AStableDirk4.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 LStableDirk4;

class AStableDirk4 : public TimeIntegrator
{
public:
  static InputParameters validParams();

  AStableDirk4(const InputParameters & parameters);

  virtual int order() override { return 4; }
  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>
  void computeTimeDerivativeHelper(T & u_dot, const T2 & u_old) const;

  // Indicates the current stage.
  unsigned int _stage;

  // Store pointers to the various stage residuals
  NumericVector<Number> * _stage_residuals[3];

  // The parameter of the method, set at construction time and cannot be changed.
  const Real _gamma; // 1.06857902130162881

  // Butcher tableau "C" parameters derived from _gamma
  // 1.06857902130162881, 0.5, -.06857902130162881
  Real _c[3];

  // Butcher tableau "A" values derived from _gamma.  We only use the
  // lower triangle of this.
  // 1.06857902130162881
  // -.56857902130162881, 1.06857902130162881
  // 2.13715804260325762, -3.27431608520651524, 1.06857902130162881
  Real _a[3][3];

  // The Butcher tableau "b" parameters derived from _gamma;
  // 1.2888640051572051e-01, 7.4222719896855893e-01, 1.2888640051572051e-01
  Real _b[3];

  // If true, we use a more expensive method (LStableDirk4) to
  // "bootstrap" the first timestep of this method and avoid
  // evaluating residuals before the initial time.
  bool _safe_start;

  // A pointer to the "bootstrapping" method to use if _safe_start==true.
  std::shared_ptr<LStableDirk4> _bootstrap_method;
};

template <typename T, typename T2>
void
AStableDirk4::computeTimeDerivativeHelper(T & u_dot, const T2 & u_old) const
{
  u_dot -= u_old;
  u_dot *= 1. / _dt;
}