KfMath.h

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/* Copyright (C) 2024 GSI Helmholtzzentrum fuer Schwerionenforschung, Darmstadt
   SPDX-License-Identifier: GPL-3.0-only
   Authors: Sergei Zharko [committer] */

 
#pragma once
 
#include <cstddef>
 
namespace cbm::algo::kf::math
{

  inline constexpr int SymIndex(int i, int j) { return (j <= i) ? i * (1 + i) / 2 + j : j * (1 + j) / 2 + i; }
 

  constexpr size_t NofPolCoefficients(size_t N, size_t M)
  {
    return M == 1 ? N + 1 : (NofPolCoefficients(N, M - 1) * (M + N)) / M;
  }

  template<size_t N, typename T>
  constexpr T Horner(const T* c, const T& x)
  {
    if constexpr (N == 0) {
      return c[0];
    }
    else {
      return Horner<N - 1, T>(c, x) * x + c[N];
    }
  }

  template<size_t N, typename T, typename... Args>
  constexpr T Horner(const T* c, const T& x1, Args... xI)
  {
    if constexpr (N == 0) {
      return c[0];
    }
    else {
      constexpr size_t M = sizeof...(Args) + 1;
      constexpr size_t k = NofPolCoefficients(N - 1, M);
      return Horner<N - 1, T>(c, x1, xI...) * x1 + Horner<N, T>(c + k, xI...);
    }
  }
}  // namespace cbm::algo::kf::math