chebyshev.hpp

Go to the documentation of this file.

/* Copyright (c) 2026 Otto Link. Distributed under the terms of the GNU General
   Public License. The full license is in the file LICENSE, distributed with
   this software. */
 
#pragma once
#include <cmath>
#include <vector>
 
#include <cassert>
 
namespace hmap
{
 
class ChebyshevEvaluator
{
public:
  using value_type = float;
 
  ChebyshevEvaluator() = default;
 
  explicit ChebyshevEvaluator(std::vector<value_type> coefficients)
      : m_coefficients(std::move(coefficients))
  {
  }
 
  void set_coefficients(std::vector<value_type> coefficients)
  {
    m_coefficients = std::move(coefficients);
  }
 
  [[nodiscard]]
  inline value_type evaluate(value_type x) const noexcept
  {
    const int n = static_cast<int>(m_coefficients.size());
 
    if (n == 0) return value_type(0);
 
    if (n == 1) return m_coefficients[0];
 
    value_type b_kplus1 = 0.0;
    value_type b_kplus2 = 0.0;
 
    const value_type two_x = 2.0 * x;
 
    for (int k = n - 1; k >= 1; --k)
    {
      const value_type b_k = m_coefficients[k] + two_x * b_kplus1 - b_kplus2;
 
      b_kplus2 = b_kplus1;
      b_kplus1 = b_k;
    }
 
    return m_coefficients[0] + x * b_kplus1 - b_kplus2;
  }
 
  template <typename InputIt, typename OutputIt>
  inline void evaluate_batch(InputIt  begin,
                             InputIt  end,
                             OutputIt out) const noexcept
  {
    for (; begin != end; ++begin, ++out)
    {
      *out = evaluate(*begin);
    }
  }
 
  [[nodiscard]]
  inline int degree() const noexcept
  {
    return static_cast<int>(m_coefficients.size()) - 1;
  }
 
  [[nodiscard]]
  inline const std::vector<value_type> &coefficients() const noexcept
  {
    return m_coefficients;
  }
 
private:
  std::vector<value_type> m_coefficients;
};
 
} // namespace hmap