// Copyright (c) 2010-2023, Lawrence Livermore National Security, LLC. Produced
// at the Lawrence Livermore National Laboratory. All Rights reserved. See files
// LICENSE and NOTICE for details. LLNL-CODE-806117.
//
// This file is part of the MFEM library. For more information and source code
// availability visit https://mfem.org.
//
// MFEM is free software; you can redistribute it and/or modify it under the
// terms of the BSD-3 license. We welcome feedback and contributions, see file
// CONTRIBUTING.md for details.

#ifndef MFEM_TMOP_HPP
#define MFEM_TMOP_HPP

#include "../linalg/invariants.hpp"
#include "nonlininteg.hpp"

namespace mfem
{

/** @brief Abstract class for local mesh quality metrics in the target-matrix
    optimization paradigm (TMOP) by P. Knupp et al. */
class TMOP_QualityMetric : public HyperelasticModel
{
protected:
   const DenseMatrix *Jtr; /**< Jacobian of the reference-element to
                                target-element transformation. */

   /** @brief The method HyperelasticModel::SetTransformation() is hidden
       for TMOP_QualityMetric%s, because it is not used. */
   void SetTransformation(ElementTransformation &) { }

public:
   TMOP_QualityMetric() : Jtr(NULL) { }
   virtual ~TMOP_QualityMetric() { }

   /** @brief Specify the reference-element -> target-element Jacobian matrix
       for the point of interest.

       The specified Jacobian matrix, #Jtr, can be used by metrics that cannot
       be written just as a function of the target->physical Jacobian matrix,
       Jpt. */
   virtual void SetTargetJacobian(const DenseMatrix &Jtr_) { Jtr = &Jtr_; }

   /** @brief Evaluates the metric in matrix form (opposed to invariant form).
       Used for validating the invariant evaluations. */
   virtual double EvalWMatrixForm(const DenseMatrix &Jpt) const
   { return -1.0; /* not implemented -> checks would fail. */ }

   /** @brief Evaluate the strain energy density function, W = W(Jpt), by using
       the 2D or 3D matrix invariants, see linalg/invariants.hpp.
       @param[in] Jpt  Represents the target->physical transformation
                       Jacobian matrix. */
   virtual double EvalW(const DenseMatrix &Jpt) const = 0;

   /** @brief Evaluate the 1st Piola-Kirchhoff stress tensor, P = P(Jpt).
       @param[in] Jpt  Represents the target->physical transformation
                       Jacobian matrix.
       @param[out]  P  The evaluated 1st Piola-Kirchhoff stress tensor. */
   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const = 0;

   /** @brief Evaluate the derivative of the 1st Piola-Kirchhoff stress tensor
       and assemble its contribution to the local gradient matrix 'A'.
       @param[in] Jpt     Represents the target->physical transformation
                          Jacobian matrix.
       @param[in] DS      Gradient of the basis matrix (dof x dim).
       @param[in] weight  Quadrature weight coefficient for the point.
       @param[in,out]  A  Local gradient matrix where the contribution from this
                          point will be added.

       Computes weight * d(dW_dxi)_d(xj) at the current point, for all i and j,
       where x1 ... xn are the FE dofs. This function is usually defined using
       the matrix invariants and their derivatives. */
   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const = 0;

   /** @brief Return the metric ID. */
   virtual int Id() const { return 0; }
};

class TargetConstructor;

/// Abstract class used to define explicit combination of metrics with constant
/// coefficients.
class TMOP_Combo_QualityMetric : public TMOP_QualityMetric
{
protected:
   Array<TMOP_QualityMetric *> tmop_q_arr; //the metrics are not owned
   Array<double> wt_arr;

public:
   virtual void AddQualityMetric(TMOP_QualityMetric *tq, double wt = 1.0)
   {
      tmop_q_arr.Append(tq);
      wt_arr.Append(wt);
   }

   virtual void SetTargetJacobian(const DenseMatrix &Jtr_)
   {
      for (int i = 0; i < tmop_q_arr.Size(); i++)
      {
         tmop_q_arr[i]->SetTargetJacobian(Jtr_);
      }
   }

   virtual double EvalWMatrixForm(const DenseMatrix &Jpt) const;

   virtual double EvalW(const DenseMatrix &Jpt) const;

   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const;

   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const;

   /// Computes the averages of all metrics (integral of metric / volume).
   /// Works in parallel when called with a ParGridFunction.
   void ComputeAvgMetrics(const GridFunction &nodes,
                          const TargetConstructor &tc,
                          Vector &averages) const;

   /// Computes weights so that the averages of all metrics are equal, and the
   /// weights sum to one. Works in parallel when called with a ParGridFunction.
   void ComputeBalancedWeights(const GridFunction &nodes,
                               const TargetConstructor &tc,
                               Vector &weights) const;

   /// Changes the weights of the metrics in the combination.
   void SetWeights(const Vector &weights)
   {
      MFEM_VERIFY(tmop_q_arr.Size() == weights.Size(), "Incorrect #weights");
      for (int i = 0; i < tmop_q_arr.Size(); i++) { wt_arr[i] = weights(i); }
   }
};

/// Simultaneous Untangler + Worst Case Improvement Metric
/// Uses a base metric mu and is defined as:
/// mu_tilde = mu_hat,                 when WorstCaseType = None,
///          = mu_hat/(beta - mu_hat), when WorstCaseType = Beta,
///          = mu_hat^p,               when WorstCaseType = PMean,
/// where beta = max(mu_hat) + muT_ep,
/// and mu_hat = (mu/2phi(tau,ep)) where
/// 2phi(tau,ep) = 1, when                                 when BarrierType = None,
///             = 2*(tau - min(alpha*min(tau)-detT_ep,0)), when BarrierType = Shifted
///             = tau^2 + sqrt(tau^2 + ep^2),              when BarrierType = Pseudo
/// where tau = det(T), and max(mu_hat) and min(tau) are computed over the
/// entire mesh.
/// Ultimately, this metric can be used for mesh untangling with the BarrierType
/// option and for worst case quality improvement with the WorstCaseType option.
class TMOP_WorstCaseUntangleOptimizer_Metric : public TMOP_QualityMetric
{
public:
   enum class BarrierType
   {
      None,
      Shifted,
      Pseudo
   };
   enum class WorstCaseType
   {
      None,
      Beta,
      PMean
   };

protected:
   TMOP_QualityMetric &tmop_metric; // non-barrier metric to use
   double min_detT;                 // minimum Jacobian in the mesh
   double max_muT;                  // max mu_k/phi(tau,ep) in the mesh
   int exponent;                    // used for p-mean metrics
   double alpha;                    // scaling factor for min(det(T))
   double detT_ep;                  // small constant subtracted from min(detT)
   double muT_ep;                   // small constant added to muT term
   BarrierType btype;
   WorstCaseType wctype;

public:
   TMOP_WorstCaseUntangleOptimizer_Metric(TMOP_QualityMetric &tmop_metric_,
                                          int exponent_ = 1,
                                          double alpha_ = 1.5,
                                          double detT_ep_ = 0.0001,
                                          double muT_ep_ = 0.0001,
                                          BarrierType btype_ = BarrierType::None,
                                          WorstCaseType wctype_ = WorstCaseType::None) :
      tmop_metric(tmop_metric_), exponent(exponent_), alpha(alpha_),
      detT_ep(detT_ep_), muT_ep(muT_ep_), btype(btype_), wctype(wctype_)
   {
      MFEM_VERIFY(wctype == WorstCaseType::None,
                  "Worst-case optimization has not been fully developed!");
      if (btype != BarrierType::None)
      {
         const int m_id = tmop_metric.Id();
         MFEM_VERIFY(m_id == 4 || m_id == 14 || m_id == 66,
                     "Incorrect input barrier metric --  must be 4 / 14 / 66");
      }
   }

   virtual double EvalW(const DenseMatrix &Jpt) const;

   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const
   { MFEM_ABORT("Not implemented"); }

   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const
   { MFEM_ABORT("Not implemented"); }

   // Compute mu_hat.
   virtual double EvalWBarrier(const DenseMatrix &Jpt) const;

   virtual void SetMinDetT(double min_detT_) { min_detT = min_detT_; }

   virtual void SetMaxMuT(double max_muT_) { max_muT = max_muT_; }

   virtual BarrierType GetBarrierType() { return btype; }

   virtual WorstCaseType GetWorstCaseType() { return wctype; }
};

/// 2D non-barrier metric without a type.
class TMOP_Metric_001 : public TMOP_QualityMetric
{
protected:
   mutable InvariantsEvaluator2D<double> ie;

public:
   // W = |J|^2.
   virtual double EvalW(const DenseMatrix &Jpt) const;

   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const;

   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const;

   virtual int Id() const { return 1; }
};

/// 2D non-barrier Skew metric.
class TMOP_Metric_skew2D : public TMOP_QualityMetric
{
public:
   // W = 0.5 (1 - cos(angle_Jpr - angle_Jtr)).
   virtual double EvalW(const DenseMatrix &Jpt) const;

   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const
   { MFEM_ABORT("Not implemented"); }

   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const
   { MFEM_ABORT("Not implemented"); }
};

/// 3D non-barrier Skew metric.
class TMOP_Metric_skew3D : public TMOP_QualityMetric
{
public:
   // W = 1/6 (3 - sum_i cos(angle_Jpr_i - angle_Jtr_i)), i = 1..3.
   virtual double EvalW(const DenseMatrix &Jpt) const;

   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const
   { MFEM_ABORT("Not implemented"); }

   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const
   { MFEM_ABORT("Not implemented"); }
};

/// 2D non-barrier Aspect ratio metric.
class TMOP_Metric_aspratio2D : public TMOP_QualityMetric
{
public:
   // W = 0.5 (ar_Jpr/ar_Jtr + ar_Jtr/ar_Jpr) - 1.
   virtual double EvalW(const DenseMatrix &Jpt) const;

   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const
   { MFEM_ABORT("Not implemented"); }

   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const
   { MFEM_ABORT("Not implemented"); }
};

/// 3D non-barrier Aspect ratio metric.
class TMOP_Metric_aspratio3D : public TMOP_QualityMetric
{
public:
   // W = 1/3 sum [0.5 (ar_Jpr_i/ar_Jtr_i + ar_Jtr_i/ar_Jpr_i) - 1], i = 1..3.
   virtual double EvalW(const DenseMatrix &Jpt) const;

   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const
   { MFEM_ABORT("Not implemented"); }

   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const
   { MFEM_ABORT("Not implemented"); }
};

/// 2D barrier shape (S) metric (polyconvex).
/// Grade - A.
class TMOP_Metric_002 : public TMOP_QualityMetric
{
protected:
   mutable InvariantsEvaluator2D<double> ie;

public:
   // W = 0.5 |J|^2 / det(J) - 1.
   virtual double EvalWMatrixForm(const DenseMatrix &Jpt) const;

   // W = 0.5 I1b - 1.
   virtual double EvalW(const DenseMatrix &Jpt) const;

   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const;

   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const;

   virtual int Id() const { return 2; }
};

/// 2D non-barrier shape (S) metric.
/// Grade - F.
class TMOP_Metric_004 : public TMOP_QualityMetric
{
protected:
   mutable InvariantsEvaluator2D<double> ie;

public:
   // W = |J|^2 - 2*det(J)
   virtual double EvalW(const DenseMatrix &Jpt) const;

   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const;

   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const;

   virtual int Id() const { return 4; }
};

/// 2D barrier Shape+Size (VS) metric (not polyconvex).
class TMOP_Metric_007 : public TMOP_QualityMetric
{
protected:
   mutable InvariantsEvaluator2D<double> ie;

public:
   // W = |J - J^-t|^2.
   virtual double EvalW(const DenseMatrix &Jpt) const;

   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const;

   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const;

   virtual int Id() const { return 7; }
};

/// 2D barrier Shape+Size (VS) metric (not polyconvex).
class TMOP_Metric_009 : public TMOP_QualityMetric
{
protected:
   mutable InvariantsEvaluator2D<double> ie;

public:
   // W = det(J) * |J - J^-t|^2.
   virtual double EvalW(const DenseMatrix &Jpt) const;

   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const;

   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const;
};

/// 2D non-barrier Shape+Size+Orientation (VOS) metric (polyconvex).
class TMOP_Metric_014 : public TMOP_QualityMetric
{
public:
   // W = |T-I|^2.
   virtual double EvalW(const DenseMatrix &Jpt) const;

   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const
   { MFEM_ABORT("Not implemented"); }

   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const
   { MFEM_ABORT("Not implemented"); }
};

/// 2D Shifted barrier form of shape metric (mu_2).
class TMOP_Metric_022 : public TMOP_QualityMetric
{
protected:
   double &min_detT;
   mutable InvariantsEvaluator2D<double> ie;

public:
   TMOP_Metric_022(double &t0): min_detT(t0) {}

   // W = 0.5(|J|^2 - 2det(J)) / (det(J) - tau0).
   virtual double EvalW(const DenseMatrix &Jpt) const;

   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const;

   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const;
};

/// 2D barrier shape metric (polyconvex).
/// Grade - A.
class TMOP_Metric_050 : public TMOP_QualityMetric
{
protected:
   mutable InvariantsEvaluator2D<double> ie;

public:
   // W = 0.5|J^t J|^2 / det(J)^2 - 1.
   virtual double EvalW(const DenseMatrix &Jpt) const;

   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const;

   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const;
};

/// 2D non-barrier size (V) metric (not polyconvex).
/// Grade - F.
class TMOP_Metric_055 : public TMOP_QualityMetric
{
protected:
   mutable InvariantsEvaluator2D<double> ie;

public:
   // W = (det(J) - 1)^2.
   virtual double EvalW(const DenseMatrix &Jpt) const;

   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const;

   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const;

};

/// 2D barrier size (V) metric (polyconvex).
/// Grade - C.
class TMOP_Metric_056 : public TMOP_QualityMetric
{
protected:
   mutable InvariantsEvaluator2D<double> ie;

public:
   // W = 0.5( sqrt(det(J)) - 1 / sqrt(det(J)) )^2
   //   = 0.5( det(J) - 1 )^2 / det(J)
   //   = 0.5( det(J) + 1/det(J) ) - 1.
   virtual double EvalW(const DenseMatrix &Jpt) const;

   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const;

   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const;
};

/// 2D barrier shape (S) metric (not polyconvex).
class TMOP_Metric_058 : public TMOP_QualityMetric
{
protected:
   mutable InvariantsEvaluator2D<double> ie;

public:
   // W = |J^t J|^2 / det(J)^2 - 2|J|^2 / det(J) + 2
   virtual double EvalWMatrixForm(const DenseMatrix &Jpt) const;

   // W = I1b (I1b - 2).
   virtual double EvalW(const DenseMatrix &Jpt) const;

   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const;

   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const;
};

/// 2D non-barrier Shape+Size (VS) metric.
/// Grade - F.
class TMOP_Metric_066 : public TMOP_Combo_QualityMetric
{
protected:
   mutable InvariantsEvaluator2D<double> ie;
   TMOP_QualityMetric *sh_metric, *sz_metric;

public:
   TMOP_Metric_066(double gamma)
      : sh_metric(new TMOP_Metric_004), sz_metric(new TMOP_Metric_055)
   {
      // (1-gamma) mu_4 + gamma mu_55
      AddQualityMetric(sh_metric, 1.-gamma);
      AddQualityMetric(sz_metric, gamma);
   }
   virtual int Id() const { return 66; }
   double GetGamma() const { return wt_arr[1]; }

   virtual ~TMOP_Metric_066() { delete sh_metric; delete sz_metric; }
};

/// 2D barrier size (V) metric (polyconvex).
/// Grade - C.
class TMOP_Metric_077 : public TMOP_QualityMetric
{
protected:
   mutable InvariantsEvaluator2D<double> ie;

public:
   // W = 0.5(det(J) - 1 / det(J))^2.
   virtual double EvalW(const DenseMatrix &Jpt) const;

   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const;

   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const;

   virtual int Id() const { return 77; }
};

/// 2D barrier Shape+Size (VS) metric (polyconvex).
/// Grade - A.
class TMOP_Metric_080 : public TMOP_Combo_QualityMetric
{
protected:
   mutable InvariantsEvaluator2D<double> ie;
   TMOP_QualityMetric *sh_metric, *sz_metric;

public:
   TMOP_Metric_080(double gamma)
      : sh_metric(new TMOP_Metric_002), sz_metric(new TMOP_Metric_077)
   {
      // (1-gamma) mu_2 + gamma mu_77
      AddQualityMetric(sh_metric, 1.0 - gamma);
      AddQualityMetric(sz_metric, gamma);
   }

   virtual int Id() const { return 80; }
   double GetGamma() const { return wt_arr[1]; }

   virtual ~TMOP_Metric_080() { delete sh_metric; delete sz_metric; }
};

/// 2D barrier Shape+Orientation (OS) metric (polyconvex).
class TMOP_Metric_085 : public TMOP_QualityMetric
{
public:
   // W = |T-T'|^2, where T'= |T|*I/sqrt(2).
   virtual double EvalW(const DenseMatrix &Jpt) const;

   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const
   { MFEM_ABORT("Not implemented"); }

   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const
   { MFEM_ABORT("Not implemented"); }
};

/// 2D barrier Shape+Size+Orientation (VOS) metric (polyconvex).
class TMOP_Metric_098 : public TMOP_QualityMetric
{
public:
   // W = 1/tau |T-I|^2.
   virtual double EvalW(const DenseMatrix &Jpt) const;

   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const
   { MFEM_ABORT("Not implemented"); }

   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const
   { MFEM_ABORT("Not implemented"); }
};

/// 2D untangling metric.
class TMOP_Metric_211 : public TMOP_QualityMetric
{
protected:
   const double eps;
   mutable InvariantsEvaluator2D<double> ie;

public:
   TMOP_Metric_211(double epsilon = 1e-4) : eps(epsilon) { }

   // W = (det(J) - 1)^2 - det(J) + sqrt(det(J)^2 + eps).
   virtual double EvalW(const DenseMatrix &Jpt) const;

   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const;

   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const;
};

/// Shifted barrier form of metric 56 (area, ideal barrier metric), 2D
class TMOP_Metric_252 : public TMOP_QualityMetric
{
protected:
   double &tau0;
   mutable InvariantsEvaluator2D<double> ie;

public:
   /// Note that @a t0 is stored by reference
   TMOP_Metric_252(double &t0): tau0(t0) {}

   // W = 0.5(det(J) - 1)^2 / (det(J) - tau0).
   virtual double EvalW(const DenseMatrix &Jpt) const;

   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const;

   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const;
};

/// 3D barrier Shape (S) metric, well-posed (polyconvex & invex).
class TMOP_Metric_301 : public TMOP_QualityMetric
{
protected:
   mutable InvariantsEvaluator3D<double> ie;

public:
   // W = 1/3 |J| |J^-1| - 1.
   virtual double EvalWMatrixForm(const DenseMatrix &Jpt) const;

   // W = 1/3 sqrt(I1b * I2b) - 1
   virtual double EvalW(const DenseMatrix &Jpt) const;

   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const;

   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const;
};

/// 3D barrier Shape (S) metric, well-posed (polyconvex & invex).
class TMOP_Metric_302 : public TMOP_QualityMetric
{
protected:
   mutable InvariantsEvaluator3D<double> ie;

public:
   // W = |J|^2 |J^{-1}|^2 / 9 - 1.
   virtual double EvalWMatrixForm(const DenseMatrix &Jpt) const;

   // W = I1b * I2b / 9 - 1.
   virtual double EvalW(const DenseMatrix &Jpt) const;

   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const;

   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const;

   virtual int Id() const { return 302; }
};

/// 3D barrier Shape (S) metric, well-posed (polyconvex & invex).
class TMOP_Metric_303 : public TMOP_QualityMetric
{
protected:
   mutable InvariantsEvaluator3D<double> ie;

public:
   // W = |J|^2 / 3 / det(J)^(2/3) - 1.
   virtual double EvalWMatrixForm(const DenseMatrix &Jpt) const;

   // W = I1b / 3 - 1.
   virtual double EvalW(const DenseMatrix &Jpt) const;

   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const;

   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const;

   virtual int Id() const { return 303; }
};

/// 3D barrier Shape (S) metric, well-posed (polyconvex & invex).
class TMOP_Metric_304 : public TMOP_QualityMetric
{
protected:
   mutable InvariantsEvaluator3D<double> ie;

public:
   // W = |J|^3 / 3^(3/2) / det(J) - 1.
   virtual double EvalWMatrixForm(const DenseMatrix &Jpt) const;

   // W = (I1b/3)^3/2 - 1.
   virtual double EvalW(const DenseMatrix &Jpt) const;

   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const;

   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const;

   virtual int Id() const { return 304; }
};

/// 3D Size (V) untangling metric.
class TMOP_Metric_311 : public TMOP_QualityMetric
{
protected:
   const double eps;
   mutable InvariantsEvaluator3D<double> ie;

public:
   TMOP_Metric_311(double epsilon = 1e-4) : eps(epsilon) { }

   // W = (det(J) - 1)^2 - det(J)  + (det(J)^2 + eps)^(1/2).
   virtual double EvalW(const DenseMatrix &Jpt) const;

   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const;

   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const;
};

/// 3D Shape (S) metric, untangling version of 303.
class TMOP_Metric_313 : public TMOP_QualityMetric
{
protected:
   double &min_detT;
   mutable InvariantsEvaluator3D<double> ie;

public:
   TMOP_Metric_313(double &mindet) : min_detT(mindet) { }

   // W = 1/3 |J|^2 / [det(J)-tau0]^(-2/3).
   virtual double EvalW(const DenseMatrix &Jpt) const;

   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const;

   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const;

   virtual int Id() const { return 313; }
};

/// 3D non-barrier metric without a type.
class TMOP_Metric_315 : public TMOP_QualityMetric
{
protected:
   mutable InvariantsEvaluator3D<double> ie;

public:
   // W = (det(J) - 1)^2.
   virtual double EvalW(const DenseMatrix &Jpt) const;

   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const;

   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const;

   virtual int Id() const { return 315; }
};

/// 3D barrier metric without a type.
class TMOP_Metric_316 : public TMOP_QualityMetric
{
protected:
   mutable InvariantsEvaluator3D<double> ie;

public:
   // W = 0.5 (det(J) + 1/det(J)) - 1.
   virtual double EvalWMatrixForm(const DenseMatrix &Jpt) const;

   // W = 0.5 (I3b + 1/I3b) - 1.
   virtual double EvalW(const DenseMatrix &Jpt) const;

   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const;

   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const;
};

/// 3D barrier Shape+Size (VS) metric, well-posed (invex).
class TMOP_Metric_321 : public TMOP_QualityMetric
{
protected:
   mutable InvariantsEvaluator3D<double> ie;

public:
   // W = |J - J^-t|^2.
   virtual double EvalWMatrixForm(const DenseMatrix &Jpt) const;

   // W = I1 + I2/I3 - 6.
   virtual double EvalW(const DenseMatrix &Jpt) const;

   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const;

   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const;

   virtual int Id() const { return 321; }
};

/// 3D barrier Shape+Size (VS) metric, well-posed (invex).
class TMOP_Metric_322 : public TMOP_QualityMetric
{
protected:
   mutable InvariantsEvaluator3D<double> ie;

public:
   // W = |J - adjJ^-t|^2.
   virtual double EvalWMatrixForm(const DenseMatrix &Jpt) const;

   // W = I1b / (I3b^-1/3) / 6 + I2b (I3b^1/3) / 6 - 1
   virtual double EvalW(const DenseMatrix &Jpt) const;

   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const;

   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const;

   virtual int Id() const { return 322; }
};

/// 3D barrier Shape+Size (VS) metric, well-posed (invex).
class TMOP_Metric_323 : public TMOP_QualityMetric
{
protected:
   mutable InvariantsEvaluator3D<double> ie;

public:
   // W = |J|^3 - 3 sqrt(3) ln(det(J)) - 3 sqrt(3).
   virtual double EvalWMatrixForm(const DenseMatrix &Jpt) const;

   // W = I1^3/2 - 3 sqrt(3) ln(I3b) - 3 sqrt(3).
   virtual double EvalW(const DenseMatrix &Jpt) const;

   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const;

   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const;

   virtual int Id() const { return 323; }
};

/// 3D barrier Shape+Size (VS) metric (polyconvex).
class TMOP_Metric_328 : public TMOP_Combo_QualityMetric
{
protected:
   mutable InvariantsEvaluator2D<double> ie;
   TMOP_QualityMetric *sh_metric, *sz_metric;

public:
   TMOP_Metric_328(double gamma)
      : sh_metric(new TMOP_Metric_301), sz_metric(new TMOP_Metric_316)
   {
      // (1-gamma) mu_301 + gamma mu_316
      AddQualityMetric(sh_metric, 1.-gamma);
      AddQualityMetric(sz_metric, gamma);
   }

   virtual ~TMOP_Metric_328() { delete sh_metric; delete sz_metric; }
};

/// 3D barrier Shape+Size (VS) metric (polyconvex).
class TMOP_Metric_332 : public TMOP_Combo_QualityMetric
{
protected:
   TMOP_QualityMetric *sh_metric, *sz_metric;

public:
   TMOP_Metric_332(double gamma)
      : sh_metric(new TMOP_Metric_302), sz_metric(new TMOP_Metric_315)
   {
      // (1-gamma) mu_302 + gamma mu_315
      AddQualityMetric(sh_metric, 1.-gamma);
      AddQualityMetric(sz_metric, gamma);
   }

   virtual int Id() const { return 332; }
   double GetGamma() const { return wt_arr[1]; }

   virtual ~TMOP_Metric_332() { delete sh_metric; delete sz_metric; }
};

/// 3D barrier Shape+Size (VS) metric, well-posed (polyconvex).
class TMOP_Metric_333 : public TMOP_Combo_QualityMetric
{
protected:
   mutable InvariantsEvaluator2D<double> ie;
   TMOP_QualityMetric *sh_metric, *sz_metric;

public:
   TMOP_Metric_333(double gamma)
      : sh_metric(new TMOP_Metric_302), sz_metric(new TMOP_Metric_316)
   {
      // (1-gamma) mu_302 + gamma mu_316
      AddQualityMetric(sh_metric, 1.-gamma);
      AddQualityMetric(sz_metric, gamma);
   }

   virtual ~TMOP_Metric_333() { delete sh_metric; delete sz_metric; }
};

/// 3D barrier Shape+Size (VS) metric, well-posed (polyconvex).
class TMOP_Metric_334 : public TMOP_Combo_QualityMetric
{
protected:
   mutable InvariantsEvaluator2D<double> ie;
   TMOP_QualityMetric *sh_metric, *sz_metric;

public:
   TMOP_Metric_334(double gamma)
      : sh_metric(new TMOP_Metric_303), sz_metric(new TMOP_Metric_316)
   {
      // (1-gamma) mu_303 + gamma mu_316
      AddQualityMetric(sh_metric, 1.-gamma);
      AddQualityMetric(sz_metric, gamma);
   }

   virtual int Id() const { return 334; }
   double GetGamma() const { return wt_arr[1]; }

   virtual ~TMOP_Metric_334() { delete sh_metric; delete sz_metric; }
};

/// 3D barrier Shape+Size (VS) metric, well-posed (polyconvex).
class TMOP_Metric_347 : public TMOP_Combo_QualityMetric
{
protected:
   mutable InvariantsEvaluator2D<double> ie;
   TMOP_QualityMetric *sh_metric, *sz_metric;

public:
   TMOP_Metric_347(double gamma)
      : sh_metric(new TMOP_Metric_304), sz_metric(new TMOP_Metric_316)
   {
      // (1-gamma) mu_304 + gamma mu_316
      AddQualityMetric(sh_metric, 1.-gamma);
      AddQualityMetric(sz_metric, gamma);
   }

   virtual int Id() const { return 347; }
   double GetGamma() const { return wt_arr[1]; }

   virtual ~TMOP_Metric_347() { delete sh_metric; delete sz_metric; }
};

/// 3D shifted barrier form of metric 316 (not typed).
class TMOP_Metric_352 : public TMOP_QualityMetric
{
protected:
   double &tau0;
   mutable InvariantsEvaluator3D<double> ie;

public:
   TMOP_Metric_352(double &t0): tau0(t0) {}

   // W = 0.5(det(J) - 1)^2 / (det(J) - tau0).
   virtual double EvalW(const DenseMatrix &Jpt) const;

   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const;

   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const;
};

/// 3D non-barrier Shape (S) metric.
class TMOP_Metric_360 : public TMOP_QualityMetric
{
protected:
   mutable InvariantsEvaluator3D<double> ie;

public:
   // W = |J|^3 / 3^(3/2) - det(J).
   virtual double EvalWMatrixForm(const DenseMatrix &Jpt) const;

   // W = (I1b/3)^3/2 - 1.
   virtual double EvalW(const DenseMatrix &Jpt) const;

   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const;

   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const;

   virtual int Id() const { return 360; }
};

/// A-metrics
/// 2D barrier Shape (S) metric (polyconvex).
class TMOP_AMetric_011 : public TMOP_QualityMetric
{
protected:
   mutable InvariantsEvaluator3D<double> ie;

public:
   // (1/4 alpha) | A - (adj A)^t W^t W / omega |^2
   virtual double EvalW(const DenseMatrix &Jpt) const;

   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const
   { MFEM_ABORT("Not implemented"); }

   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const
   { MFEM_ABORT("Not implemented"); }
};

/// 2D barrier Size (V) metric (polyconvex).
class TMOP_AMetric_014a : public TMOP_QualityMetric
{
protected:
   mutable InvariantsEvaluator3D<double> ie;

public:
   // 0.5 * ( sqrt(alpha/omega) - sqrt(omega/alpha) )^2
   virtual double EvalW(const DenseMatrix &Jpt) const;

   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const
   { MFEM_ABORT("Not implemented"); }

   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const
   { MFEM_ABORT("Not implemented"); }
};

/// 2D barrier Shape+Size+Orientation (VOS) metric (polyconvex).
class TMOP_AMetric_036 : public TMOP_QualityMetric
{
protected:
   mutable InvariantsEvaluator3D<double> ie;

public:
   // (1/alpha) | A - W |^2
   virtual double EvalW(const DenseMatrix &Jpt) const;

   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const
   { MFEM_ABORT("Not implemented"); }

   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const
   { MFEM_ABORT("Not implemented"); }
};

/// 2D barrier Shape+Orientation (OS) metric (polyconvex).
class TMOP_AMetric_107a : public TMOP_QualityMetric
{
protected:
   mutable InvariantsEvaluator3D<double> ie;

public:
   // (1/2 alpha) | A - (|A|/|W|) W |^2
   virtual double EvalW(const DenseMatrix &Jpt) const;

   virtual void EvalP(const DenseMatrix &Jpt, DenseMatrix &P) const
   { MFEM_ABORT("Not implemented"); }

   virtual void AssembleH(const DenseMatrix &Jpt, const DenseMatrix &DS,
                          const double weight, DenseMatrix &A) const
   { MFEM_ABORT("Not implemented"); }
};

/// 2D barrier Shape+Size (VS) metric (polyconvex).
class TMOP_AMetric_126 : public TMOP_Combo_QualityMetric
{
protected:
   mutable InvariantsEvaluator2D<double> ie;
   TMOP_QualityMetric *sh_metric, *sz_metric;

public:
   TMOP_AMetric_126(double gamma)
      : sh_metric(new TMOP_AMetric_011), sz_metric(new TMOP_AMetric_014a)
   {
      // (1-gamma) nu_11 + gamma nu_14
      AddQualityMetric(sh_metric, 1.-gamma);
      AddQualityMetric(sz_metric, gamma);
   }

   virtual ~TMOP_AMetric_126() { delete sh_metric; delete sz_metric; }
};

/// Base class for limiting functions to be used in class TMOP_Integrator.
/** This class represents a scalar function f(x, x0, d), where x and x0 are
    positions in physical space, and d is a reference physical distance
    associated with the point x0. */
class TMOP_LimiterFunction
{
public:
   /// Returns the limiting function, f(x, x0, d).
   virtual double Eval(const Vector &x, const Vector &x0, double d) const = 0;

   /** @brief Returns the gradient of the limiting function f(x, x0, d) with
       respect to x. */
   virtual void Eval_d1(const Vector &x, const Vector &x0, double dist,
                        Vector &d1) const = 0;

   /** @brief Returns the Hessian of the limiting function f(x, x0, d) with
       respect to x. */
   virtual void Eval_d2(const Vector &x, const Vector &x0, double dist,
                        DenseMatrix &d2) const = 0;

   /// Virtual destructor.
   virtual ~TMOP_LimiterFunction() { }
};

/// Default limiter function in TMOP_Integrator.
class TMOP_QuadraticLimiter : public TMOP_LimiterFunction
{
public:
   virtual double Eval(const Vector &x, const Vector &x0, double dist) const
   {
      MFEM_ASSERT(x.Size() == x0.Size(), "Bad input.");

      return 0.5 * x.DistanceSquaredTo(x0) / (dist * dist);
   }

   virtual void Eval_d1(const Vector &x, const Vector &x0, double dist,
                        Vector &d1) const
   {
      MFEM_ASSERT(x.Size() == x0.Size(), "Bad input.");

      d1.SetSize(x.Size());
      subtract(1.0 / (dist * dist), x, x0, d1);
   }

   virtual void Eval_d2(const Vector &x, const Vector &x0, double dist,
                        DenseMatrix &d2) const
   {
      MFEM_ASSERT(x.Size() == x0.Size(), "Bad input.");

      d2.Diag(1.0 / (dist * dist), x.Size());
   }

   virtual ~TMOP_QuadraticLimiter() { }
};

/// Exponential limiter function in TMOP_Integrator.
class TMOP_ExponentialLimiter : public TMOP_LimiterFunction
{
public:
   virtual double Eval(const Vector &x, const Vector &x0, double dist) const
   {
      MFEM_ASSERT(x.Size() == x0.Size(), "Bad input.");

      return  exp(10.0*((x.DistanceSquaredTo(x0) / (dist * dist))-1.0));
   }

   virtual void Eval_d1(const Vector &x, const Vector &x0, double dist,
                        Vector &d1) const
   {
      MFEM_ASSERT(x.Size() == x0.Size(), "Bad input.");

      d1.SetSize(x.Size());
      double dist_squared = dist*dist;
      subtract(20.0*exp(10.0*((x.DistanceSquaredTo(x0) / dist_squared) - 1.0)) /
               dist_squared, x, x0, d1);
   }

   virtual void Eval_d2(const Vector &x, const Vector &x0, double dist,
                        DenseMatrix &d2) const
   {
      MFEM_ASSERT(x.Size() == x0.Size(), "Bad input.");
      Vector tmp;
      tmp.SetSize(x.Size());
      double dist_squared = dist*dist;
      double dist_squared_squared = dist_squared*dist_squared;
      double f = exp(10.0*((x.DistanceSquaredTo(x0) / dist_squared)-1.0));

      subtract(x,x0,tmp);
      d2.SetSize(x.Size());
      d2(0,0) = ((400.0*tmp(0)*tmp(0)*f)/dist_squared_squared)+(20.0*f/dist_squared);
      d2(1,1) = ((400.0*tmp(1)*tmp(1)*f)/dist_squared_squared)+(20.0*f/dist_squared);
      d2(0,1) = (400.0*tmp(0)*tmp(1)*f)/dist_squared_squared;
      d2(1,0) = d2(0,1);

      if (x.Size() == 3)
      {
         d2(0,2) = (400.0*tmp(0)*tmp(2)*f)/dist_squared_squared;
         d2(1,2) = (400.0*tmp(1)*tmp(2)*f)/dist_squared_squared;
         d2(2,0) = d2(0,2);
         d2(2,1) = d2(1,2);
         d2(2,2) = ((400.0*tmp(2)*tmp(2)*f)/dist_squared_squared)+(20.0*f/dist_squared);
      }

   }

   virtual ~TMOP_ExponentialLimiter() { }
};

class FiniteElementCollection;
class FiniteElementSpace;
class ParFiniteElementSpace;

class AdaptivityEvaluator
{
protected:
   // Owned.
   Mesh *mesh;
   FiniteElementSpace *fes;

#ifdef MFEM_USE_MPI
   // Owned.
   ParMesh *pmesh;
   ParFiniteElementSpace *pfes;
#endif

public:
   AdaptivityEvaluator() : mesh(NULL), fes(NULL)
   {
#ifdef MFEM_USE_MPI
      pmesh = NULL;
      pfes = NULL;
#endif
   }
   virtual ~AdaptivityEvaluator();

   /** Specifies the Mesh and FiniteElementSpace of the solution that will
       be evaluated. The given mesh will be copied into the internal object. */
   void SetSerialMetaInfo(const Mesh &m,
                          const FiniteElementSpace &f);

#ifdef MFEM_USE_MPI
   /// Parallel version of SetSerialMetaInfo.
   void SetParMetaInfo(const ParMesh &m,
                       const ParFiniteElementSpace &f);
#endif

   // TODO use GridFunctions to make clear it's on the ldofs?
   virtual void SetInitialField(const Vector &init_nodes,
                                const Vector &init_field) = 0;

   virtual void ComputeAtNewPosition(const Vector &new_nodes,
                                     Vector &new_field,
                                     int new_nodes_ordering = Ordering::byNODES) = 0;

   void ClearGeometricFactors();
};

/** @brief Base class representing target-matrix construction algorithms for
    mesh optimization via the target-matrix optimization paradigm (TMOP). */
/** This class is used by class TMOP_Integrator to construct the target Jacobian
    matrices (reference-element to target-element) at quadrature points. It
    supports a set of algorithms chosen by the #TargetType enumeration.

    New target-matrix construction algorithms can be defined by deriving new
    classes and overriding the methods ComputeElementTargets() and
    ContainsVolumeInfo(). */
class TargetConstructor
{
public:
   /// Target-matrix construction algorithms supported by this class.
   enum TargetType
   {
      IDEAL_SHAPE_UNIT_SIZE, /**<
         Ideal shape, unit size; the nodes are not used. */
      IDEAL_SHAPE_EQUAL_SIZE, /**<
         Ideal shape, equal size/volume; the given nodes define the total target
         volume; for each mesh element, the target volume is the average volume
         multiplied by the volume scale, set with SetVolumeScale(). */
      IDEAL_SHAPE_GIVEN_SIZE, /**<
         Ideal shape, given size/volume; the given nodes define the target
         volume at all quadrature points. */
      GIVEN_SHAPE_AND_SIZE, /**<
         Given shape, given size/volume; the given nodes define the exact target
         Jacobian matrix at all quadrature points. */
      GIVEN_FULL /**<
         Full target tensor is specified at every quadrature point. */
   };

protected:
   // Nodes that are used in ComputeElementTargets(), depending on target_type.
   const GridFunction *nodes; // not owned
   mutable double avg_volume;
   double volume_scale;
   const TargetType target_type;
   bool uses_phys_coords; // see UsesPhysicalCoordinates()

#ifdef MFEM_USE_MPI
   MPI_Comm comm;
#endif

   // should be called only if avg_volume == 0.0, i.e. avg_volume is not
   // computed yet
   void ComputeAvgVolume() const;

   template<int DIM>
   bool ComputeAllElementTargets(const FiniteElementSpace &fes,
                                 const IntegrationRule &ir,
                                 const Vector &xe,
                                 DenseTensor &Jtr) const;

   // CPU fallback that uses ComputeElementTargets()
   void ComputeAllElementTargets_Fallback(const FiniteElementSpace &fes,
                                          const IntegrationRule &ir,
                                          const Vector &xe,
                                          DenseTensor &Jtr) const;

public:
   /// Constructor for use in serial
   TargetConstructor(TargetType ttype)
      : nodes(NULL), avg_volume(), volume_scale(1.0), target_type(ttype),
        uses_phys_coords(false)
   {
#ifdef MFEM_USE_MPI
      comm = MPI_COMM_NULL;
#endif
   }
#ifdef MFEM_USE_MPI
   /// Constructor for use in parallel
   TargetConstructor(TargetType ttype, MPI_Comm mpicomm)
      : nodes(NULL), avg_volume(), volume_scale(1.0), target_type(ttype),
        uses_phys_coords(false), comm(mpicomm) { }
#endif
   virtual ~TargetConstructor() { }

#ifdef MFEM_USE_MPI
   bool Parallel() const { return (comm != MPI_COMM_NULL); }
   MPI_Comm GetComm() const { return comm; }
#else
   bool Parallel() const { return false; }
#endif

   /** @brief Set the nodes to be used in the target-matrix construction.

       This method should be called every time the target nodes are updated
       externally and recomputation of the target average volume is needed. The
       nodes are used by all target types except IDEAL_SHAPE_UNIT_SIZE. */
   void SetNodes(const GridFunction &n) { nodes = &n; avg_volume = 0.0; }

   /** @brief Get the nodes to be used in the target-matrix construction. */
   const GridFunction *GetNodes() const { return nodes; }

   /// Used by target type IDEAL_SHAPE_EQUAL_SIZE. The default volume scale is 1.
   void SetVolumeScale(double vol_scale) { volume_scale = vol_scale; }

   TargetType GetTargetType() const { return target_type; }

   /** @brief Return true if the methods ComputeElementTargets(),
       ComputeAllElementTargets(), and ComputeElementTargetsGradient() use the
       physical node coordinates provided by the parameters 'elfun', or 'xe'. */
   bool UsesPhysicalCoordinates() const { return uses_phys_coords; }

   /// Checks if the target matrices contain non-trivial size specification.
   virtual bool ContainsVolumeInfo() const;

   /** @brief Given an element and quadrature rule, computes ref->target
       transformation Jacobians for each quadrature point in the element.
       The physical positions of the element's nodes are given by @a elfun. */
   virtual void ComputeElementTargets(int e_id, const FiniteElement &fe,
                                      const IntegrationRule &ir,
                                      const Vector &elfun,
                                      DenseTensor &Jtr) const;

   /** @brief Computes reference-to-target transformation Jacobians for all
       quadrature points in all elements.

       @param[in] fes  The nodal FE space
       @param[in] ir   The quadrature rule to use for all elements
       @param[in] xe   E-vector with the current physical coordinates/positions;
                       this parameter is used only when needed by the target
                       constructor, see UsesPhysicalCoordinates()
       @param[out] Jtr The computed ref->target Jacobian matrices. */
   virtual void ComputeAllElementTargets(const FiniteElementSpace &fes,
                                         const IntegrationRule &ir,
                                         const Vector &xe,
                                         DenseTensor &Jtr) const;

   virtual void ComputeElementTargetsGradient(const IntegrationRule &ir,
                                              const Vector &elfun,
                                              IsoparametricTransformation &Tpr,
                                              DenseTensor &dJtr) const;
};

class TMOPMatrixCoefficient : public MatrixCoefficient
{
public:
   explicit TMOPMatrixCoefficient(int dim) : MatrixCoefficient(dim, dim) { }

   /** @brief Evaluate the derivative of the matrix coefficient with respect to
       @a comp in the element described by @a T at the point @a ip, storing the
       result in @a K. */
   virtual void EvalGrad(DenseMatrix &K, ElementTransformation &T,
                         const IntegrationPoint &ip, int comp) = 0;

   virtual ~TMOPMatrixCoefficient() { }
};

class AnalyticAdaptTC : public TargetConstructor
{
protected:
   // Analytic target specification.
   Coefficient *scalar_tspec;
   VectorCoefficient *vector_tspec;
   TMOPMatrixCoefficient *matrix_tspec;

public:
   AnalyticAdaptTC(TargetType ttype)
      : TargetConstructor(ttype),
        scalar_tspec(NULL), vector_tspec(NULL), matrix_tspec(NULL)
   { uses_phys_coords = true; }

   virtual void SetAnalyticTargetSpec(Coefficient *sspec,
                                      VectorCoefficient *vspec,
                                      TMOPMatrixCoefficient *mspec);

   /** @brief Given an element and quadrature rule, computes ref->target
       transformation Jacobians for each quadrature point in the element.
       The physical positions of the element's nodes are given by @a elfun. */
   virtual void ComputeElementTargets(int e_id, const FiniteElement &fe,
                                      const IntegrationRule &ir,
                                      const Vector &elfun,
                                      DenseTensor &Jtr) const;

   virtual void ComputeAllElementTargets(const FiniteElementSpace &fes,
                                         const IntegrationRule &ir,
                                         const Vector &xe,
                                         DenseTensor &Jtr) const;

   virtual void ComputeElementTargetsGradient(const IntegrationRule &ir,
                                              const Vector &elfun,
                                              IsoparametricTransformation &Tpr,
                                              DenseTensor &dJtr) const;
};

#ifdef MFEM_USE_MPI
class ParGridFunction;
#endif

class DiscreteAdaptTC : public TargetConstructor
{
protected:
   // Discrete target specification.
   // Data is owned, updated by UpdateTargetSpecification.
   int ncomp, sizeidx, skewidx, aspectratioidx, orientationidx;
   Vector tspec;             //eta(x) - we enforce Ordering::byNODES
   Vector tspec_sav;
   Vector tspec_pert1h;      //eta(x+h)
   Vector tspec_pert2h;      //eta(x+2*h)
   Vector tspec_pertmix;     //eta(x+h,y+h)
   // The order inside these perturbation vectors (e.g. in 2D) is
   // eta1(x+h,y), eta2(x+h,y) ... etan(x+h,y), eta1(x,y+h), eta2(x,y+h) ...
   // same for tspec_pert2h and tspec_pertmix.

   // DenseMatrix to hold target_spec values for the (children of the)
   // element being refined to consider for h-refinement.
   DenseMatrix tspec_refine;
   // Vector to hold the target_spec values for the coarse version of the
   // current mesh. Used for derefinement decision with hr-adaptivity.
   Vector tspec_derefine;

   // Components of Target Jacobian at each quadrature point of an element. This
   // is required for computation of the derivative using chain rule.
   mutable DenseTensor Jtrcomp;

   // Note: do not use the Nodes of this space as they may not be on the
   // positions corresponding to the values of tspec.
   FiniteElementSpace *tspec_fesv;         //owned
   FiniteElementSpace *coarse_tspec_fesv;  //not owned, derefinement FESpace
   GridFunction *tspec_gf;                 //owned, uses tspec and tspec_fes
   // discrete adaptivity
#ifdef MFEM_USE_MPI
   ParFiniteElementSpace *ptspec_fesv;     //owned, needed for derefinement to
   // get update operator.
   ParGridFunction *tspec_pgf;             // similar to tspec_gf
#endif

   int amr_el;
   double lim_min_size;

   // These flags can be used by outside functions to avoid recomputing the
   // tspec and tspec_perth fields again on the same mesh.
   bool good_tspec, good_tspec_grad, good_tspec_hess;

   // Evaluation of the discrete target specification on different meshes.
   // Owned.
   AdaptivityEvaluator *adapt_eval;

   void SetDiscreteTargetBase(const GridFunction &tspec_);
   void SetTspecAtIndex(int idx, const GridFunction &tspec_);
   void FinalizeSerialDiscreteTargetSpec(const GridFunction &tspec_);
#ifdef MFEM_USE_MPI
   void SetTspecAtIndex(int idx, const ParGridFunction &tspec_);
   void FinalizeParDiscreteTargetSpec(const ParGridFunction &tspec_);
#endif

public:
   DiscreteAdaptTC(TargetType ttype)
      : TargetConstructor(ttype),
        ncomp(0),
        sizeidx(-1), skewidx(-1), aspectratioidx(-1), orientationidx(-1),
        tspec(), tspec_sav(), tspec_pert1h(), tspec_pert2h(), tspec_pertmix(),
        tspec_refine(), tspec_derefine(),
        tspec_fesv(NULL), coarse_tspec_fesv(NULL), tspec_gf(NULL),
#ifdef MFEM_USE_MPI
        ptspec_fesv(NULL), tspec_pgf(NULL),
#endif
        amr_el(-1), lim_min_size(-0.1),
        good_tspec(false), good_tspec_grad(false), good_tspec_hess(false),
        adapt_eval(NULL) { }

   virtual ~DiscreteAdaptTC();

   /** @name Target specification methods.
       The following methods are used to specify geometric parameters of the
       targets when these parameters are given by discrete FE functions.
       Note that every GridFunction given to the Set methods must use a
       H1_FECollection of the same order. The number of components must
       correspond to the type of geometric parameter and dimension.

       @param[in] tspec_  Input values of a geometric parameter. Note that
                          the methods in this class support only functions that
                          use H1_FECollection collection of the same order. */
   ///@{
   virtual void SetSerialDiscreteTargetSpec(const GridFunction &tspec_);
   virtual void SetSerialDiscreteTargetSize(const GridFunction &tspec_);
   virtual void SetSerialDiscreteTargetSkew(const GridFunction &tspec_);
   virtual void SetSerialDiscreteTargetAspectRatio(const GridFunction &tspec_);
   virtual void SetSerialDiscreteTargetOrientation(const GridFunction &tspec_);
#ifdef MFEM_USE_MPI
   virtual void SetParDiscreteTargetSpec(const ParGridFunction &tspec_);
   virtual void SetParDiscreteTargetSize(const ParGridFunction &tspec_);
   virtual void SetParDiscreteTargetSkew(const ParGridFunction &tspec_);
   virtual void SetParDiscreteTargetAspectRatio(const ParGridFunction &tspec_);
   virtual void SetParDiscreteTargetOrientation(const ParGridFunction &tspec_);
#endif
   ///@}

   /// Used in combination with the Update methods to avoid extra computations.
   void ResetUpdateFlags()
   { good_tspec = good_tspec_grad = good_tspec_hess = false; }

   /// Get one of the discrete fields from tspec.
   void GetDiscreteTargetSpec(GridFunction &tspec_, int idx);
   /// Get the FESpace associated with tspec.
   FiniteElementSpace *GetTSpecFESpace() { return tspec_fesv; }
   /// Get the entire tspec.
   GridFunction *GetTSpecData() { return tspec_gf; }
   /// Update all discrete fields based on tspec and update for AMR
   void UpdateAfterMeshTopologyChange();

#ifdef MFEM_USE_MPI
   ParFiniteElementSpace *GetTSpecParFESpace() { return ptspec_fesv; }
   void ParUpdateAfterMeshTopologyChange();
#endif

   /** Used to update the target specification after the mesh has changed. The
       new mesh positions are given by new_x. If @a use_flags is true, repeated
       calls won't do anything until ResetUpdateFlags() is called. */
   void UpdateTargetSpecification(const Vector &new_x, bool use_flag = false,
                                  int new_x_ordering=Ordering::byNODES);

   void UpdateTargetSpecification(Vector &new_x, Vector &IntData,
                                  int new_x_ordering=Ordering::byNODES);

   void UpdateTargetSpecificationAtNode(const FiniteElement &el,
                                        ElementTransformation &T,
                                        int nodenum, int idir,
                                        const Vector &IntData);

   void RestoreTargetSpecificationAtNode(ElementTransformation &T, int nodenum);

   /** Used for finite-difference based computations. Computes the target
       specifications after a mesh perturbation in x or y direction.
       If @a use_flags is true, repeated calls won't do anything until
       ResetUpdateFlags() is called. */
   void UpdateGradientTargetSpecification(const Vector &x, double dx,
                                          bool use_flag = false,
                                          int x_ordering = Ordering::byNODES);
   /** Used for finite-difference based computations. Computes the target
       specifications after two mesh perturbations in x and/or y direction.
       If @a use_flags is true, repeated calls won't do anything until
       ResetUpdateFlags() is called. */
   void UpdateHessianTargetSpecification(const Vector &x, double dx,
                                         bool use_flag = false,
                                         int x_ordering = Ordering::byNODES);

   void SetAdaptivityEvaluator(AdaptivityEvaluator *ae)
   {
      if (adapt_eval) { delete adapt_eval; }
      adapt_eval = ae;
   }

   const AdaptivityEvaluator *GetAdaptivityEvaluator() const
   {
      return adapt_eval;
   }

   const Vector &GetTspecPert1H()   { return tspec_pert1h; }
   const Vector &GetTspecPert2H()   { return tspec_pert2h; }
   const Vector &GetTspecPertMixH() { return tspec_pertmix; }

   /** @brief Given an element and quadrature rule, computes ref->target
       transformation Jacobians for each quadrature point in the element.
       The physical positions of the element's nodes are given by @a elfun.
       Note that this function assumes that UpdateTargetSpecification() has
       been called with the position vector corresponding to @a elfun. */
   virtual void ComputeElementTargets(int e_id, const FiniteElement &fe,
                                      const IntegrationRule &ir,
                                      const Vector &elfun,
                                      DenseTensor &Jtr) const;

   virtual void ComputeAllElementTargets(const FiniteElementSpace &fes,
                                         const IntegrationRule &ir,
                                         const Vector &xe,
                                         DenseTensor &Jtr) const;

   virtual void ComputeElementTargetsGradient(const IntegrationRule &ir,
                                              const Vector &elfun,
                                              IsoparametricTransformation &Tpr,
                                              DenseTensor &dJtr) const;

   // Generates tspec_vals for target construction using intrule
   // Used for the refinement component in hr-adaptivity.
   void SetTspecFromIntRule(int e_id, const IntegrationRule &intrule);

   // Targets based on discrete functions can result in invalid (negative)
   // size at the quadrature points. This method can be used to set a
   // minimum target size.
   void SetMinSizeForTargets(double min_size_) { lim_min_size = min_size_; }

   /// Computes target specification data with respect to the coarse FE space.
   void SetTspecDataForDerefinement(FiniteElementSpace *fes);

   // Reset refinement data associated with h-adaptivity component.
   void ResetRefinementTspecData()
   {
      tspec_refine.Clear();
      amr_el = -1;
   }

   // Reset derefinement data associated with h-adaptivity component.
   void ResetDerefinementTspecData()
   {
      tspec_derefine.Destroy();
      coarse_tspec_fesv = NULL;
   }

   // Used to specify the fine element for determining energy of children of a
   // parent element.
   void SetRefinementSubElement(int amr_el_) { amr_el = amr_el_; }
};

class TMOPNewtonSolver;

/** @brief A TMOP integrator class based on any given TMOP_QualityMetric and
    TargetConstructor.

    Represents @f$ \int W(Jpt) dx @f$ over a target zone, where W is the
    metric's strain energy density function, and Jpt is the Jacobian of the
    target->physical coordinates transformation. The virtual target zone is
    defined by the TargetConstructor. */
class TMOP_Integrator : public NonlinearFormIntegrator
{
protected:
   friend class TMOPNewtonSolver;
   friend class TMOPComboIntegrator;

   TMOP_QualityMetric *h_metric;
   TMOP_QualityMetric *metric;        // not owned
   const TargetConstructor *targetC;  // not owned

   // Custom integration rules.
   IntegrationRules *IntegRules;
   int integ_order;

   // Weight Coefficient multiplying the quality metric term.
   Coefficient *metric_coeff; // not owned, if NULL -> metric_coeff is 1.
   // Normalization factor for the metric term.
   double metric_normal;

   // Nodes and weight Coefficient used for "limiting" the TMOP_Integrator.
   // These are both NULL when there is no limiting.
   // The class doesn't own lim_nodes0 and lim_coeff.
   const GridFunction *lim_nodes0;
   Coefficient *lim_coeff;
   // Limiting reference distance. Not owned.
   const GridFunction *lim_dist;
   // Limiting function. Owned.
   TMOP_LimiterFunction *lim_func;
   // Normalization factor for the limiting term.
   double lim_normal;

   // Adaptive limiting.
   const GridFunction *adapt_lim_gf0;    // Not owned.
#ifdef MFEM_USE_MPI
   const ParGridFunction *adapt_lim_pgf0;
#endif
   GridFunction *adapt_lim_gf;           // Owned. Updated by adapt_lim_eval.
   Coefficient *adapt_lim_coeff;         // Not owned.
   AdaptivityEvaluator *adapt_lim_eval;  // Not owned.

   // Surface fitting.
   GridFunction *surf_fit_gf;       // Owned, Updated by surf_fit_eval.
   const Array<bool> *surf_fit_marker;  // Not owned.
   Coefficient *surf_fit_coeff;         // Not owned.
   AdaptivityEvaluator *surf_fit_eval;  // Not owned.
   double surf_fit_normal;

   DiscreteAdaptTC *discr_tc;

   // Parameters for FD-based Gradient & Hessian calculation.
   bool fdflag;
   double dx;
   double dxscale;
   // Specifies that ComputeElementTargets is being called by a FD function.
   // It's used to skip terms that have exact derivative calculations.
   bool fd_call_flag;
   // Compute the exact action of the Integrator (includes derivative of the
   // target with respect to spatial position)
   bool exact_action;

   Array <Vector *> ElemDer;        //f'(x)
   Array <Vector *> ElemPertEnergy; //f(x+h)

   //   Jrt: the inverse of the ref->target Jacobian, Jrt = Jtr^{-1}.
   //   Jpr: the ref->physical transformation Jacobian, Jpr = PMatI^t DS.
   //   Jpt: the target->physical transformation Jacobian, Jpt = Jpr Jrt.
   //     P: represents dW_d(Jtp) (dim x dim).
   //   DSh: gradients of reference shape functions (dof x dim).
   //    DS: gradients of the shape functions in the target configuration,
   //        DS = DSh Jrt (dof x dim).
   // PMatI: current coordinates of the nodes (dof x dim).
   // PMat0: reshaped view into the local element contribution to the operator
   //        output - the result of AssembleElementVector() (dof x dim).
   DenseMatrix DSh, DS, Jrt, Jpr, Jpt, P, PMatI, PMatO;

   // PA extension
   // ------------
   //  E: Q-vector for TMOP-energy
   //  O: Q-Vector of 1.0, used to compute sums using the dot product kernel.
   // X0: E-vector for initial nodal coordinates used for limiting.
   //  H: Q-Vector for Hessian associated with the metric term.
   // C0: Q-Vector for spatial weight used for the limiting term.
   // LD: E-Vector constructed using limiting distance grid function (delta).
   // H0: Q-Vector for Hessian associated with the limiting term.
   //
   // maps:     Dof2Quad map for fespace associate with nodal coordinates.
   // maps_lim: Dof2Quad map for fespace associated with the limiting distance
   //            grid function.
   //
   // Jtr_debug_grad
   //     We keep track if Jtr was set by AssembleGradPA() in Jtr_debug_grad: it
   //     is set to true by AssembleGradPA(); any other call to
   //     ComputeAllElementTargets() will set the flag to false. This flag will
   //     be used to check that Jtr is the one set by AssembleGradPA() when
   //     performing operations with the gradient like AddMultGradPA() and
   //     AssembleGradDiagonalPA().
   //
   // TODO:
   //   * Merge LD, C0, H0 into one scalar Q-vector
   struct
   {
      bool enabled;
      int dim, ne, nq;
      mutable DenseTensor Jtr;
      mutable bool Jtr_needs_update;
      mutable bool Jtr_debug_grad;
      mutable Vector E, O, X0, H, C0, LD, H0;
      const DofToQuad *maps;
      const DofToQuad *maps_lim = nullptr;
      const GeometricFactors *geom;
      const FiniteElementSpace *fes;
      const IntegrationRule *ir;
   } PA;

   void ComputeNormalizationEnergies(const GridFunction &x,
                                     double &metric_energy, double &lim_energy,
                                     double &surf_fit_gf_energy);

   void AssembleElementVectorExact(const FiniteElement &el,
                                   ElementTransformation &T,
                                   const Vector &elfun, Vector &elvect);

   void AssembleElementGradExact(const FiniteElement &el,
                                 ElementTransformation &T,
                                 const Vector &elfun, DenseMatrix &elmat);

   void AssembleElementVectorFD(const FiniteElement &el,
                                ElementTransformation &T,
                                const Vector &elfun, Vector &elvect);

   // Assumes that AssembleElementVectorFD has been called.
   void AssembleElementGradFD(const FiniteElement &el,
                              ElementTransformation &T,
                              const Vector &elfun, DenseMatrix &elmat);

   void AssembleElemVecAdaptLim(const FiniteElement &el,
                                IsoparametricTransformation &Tpr,
                                const IntegrationRule &ir,
                                const Vector &weights, DenseMatrix &mat);
   void AssembleElemGradAdaptLim(const FiniteElement &el,
                                 IsoparametricTransformation &Tpr,
                                 const IntegrationRule &ir,
                                 const Vector &weights, DenseMatrix &m);

   // First derivative of the surface fitting term.
   void AssembleElemVecSurfFit(const FiniteElement &el_x,
                               IsoparametricTransformation &Tpr,
                               DenseMatrix &mat);

   // Second derivative of the surface fitting term.
   void AssembleElemGradSurfFit(const FiniteElement &el_x,
                                IsoparametricTransformation &Tpr,
                                DenseMatrix &mat);

   double GetFDDerivative(const FiniteElement &el,
                          ElementTransformation &T,
                          Vector &elfun, const int nodenum,const int idir,
                          const double baseenergy, bool update_stored);

   /** @brief Determines the perturbation, h, for FD-based approximation. */
   void ComputeFDh(const Vector &x, const FiniteElementSpace &fes);
   void ComputeMinJac(const Vector &x, const FiniteElementSpace &fes);

   void UpdateAfterMeshPositionChange(const Vector &new_x,
                                      int new_x_ordering = Ordering::byNODES);

   void DisableLimiting()
   {
      lim_nodes0 = NULL; lim_coeff = NULL; lim_dist = NULL;
      delete lim_func; lim_func = NULL;
   }

   const IntegrationRule &EnergyIntegrationRule(const FiniteElement &el) const
   {
      if (IntegRules)
      {
         return IntegRules->Get(el.GetGeomType(), integ_order);
      }
      return (IntRule) ? *IntRule
             /*     */ : IntRules.Get(el.GetGeomType(), 2*el.GetOrder() + 3);
   }
   const IntegrationRule &ActionIntegrationRule(const FiniteElement &el) const
   {
      // TODO the energy most likely needs less integration points.
      return EnergyIntegrationRule(el);
   }
   const IntegrationRule &GradientIntegrationRule(const FiniteElement &el) const
   {
      // TODO the action and energy most likely need less integration points.
      return EnergyIntegrationRule(el);
   }

   // Auxiliary PA methods
   void AssembleGradPA_2D(const Vector&) const;
   void AssembleGradPA_3D(const Vector&) const;
   void AssembleGradPA_C0_2D(const Vector&) const;
   void AssembleGradPA_C0_3D(const Vector&) const;

   double GetLocalStateEnergyPA_2D(const Vector&) const;
   double GetLocalStateEnergyPA_C0_2D(const Vector&) const;
   double GetLocalStateEnergyPA_3D(const Vector&) const;
   double GetLocalStateEnergyPA_C0_3D(const Vector&) const;

   void AddMultPA_2D(const Vector&, Vector&) const;
   void AddMultPA_3D(const Vector&, Vector&) const;
   void AddMultPA_C0_2D(const Vector&, Vector&) const;
   void AddMultPA_C0_3D(const Vector&, Vector&) const;

   void AddMultGradPA_2D(const Vector&, Vector&) const;
   void AddMultGradPA_3D(const Vector&, Vector&) const;
   void AddMultGradPA_C0_2D(const Vector&, Vector&) const;
   void AddMultGradPA_C0_3D(const Vector&, Vector&) const;

   void AssembleDiagonalPA_2D(Vector&) const;
   void AssembleDiagonalPA_3D(Vector&) const;
   void AssembleDiagonalPA_C0_2D(Vector&) const;
   void AssembleDiagonalPA_C0_3D(Vector&) const;

   void AssemblePA_Limiting();
   void ComputeAllElementTargets(const Vector &xe = Vector()) const;

   // Compute Min(Det(Jpt)) in the mesh, does not reduce over MPI.
   double ComputeMinDetT(const Vector &x, const FiniteElementSpace &fes);
   // Compute Max(mu_hat) for the TMOP_WorstCaseUntangleOptimizer_Metric,
   // does not reduce over MPI.
   double ComputeUntanglerMaxMuBarrier(const Vector &x,
                                       const FiniteElementSpace &fes);

public:
   /** @param[in] m    TMOP_QualityMetric for r-adaptivity (not owned).
       @param[in] tc   Target-matrix construction algorithm to use (not owned).
       @param[in] hm   TMOP_QualityMetric for h-adaptivity (not owned). */
   TMOP_Integrator(TMOP_QualityMetric *m, TargetConstructor *tc,
                   TMOP_QualityMetric *hm)
      : h_metric(hm), metric(m), targetC(tc), IntegRules(NULL),
        integ_order(-1), metric_coeff(NULL), metric_normal(1.0),
        lim_nodes0(NULL), lim_coeff(NULL),
        lim_dist(NULL), lim_func(NULL), lim_normal(1.0),
        adapt_lim_gf0(NULL), adapt_lim_gf(NULL), adapt_lim_coeff(NULL),
        adapt_lim_eval(NULL),
        surf_fit_gf(NULL), surf_fit_marker(NULL),
        surf_fit_coeff(NULL),
        surf_fit_eval(NULL), surf_fit_normal(1.0),
        discr_tc(dynamic_cast<DiscreteAdaptTC *>(tc)),
        fdflag(false), dxscale(1.0e3), fd_call_flag(false), exact_action(false)
   { PA.enabled = false; }

   TMOP_Integrator(TMOP_QualityMetric *m, TargetConstructor *tc)
      : TMOP_Integrator(m, tc, m) { }

   ~TMOP_Integrator();

   /// Release the device memory of large PA allocations. This will copy device
   /// memory back to the host before releasing.
   void ReleasePADeviceMemory(bool copy_to_host = true);

   /// Prescribe a set of integration rules; relevant for mixed meshes.
   /** This function has priority over SetIntRule(), if both are called. */
   void SetIntegrationRules(IntegrationRules &irules, int order)
   {
      IntegRules = &irules;
      integ_order = order;
   }

   /// Sets a scaling Coefficient for the quality metric term of the integrator.
   /** With this addition, the integrator becomes
          @f$ \int w1 W(Jpt) dx @f$.

       Note that the Coefficient is evaluated in the physical configuration and
       not in the target configuration which may be undefined. */
   void SetCoefficient(Coefficient &w1) { metric_coeff = &w1; }

   /** @brief Limiting of the mesh displacements (general version).

       Adds the term @f$ \int w_0 f(x, x_0, d) dx @f$, where f is a measure of
       the displacement between x and x_0, given the max allowed displacement d.

       @param[in] n0     Original mesh node coordinates (x0 above).
       @param[in] dist   Allowed displacement in physical space (d above).
       @param[in] w0     Coefficient scaling the limiting integral.
       @param[in] lfunc  TMOP_LimiterFunction defining the function f. If
                         NULL, a TMOP_QuadraticLimiter will be used. The
                         TMOP_Integrator assumes ownership of this pointer. */
   void EnableLimiting(const GridFunction &n0, const GridFunction &dist,
                       Coefficient &w0, TMOP_LimiterFunction *lfunc = NULL);

   /** @brief Adds a limiting term to the integrator with limiting distance
       function (@a dist in the general version of the method) equal to 1. */
   void EnableLimiting(const GridFunction &n0, Coefficient &w0,
                       TMOP_LimiterFunction *lfunc = NULL);

   /** @brief Restriction of the node positions to certain regions.

       Adds the term @f$ \int c (z(x) - z_0(x_0))^2 @f$, where z0(x0) is a given
       function on the starting mesh, and z(x) is its image on the new mesh.
       Minimizing this term means that a node at x0 is allowed to move to a
       position x(x0) only if z(x) ~ z0(x0).
       Such term can be used for tangential mesh relaxation.

       @param[in] z0     Function z0 that controls the adaptive limiting.
       @param[in] coeff  Coefficient c for the above integral.
       @param[in] ae     AdaptivityEvaluator to compute z(x) from z0(x0). */
   void EnableAdaptiveLimiting(const GridFunction &z0, Coefficient &coeff,
                               AdaptivityEvaluator &ae);
#ifdef MFEM_USE_MPI
   /// Parallel support for adaptive limiting.
   void EnableAdaptiveLimiting(const ParGridFunction &z0, Coefficient &coeff,
                               AdaptivityEvaluator &ae);
#endif

   /** @brief Fitting of certain DOFs to the zero level set of a function.

       Having a level set function s0(x0) on the starting mesh, and a set of
       marked nodes (or DOFs), we move these nodes to the zero level set of s0.
       If s(x) is the image of s0(x0) on the current mesh, this function adds to
       the TMOP functional the term @f$ \int c \bar{s}(x))^2 @f$, where
       @f$\bar{s}(x)@f$ is the restriction of s(x) on the aligned DOFs.
       Minimizing this term means that a marked node at x0 is allowed to move to
       a position x(x0) only if s(x) ~ 0.
       Such term can be used for surface fitting and tangential relaxation.

       @param[in] s0      The level set function on the initial mesh.
       @param[in] smarker Indicates which DOFs will be aligned.
       @param[in] coeff   Coefficient c for the above integral.
       @param[in] ae      AdaptivityEvaluator to compute s(x) from s0(x0). */
   void EnableSurfaceFitting(const GridFunction &s0,
                             const Array<bool> &smarker, Coefficient &coeff,
                             AdaptivityEvaluator &ae);
#ifdef MFEM_USE_MPI
   /// Parallel support for surface fitting.
   void EnableSurfaceFitting(const ParGridFunction &s0,
                             const Array<bool> &smarker, Coefficient &coeff,
                             AdaptivityEvaluator &ae);
#endif
   void GetSurfaceFittingErrors(double &err_avg, double &err_max);
   bool IsSurfaceFittingEnabled() { return (surf_fit_gf != NULL); }

   /// Update the original/reference nodes used for limiting.
   void SetLimitingNodes(const GridFunction &n0) { lim_nodes0 = &n0; }

   /** @brief Computes the integral of W(Jacobian(Trt)) over a target zone.
       @param[in] el     Type of FiniteElement.
       @param[in] T      Mesh element transformation.
       @param[in] elfun  Physical coordinates of the zone. */
   virtual double GetElementEnergy(const FiniteElement &el,
                                   ElementTransformation &T,
                                   const Vector &elfun);

   /** @brief Computes the mean of the energies of the given element's children.

       In addition to the inputs for GetElementEnergy, this function requires an
       IntegrationRule to be specified that will give the decomposition of the
       given element based on the refinement type being considered. */
   virtual double GetRefinementElementEnergy(const FiniteElement &el,
                                             ElementTransformation &T,
                                             const Vector &elfun,
                                             const IntegrationRule &irule);

   /// This function is similar to GetElementEnergy, but ignores components
   /// such as limiting etc. to compute the element energy.
   virtual double GetDerefinementElementEnergy(const FiniteElement &el,
                                               ElementTransformation &T,
                                               const Vector &elfun);

   virtual void AssembleElementVector(const FiniteElement &el,
                                      ElementTransformation &T,
                                      const Vector &elfun, Vector &elvect);

   virtual void AssembleElementGrad(const FiniteElement &el,
                                    ElementTransformation &T,
                                    const Vector &elfun, DenseMatrix &elmat);

   TMOP_QualityMetric &GetAMRQualityMetric() { return *h_metric; }

   void UpdateAfterMeshTopologyChange();
#ifdef MFEM_USE_MPI
   void ParUpdateAfterMeshTopologyChange();
#endif

   // PA extension
   using NonlinearFormIntegrator::AssemblePA;
   virtual void AssemblePA(const FiniteElementSpace&);

   virtual void AssembleGradPA(const Vector&, const FiniteElementSpace&);

   virtual double GetLocalStateEnergyPA(const Vector&) const;

   virtual void AddMultPA(const Vector&, Vector&) const;

   virtual void AddMultGradPA(const Vector&, Vector&) const;

   virtual void AssembleGradDiagonalPA(Vector&) const;

   DiscreteAdaptTC *GetDiscreteAdaptTC() const { return discr_tc; }

   /** @brief Computes the normalization factors of the metric and limiting
       integrals using the mesh position given by @a x. */
   void EnableNormalization(const GridFunction &x);
#ifdef MFEM_USE_MPI
   void ParEnableNormalization(const ParGridFunction &x);
#endif

   /** @brief Enables FD-based approximation and computes dx. */
   void EnableFiniteDifferences(const GridFunction &x);
#ifdef MFEM_USE_MPI
   void EnableFiniteDifferences(const ParGridFunction &x);
#endif

   void   SetFDhScale(double dxscale_) { dxscale = dxscale_; }
   bool   GetFDFlag() const { return fdflag; }
   double GetFDh()    const { return dx; }

   /** @brief Flag to control if exact action of Integration is effected. */
   void SetExactActionFlag(bool flag_) { exact_action = flag_; }

   /// Update the surface fitting weight as surf_fit_coeff *= factor;
   void UpdateSurfaceFittingWeight(double factor);

   /// Get the surface fitting weight.
   double GetSurfaceFittingWeight();

   /// Computes quantiles needed for UntangleMetrics. Note that in parallel,
   /// the ParFiniteElementSpace must be passed as argument for consistency
   /// across MPI ranks.
   void ComputeUntangleMetricQuantiles(const Vector &x,
                                       const FiniteElementSpace &fes);
};

class TMOPComboIntegrator : public NonlinearFormIntegrator
{
protected:
   // Integrators in the combination. Owned.
   Array<TMOP_Integrator *> tmopi;

public:
   TMOPComboIntegrator() : tmopi(0) { }

   ~TMOPComboIntegrator()
   {
      for (int i = 0; i < tmopi.Size(); i++) { delete tmopi[i]; }
   }

   /// Adds a new TMOP_Integrator to the combination.
   void AddTMOPIntegrator(TMOP_Integrator *ti) { tmopi.Append(ti); }

   const Array<TMOP_Integrator *> &GetTMOPIntegrators() const { return tmopi; }

   /// Adds the limiting term to the first integrator. Disables it for the rest.
   void EnableLimiting(const GridFunction &n0, const GridFunction &dist,
                       Coefficient &w0, TMOP_LimiterFunction *lfunc = NULL);

   /** @brief Adds the limiting term to the first integrator. Disables it for
       the rest (@a dist in the general version of the method) equal to 1. */
   void EnableLimiting(const GridFunction &n0, Coefficient &w0,
                       TMOP_LimiterFunction *lfunc = NULL);

   /// Update the original/reference nodes used for limiting.
   void SetLimitingNodes(const GridFunction &n0);

   virtual double GetElementEnergy(const FiniteElement &el,
                                   ElementTransformation &T,
                                   const Vector &elfun);
   virtual void AssembleElementVector(const FiniteElement &el,
                                      ElementTransformation &T,
                                      const Vector &elfun, Vector &elvect);
   virtual void AssembleElementGrad(const FiniteElement &el,
                                    ElementTransformation &T,
                                    const Vector &elfun, DenseMatrix &elmat);

   virtual double GetRefinementElementEnergy(const FiniteElement &el,
                                             ElementTransformation &T,
                                             const Vector &elfun,
                                             const IntegrationRule &irule);

   virtual double GetDerefinementElementEnergy(const FiniteElement &el,
                                               ElementTransformation &T,
                                               const Vector &elfun);

   /// Normalization factor that considers all integrators in the combination.
   void EnableNormalization(const GridFunction &x);
#ifdef MFEM_USE_MPI
   void ParEnableNormalization(const ParGridFunction &x);
#endif

   // PA extension
   using NonlinearFormIntegrator::AssemblePA;
   virtual void AssemblePA(const FiniteElementSpace&);
   virtual void AssembleGradPA(const Vector&, const FiniteElementSpace&);
   virtual double GetLocalStateEnergyPA(const Vector&) const;
   virtual void AddMultPA(const Vector&, Vector&) const;
   virtual void AddMultGradPA(const Vector&, Vector&) const;
   virtual void AssembleGradDiagonalPA(Vector&) const;
};

/// Interpolates the @a metric's values at the nodes of @a metric_gf.
/** Assumes that @a metric_gf's FiniteElementSpace is initialized. */
void InterpolateTMOP_QualityMetric(TMOP_QualityMetric &metric,
                                   const TargetConstructor &tc,
                                   const Mesh &mesh, GridFunction &metric_gf);
}

#endif
