ExplicitRK2.h

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//* This file is part of the MOOSE framework
//* https://mooseframework.inl.gov
//*
//* All rights reserved, see COPYRIGHT for full restrictions
//* https://github.com/idaholab/moose/blob/master/COPYRIGHT
//*
//* Licensed under LGPL 2.1, please see LICENSE for details
//* https://www.gnu.org/licenses/lgpl-2.1.html

#pragma once

#include "TimeIntegrator.h"

class ExplicitRK2 : public TimeIntegrator
{
public:
  static InputParameters validParams();

  ExplicitRK2(const InputParameters & parameters);

  virtual void preSolve() override;
  virtual int order() override { return 2; }

  virtual void computeTimeDerivatives() override;
  void computeADTimeDerivatives(ADReal & ad_u_dot,
                                const dof_id_type & dof,
                                ADReal & ad_u_dotdot) const override;
  virtual void solve() override;
  virtual void postResidual(NumericVector<Number> & residual) override;
  virtual bool overridesSolve() const override { return true; }

protected:
  template <typename T, typename T2, typename T3>
  void computeTimeDerivativeHelper(T & u_dot, const T2 & u_old, const T3 & u_older) const;

  unsigned int _stage;

  NumericVector<Number> * _residual_old;

  const NumericVector<Number> & _solution_older;

  virtual Real a() const = 0;
  virtual Real b1() const = 0;
  virtual Real b2() const = 0;
};

template <typename T, typename T2, typename T3>
void
ExplicitRK2::computeTimeDerivativeHelper(T & u_dot, const T2 & u_old, const T3 & u_older) const
{
  if (_stage < 3)
    u_dot -= u_old;
  else
    u_dot -= u_older;

  u_dot *= 1. / _dt;
}