low_pass_filter.h

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// SPDX-License-Identifier: Apache-2.0
// Copyright 2022 Stéphane Caron
 
#pragma once
 
#include <stdexcept>
#include <string>
 
#include "vulp/exceptions/FilterError.h"
 
namespace vulp::utils {
 
using vulp::exceptions::FilterError;
 
inline double low_pass_filter(double prev_output, double cutoff_period,
                              double new_input, double dt) {
  // Make sure the cutoff period is not too small
  if (cutoff_period <= 2.0 * dt) {
    auto message =
        std::string("[low_pass_filter] Cutoff period ") +
        std::to_string(cutoff_period) +
        " s is less than 2 * dt = " + std::to_string(2.0 * dt) +
        " s, causing information loss (Nyquist–Shannon sampling theorem)";
    throw FilterError(message);
  }
 
  // Actual filtering ;)
  const double alpha = dt / cutoff_period;
  return prev_output + alpha * (new_input - prev_output);
}
 
}  // namespace vulp::utils