vector<RealType>& inWeights, RealType frequency, RealType phaseOffset,
                            RealType freqOffset, int branches) noexcept;
 
        /**
         * @brief Generates a clock model noise sample.
         *
         * @return A noise sample of type RealType.
         */
        RealType getSample() override;
 
        /**
         * @brief Skips a number of samples in the noise sequence.
         *
         * @param samples The number of samples to skip.
         */
        void skipSamples(std::size_t samples);
 
        /**
         * @brief Resets the noise generator state.
         */
        void reset();
 
        /**
         * @brief Checks if the noise generator is enabled.
         *
         * @return True if the generator is enabled, false otherwise.
         */
        [[nodiscard]] bool enabled() const;
 
    private:
        std::reference_wrapper<std::mt19937> _rng_engine; ///< Reference to the RNG engine.
        std::vector<std::unique_ptr<MultirateGenerator>> _generators; ///< Multirate noise generators.
        std::vector<RealType> _weights; ///< Weights for each rate process.
        RealType _phase_offset; ///< Phase offset for the noise.
        RealType _freq_offset; ///< Frequency offset for the noise.
        RealType _frequency; ///< Base frequency of the clock model.
        std::size_t _count = 0; ///< Sample counter.
    };
}