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#ifndef TPETRA_OPERATOR_HPP
#define TPETRA_OPERATOR_HPP

#include <Tpetra_MultiVector_decl.hpp>
#include <Teuchos_Describable.hpp>
#include <Teuchos_BLAS_types.hpp>
#include <Teuchos_ScalarTraits.hpp>

namespace Tpetra {
  /// \class Operator
  /// \brief Abstract interface for operators (e.g., matrices and
  ///   preconditioners).
  ///
  /// \tparam Scalar The type of the entries of the input and output
  ///   MultiVector objects.  See the documentation of MultiVector for
  ///   details and requirements.
  /// \tparam LocalOrdinal The type of local indices.  See the
  ///   documentation of Map for requirements.
  /// \tparam GlobalOrdinal The type of global indices.  See the
  ///   documentation of Map for requirements.
  /// \tparam Node The Kokkos Node type.  See the documentation of Map
  ///   for requirements.
  ///
  /// An Operator takes a MultiVector as input, and fills a given
  /// output MultiVector with the result.  The input and output
  /// MultiVector objects must have the same number of columns
  /// (vectors).  However, they need not have the same numbers of rows
  /// or the same parallel distributions.  The <i>domain</i> of the
  /// Operator describes the parallel distribution of valid input
  /// MultiVector objects, and the <i>range</i> of the Operator
  /// describes the parallel distribution of valid output MultiVector
  /// objects.
  ///
  /// Operator is just an interface, not an implementation.  Many
  /// different classes implement this interface, including sparse
  /// matrices, direct solvers, iterative solvers, and
  /// preconditioners.
  template <class Scalar = Details::DefaultTypes::scalar_type,
            class LocalOrdinal = Details::DefaultTypes::local_ordinal_type,
            class GlobalOrdinal = Details::DefaultTypes::global_ordinal_type,
            class Node = Details::DefaultTypes::node_type>
  class Operator : virtual public Teuchos::Describable {
  public:
    /** \name Typedefs that give access to the template parameters. */
    //@{

    //! The type of the entries of the input and output multivectors.
    typedef Scalar scalar_type;

    //! The local index type.
    typedef LocalOrdinal local_ordinal_type;

    //! The global index type.
    typedef GlobalOrdinal global_ordinal_type;

    //! The Kokkos Node type.
    typedef Node node_type;

    //@}
    /** \name Pure virtual functions to be overridden by subclasses. */
    //@{

    //! The Map associated with the domain of this operator, which must be compatible with X.getMap().
    virtual Teuchos::RCP<const Map<LocalOrdinal,GlobalOrdinal,Node> > getDomainMap() const = 0;

    //! The Map associated with the range of this operator, which must be compatible with Y.getMap().
    virtual Teuchos::RCP<const Map<LocalOrdinal,GlobalOrdinal,Node> > getRangeMap() const = 0;

    //! \brief Computes the operator-multivector application.
    /*! Loosely, performs \f$Y = \alpha \cdot A^{\textrm{mode}} \cdot X + \beta \cdot Y\f$. However, the details of operation
        vary according to the values of \c alpha and \c beta. Specifically
        - if <tt>beta == 0</tt>, apply() <b>must</b> overwrite \c Y, so that any values in \c Y (including NaNs) are ignored.
        - if <tt>alpha == 0</tt>, apply() <b>may</b> short-circuit the operator, so that any values in \c X (including NaNs) are ignored.
     */
    virtual void
    apply (const MultiVector<Scalar,LocalOrdinal,GlobalOrdinal,Node> &X,
           MultiVector<Scalar,LocalOrdinal,GlobalOrdinal,Node> &Y,
           Teuchos::ETransp mode = Teuchos::NO_TRANS,
           Scalar alpha = Teuchos::ScalarTraits<Scalar>::one(),
           Scalar beta = Teuchos::ScalarTraits<Scalar>::zero()) const = 0;

    /// \brief Whether this operator supports applying the transpose or conjugate transpose.
    ///
    /// By default, this returns false.  Subclasses must override this
    /// method if they can support apply() with
    /// <tt>mode=Teuchos::TRANS</tt> or
    /// <tt>mode=Teuchos::CONJ_TRANS</tt>.
    virtual bool hasTransposeApply() const;

    //@}
  };

  template <class Scalar, class LocalOrdinal, class GlobalOrdinal, class Node>
  bool Operator<Scalar,LocalOrdinal,GlobalOrdinal,Node>::hasTransposeApply() const {
    return false;
  }

} // Tpetra namespace

#endif // TPETRA_OPERATOR_HPP
