hpf_2.h

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 * @details
 * This file defines the @ref filter::high_pass::HPF_2 class, which
 * implements a discrete-time second-order high-pass filter.
 *
 * The filter attenuates low-frequency and DC components while allowing
 * high-frequency components of the input signal to pass.
 *
 * The filter is configured using the cutoff frequency and sampling time.
 * The cutoff frequency is prewarped during filter design to compensate
 * for the frequency transformation associated with the bilinear
 * transformation.
 *
 * The implementation maintains previous input and output samples as
 * internal filter state.
 *
 * The class is templated to support different numeric types such as
 * @c float and @c double.
 *
 * @tparam T Numeric data type used for the filter calculations.
 *
 * @author Abhinay Kumar
 * @version 1.0.0
 *
 * @copyright
 * Copyright (c) 2026 Abhinay Kumar
 */
 
#ifndef FILTER_HPF_2_H
#define FILTER_HPF_2_H
 
#include <cstdint>
 
namespace filter
{
 
    namespace high_pass
    {


         * while allowing higher-frequency components to pass.
         *
         * The filter is configured using the cutoff frequency and
         * sampling time. During configuration, the cutoff frequency
         * is prewarped to compensate for frequency distortion introduced
         * by the bilinear transformation.
         *
         * The filter maintains two previous input samples and two
         * previous output samples as its internal state.
         *
         * @tparam T Numeric data type used for the filter.
         *
         * @par Example
         * @code{.cpp}
         * filter::high_pass::HPF_2<double> filter;
         *
         * filter.config(2.5, 0.001);
         *
         * double output = filter.update(input);
         * @endcode
         */
        template <typename T>
        class HPF_2
        {
        public:

             * @details
             * Constructs the filter with its parameters and internal
             * state initialized to their default values.
             */
            HPF_2();

            /**
             * @brief Configures the high-pass filter.
             *
             * @param[in] fc_ Cutoff frequency of the filter in Hz.
             * @param[in] dt_ Sampling time of the filter in seconds.
             *
             * @details
             * Configures the filter using the specified cutoff frequency
             * and sampling time and calculates the required filter
             * coefficients.
             */
            void config(T fc_, T dt_);

            void set_param(T fc_, T dt_);

            T update(T x_k);

            void reset();

            void set_fc(T fc_);

            void set_dt(T dt_);

            T get_fc();

            T get_dt();

            T get_fs();
 
        private:
            T fc = 0.0;

            T dt = 0.0;

            T lambda_1 = 0.0;

            T lambda_2 = 0.0;

            T lambda_3 = 0.0;

            T lambda_4 = 0.0;

            T lambda_5 = 0.0;

            T y_k_1 = 0.0;

            T y_k_2 = 0.0;

            T x_k_1 = 0.0;

            T x_k_2 = 0.0;

            uint8_t start_counter = 0;

            T prewarp_freq(T wc_, T dt_);
        };
 
#include "../../src/high_pass/hpf_2.tpp"
 
    } // namespace high_pass
 
} // namespace filter
 
#endif // FILTER_HPF_2_H