flux_function.h

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//
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// Copyright (C) 2023 - 2026 by the ryujin authors
//
 
#pragma once
 
#include <ryujin/base/compile_time_options.h>
#include <ryujin/scalar_conservation/flux.h>
 
#include <deal.II/base/function_parser.h>
 
#include <boost/algorithm/string/classification.hpp>
#include <boost/algorithm/string/split.hpp>
 
#include <numeric>
 
namespace ryujin
{
  namespace Fluxes
  {
    class Function : public Flux
    {
    public:
      Function(const std::string &subsection)
          : Flux("function", subsection)
      {
        expression_ = "0.5*u*u";
        add_parameter("expression",
                      expression_,
                      "A function expression of the flux as a function of "
                      "state used to create a muparser object to evaluate the "
                      "flux. For two, or three dimensional fluxes, components "
                      "are separated with a semicolon (;).");
 
        this->derivative_approximation_delta_ = 1.0e-10;
        add_parameter("derivative approximation delta",
                      this->derivative_approximation_delta_,
                      "Step size of the central difference quotient to compute "
                      "an approximation of the flux derivative");
 
        /*
         * Set up the muparser object with the final flux description from
         * the parameter file:
         */
        const auto set_up_muparser = [this] {
          std::vector<std::string> split_expressions;
          boost::split(split_expressions, expression_, boost::is_any_of(";"));
 
          const auto size = split_expressions.size();
 
          Assert(0 < size && size <= 3,
                 dealii::ExcMessage(
                     "user specified flux description must be either one, two, "
                     "or three strings separated by a comma"));
          flux_function_ = std::make_unique<dealii::FunctionParser<1>>(
              size, 0.0, this->derivative_approximation_delta_);
          flux_function_->initialize({"u"}, split_expressions, {});
 
          flux_formula_ = "f(u)={" + expression_ + "}";
        };
 
        set_up_muparser();
        ParameterAcceptor::parse_parameters_call_back.connect(set_up_muparser);
      }
 
 
      double value(const double state,
                   const unsigned int direction) const override
      {
        return flux_function_->value(dealii::Point<1>(state), direction);
      }
 
 
      double gradient(const double state,
                      const unsigned int direction) const override
      {
        return flux_function_->gradient(dealii::Point<1>(state), direction)[0];
      }
 
 
    private:
      std::string expression_;
 
      std::unique_ptr<dealii::FunctionParser<1>> flux_function_;
    };
  } // namespace Fluxes
} // namespace ryujin