FoamIntegrator.cpp

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/*
 *  CepGen: a central exclusive processes event generator
 *  Copyright (C) 2020-2024  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 <TFoam.h>
#include <TFoamIntegrand.h>
 
#include "CepGen/Core/Exception.h"
#include "CepGen/Integration/Integrator.h"
#include "CepGen/Integration/ProcessIntegrand.h"
#include "CepGen/Modules/IntegratorFactory.h"
#include "CepGen/Modules/RandomGeneratorFactory.h"
#include "CepGen/Utils/Histogram.h"
#include "CepGen/Utils/ProcessVariablesAnalyser.h"
 
namespace cepgen {
  class FoamIntegrator final : public Integrator, public TFoamIntegrand {
  public:
    explicit FoamIntegrator(const ParametersList& params) : Integrator(params) {}
 
    static ParametersDescription description() {
      auto desc = Integrator::description();
      desc.setDescription("FOAM general purpose MC integrator");
      desc.add("randomGenerator", RandomGeneratorFactory::get().describeParameters("root"));
      desc.add<int>("nCalls", 100'000).setDescription("number of calls for the cell evaluation");
      desc.add<int>("nCells", 1000);
      desc.add<int>("nSampl", 200);
      desc.add<int>("nBin", 8);
      desc.add<int>("EvPerBin", 25);
      desc.add<int>("verbose", 0).setDescription("Verbosity level");
      return desc;
    }
 
    Value integrate(Integrand& integrand) override {
      integrand_ = &integrand;
      std::unique_ptr<TFoam> foam(new TFoam("Foam"));
      CG_DEBUG("Integrator:integrate") << "FOAM integrator built\n\t"
                                       << "Version: " << foam->GetVersion() << ".";
      foam->SetPseRan(rnd_gen_->engine<TRandom>());
      foam->SetnCells(steer<int>("nCells"));
      foam->SetnSampl(steer<int>("nSampl"));
      foam->SetnBin(steer<int>("nBin"));
      foam->SetEvPerBin(steer<int>("EvPerBin"));
      foam->SetChat(std::max(verbosity_, 0));
      foam->SetRho(this);
      foam->SetkDim(integrand_->size());
      Integrator::checkLimits(*integrand_);
      coord_.resize(integrand_->size());
      foam->Initialize();
      std::unique_ptr<utils::ProcessVariablesAnalyser> analyser;
      if (integrand.hasProcess())
        analyser.reset(new utils::ProcessVariablesAnalyser(dynamic_cast<ProcessIntegrand&>(integrand).process(),
                                                           ParametersList{}));
      const auto num_calls = steer<int>("nCalls");
      for (int i = 0; i < num_calls; ++i) {
        foam->MakeEvent();
        if (analyser)
          analyser->feed(foam->GetMCwt() / num_calls);
      }
      if (analyser)
        analyser->analyse();
      //--- launch integration
      double norm, err;
      foam->Finalize(norm, err);
 
      double result, abs_error;
      foam->GetIntegMC(result, abs_error);
      for (const auto& lim : limits_) {
        result *= lim.range();
        abs_error *= lim.range();
      }
      const auto res = Value{result, abs_error};
 
      CG_DEBUG("FoamIntegrator").log([&](auto& log) {
        double eps = 5.e-4, avewt, wtmax, sigma;
        foam->GetWtParams(eps, avewt, wtmax, sigma);
        const double ncalls = foam->GetnCalls();
        const double effic = wtmax > 0 ? avewt / wtmax : 0.;
        log << "Result: " << res << "\n\t"
            << "Relative error: " << res.relativeUncertainty() * 100. << "%\n\t"
            << "Dispersion/<wt> = " << sigma << ", <wt> = " << avewt << ", <wt>/wtmax = " << effic << ",\n\t"
            << " for epsilon = " << eps << "\n\t"
            << " nCalls (initialisation only)= " << ncalls << ".";
      });
      return res;
    }
 
    inline double Density(int ndim, double* x) override {
      if (!integrand_)
        throw CG_FATAL("FoamDensity") << "Integrand object not yet initialised!";
      for (int i = 0; i < ndim; ++i)
        coord_[i] = limits_.at(i).x(x[i]);
      return integrand_->eval(coord_);
    }
 
  private:
    Integrand* integrand_{nullptr};
    std::vector<double> coord_;
  };
}  // namespace cepgen
REGISTER_INTEGRATOR("Foam", FoamIntegrator);