Derivator.cpp

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/*
 *  CepGen: a central exclusive processes event generator
 *  Copyright (C) 2022  Laurent Forthomme
 *
 *  This program is free software: you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation, either version 3 of the License, or
 *  any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
 
#include <TF1.h>
 
#include "CepGen/Core/Exception.h"
#include "CepGen/Modules/DerivatorFactory.h"
#include "CepGen/Utils/Derivator.h"
#include "CepGen/Utils/FunctionsWrappers.h"
 
namespace cepgen {
  class ROOTDerivator : public utils::Derivator {
  public:
    explicit ROOTDerivator(const ParametersList& params) : Derivator(params), order_(steer<int>("order")) {}
 
    static ParametersDescription description() {
      auto desc = Derivator::description();
      desc.setDescription("ROOT derivation algorithm (Richardson's extrapolation method)");
      desc.add<int>("order", 1).setDescription("order of the derivation");
      return desc;
    }
 
    double derivate(const utils::Function1D& func, double x, double h = -1.) const override {
      auto rfunc = TF1(
          "cepgen_functional",
          [&func](double vars[1], double* pars) { return func(vars[0], static_cast<void*>(pars)); },
          0.,
          1.,
          0);
      const auto epsilon = h < 0. ? h_ : h;
      switch (order_) {
        case 1:
          return rfunc.Derivative(x, nullptr, epsilon);
        case 2:
          return rfunc.Derivative2(x, nullptr, epsilon);
        case 3:
          return rfunc.Derivative3(x, nullptr, epsilon);
        default:
          throw CG_FATAL("ROOTDerivator") << "Invalid derivation order requested: " << order_ << ".";
      }
    }
 
  private:
    const int order_;
  };
}  // namespace cepgen
 
REGISTER_DERIVATOR("root", ROOTDerivator);