/*=========================================================================
 *
 *  Copyright Insight Software Consortium
 *
 *  Licensed under the Apache License, Version 2.0 (the "License");
 *  you may not use this file except in compliance with the License.
 *  You may obtain a copy of the License at
 *
 *         http://www.apache.org/licenses/LICENSE-2.0.txt
 *
 *  Unless required by applicable law or agreed to in writing, software
 *  distributed under the License is distributed on an "AS IS" BASIS,
 *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *  See the License for the specific language governing permissions and
 *  limitations under the License.
 *
 *=========================================================================*/
/*=========================================================================
 *
 *  Portions of this file are subject to the VTK Toolkit Version 3 copyright.
 *
 *  Copyright (c) Ken Martin, Will Schroeder, Bill Lorensen
 *
 *  For complete copyright, license and disclaimer of warranty information
 *  please refer to the NOTICE file at the top of the ITK source tree.
 *
 *=========================================================================*/
#ifndef itkProcessObject_h
#define itkProcessObject_h

#include "itkDataObject.h"
#include "itkDomainThreader.h"
#include "itkMultiThreaderBase.h"
#include "itkObjectFactory.h"
#include "itkNumericTraits.h"
#include <vector>
#include <map>
#include <set>
#include <algorithm>

namespace itk
{
/** \class ProcessObject
 * \brief The base class for all process objects (source,
 *        filters, mappers) in the Insight data processing pipeline.
 *
 * ProcessObject is an abstract object that specifies behavior and
 * interface of network process objects (sources, filters,
 * mappers). Source objects are creators of visualization data;
 * filters input, process, and output image data; and mappers
 * transform data into another form (like transforming coordinates or
 * writing data to a file).
 *
 * A major role of ProcessObject is to define the inputs and outputs
 * of a filter. More than one input and/or output may exist for a given
 * filter. Some classes (e.g., source objects or mapper objects) will
 * not use inputs (the source) or outputs (mappers). In this case, the
 * inputs or outputs is just ignored.
 *
 * VOCABULARY:
 *   - named entry - an entry indexed by a DataObjectIdentifierType or string.
 *   - index entry - an entry indexed by an integer, which also always has a
 *      string identifier.
 *   - define an input/output - adds a named entry or a indexed entry.
 *   - required input - a precondition that the inputs is set before updating.
 *   - set the value - set the value of an input or output, and
 *       automatically define the entry if it does not exist.
 *
 * \note
 *  - The Primary Input is always defined internally, and is handled as
 *    a special case for many methods.
 *  - Some inputs can be defined as required. Either explicitly by
 *   name or the older ITKv3 style where a certain number of index
 *   inputs are required.
 *
 * The inputs and outputs are referenced by name and optionally by an
 * integer index. The \b Primary input and the \b Primary output play
 * a special role: they drive the pipeline.
 *
 * In addition to the reference by name, it is possible to access the
 * inputs and outputs with an index. The index by default is mapped
 * internally to a name with  '_' followed by the index number. This
 * default name can be changed with the AddRequiredInputName
 * method. The indexed input or output 0 is mapped to the Primary
 * input or output. The name of the  Primary input or output defaults
 * to "Primary", but this can be changed  with SetPrimaryInputName and
 * SetPrimaryOutputName.
 *
 * For complicated filters which have optional, or varied required
 * inputs, named input access is preferred. However, indexed input
 * access provides constant time access to input and output
 * DataObjects, and so are more efficient. A name can also be
 * associated with an indexed input. Neither type of input or output
 * should be accessed in a tight loop.
 *
 * ProcessObject invokes the following events:
 * Command::StartEvent, Command::EndEvent
 * These are convenience events you can use for any purpose
 * (e.g., debugging info, highlighting/notifying user interface, etc.)
 * See Command and LightObject for information on using AddObserver.
 *
 * Another event Command::ProgressEvent can be observed. Some filters invoke
 * this event periodically during their execution (with the progress,
 * parameter, the fraction of work done). The use is similar to that of
 * StartEvent and EndEvent. Filters may also check their
 * AbortGenerateData flag to determine whether to prematurally end their
 * execution.
 *
 * An important feature of subclasses of ProcessObject is that it is
 * possible to control the memory-management model (i.e., retain
 * output versus delete output data). The ReleaseDataFlag enables the
 * deletion of the output data once the downstream process object
 * finishes processing the data (please see text). The
 * ReleaseDataBeforeUpdateFlag enables the deletion of the
 * ProcessObject's output data from a previous update if that output
 * data is slated to be regenerated by the pipeline process.  Setting
 * this flag can control peak memory usage during a subsequent
 * pipeline update.  For a ProcessObject, the ReleaseDataFlag defaults
 * to false and the ReleaseDataBeforeUpdateFlag defaults to true.
 * Some subclasses of ProcessObject, for example ImageSource, use a
 * default setting of false for the ReleaseDataBeforeUpdateFlag.
 *
 * Subclasses of ProcessObject may override 4 of the methods of this class
 * to control how a given filter may interact with the pipeline (dataflow).
 * These methods are: GenerateOutputInformation(),
 * EnlargeOutputRequestedRegion(), GenerateInputRequestedRegion(), and
 * GenerateOutputRequestedRegion(). By overriding these methods, a filter
 * can deviate from the base assumptions of the pipeline execution model.
 *
 * \ingroup ITKSystemObjects
 * \ingroup DataProcessing
 *
 * \ingroup ITKCommon
 */
class ITKCommon_EXPORT ProcessObject:public Object
{
public:
  ITK_DISALLOW_COPY_AND_ASSIGN(ProcessObject);

  /** Standard class type aliases. */
  using Self = ProcessObject;
  using Superclass = Object;
  using Pointer = SmartPointer< Self >;
  using ConstPointer = SmartPointer< const Self >;

  /** Run-time type information (and related methods). */
  itkTypeMacro(ProcessObject, Object);

  /** Smart Pointer type to a DataObject. */
  using DataObjectPointer = DataObject::Pointer;

  /** STL array of const pointer to the data objects */
  // using ConstDataObjectPointerArray = std::vector< const DataObject * >;

  /** STL array of pointer to the data objects */
  using DataObjectPointerArray = std::vector< DataObjectPointer >;

  using DataObjectIdentifierType = DataObject::DataObjectIdentifierType;

  /** STL array of data object names */
  using NameArray = std::vector< DataObjectIdentifierType >;

  //** Type of general multi-threader interface */
  using MultiThreaderType = MultiThreaderBase;

  /** \brief Return an array with the names of the inputs defined.
   *
   * The names are ordered lexicographically, and match the order of the
   * data object produced by GetInputs().
   */
  NameArray GetInputNames() const;

  /** Return an array with the names of the required inputs. */
  NameArray GetRequiredInputNames() const;

  /** \brief Return an array of DataObjects with the defined named inputs.
   *
   * The order of the inputs matches the order of the input names produced
   * by GetInputNames().
   */
  DataObjectPointerArray GetInputs();

  /** Return true if the input with this name is defined */
  bool HasInput( const DataObjectIdentifierType & key ) const;

  using DataObjectPointerArraySizeType = DataObjectPointerArray::size_type;

  /** \brief Get the size of the input container.
   *
   * If the Primary input is nullptr it is not counted. This is
   * not the number of inputs that have valid DataObject's
   * assigned. Use GetNumberOfValidRequiredInputs() to determine how
   * many inputs are non-null.
   *
   * \sa GetNumberOfValidRequiredInputs
   * \sa GetNumberOfIndexedInputs
   */
  DataObjectPointerArraySizeType GetNumberOfInputs() const;

  /** \brief Get the size of the output container. */
  DataObjectPointerArraySizeType GetNumberOfOutputs() const;

  /** \brief Return an array with the defined names of the outputs.
   *
   * The names are ordered lexicographically, and match the order of the
   * data object produced by GetOutputs()
   */
  NameArray GetOutputNames() const;

  /** \brief Return an array with the defined named outputs.
   *
   * The order of the outputs match the order of the input names produced
   * by GetOutputNames().
   */
  DataObjectPointerArray GetOutputs();

  /** \brief Return true if the output with this name is defined. */
  bool HasOutput( const DataObjectIdentifierType & key ) const;

  /** \brief Return an array with all the indexed inputs. */
  DataObjectPointerArray GetIndexedInputs();

  /** \brief Get the number of defined Indexed inputs.
   *
   * This is merely the size of the index input vector, not the number
   * of inputs that have valid DataObject's assigned. The \b Primary
   * inputs is handled as a special case. Use
   * GetNumberOfValidRequiredInputs() to determine how many indexed
   * inputs are non-null.
   *
   * All indexed inputs up to this value are defined.
   *
   * \sa GetNumberOfInputs
   * \sa GetNumberOfValidRequiredInputs
   */
  DataObjectPointerArraySizeType GetNumberOfIndexedInputs() const;

  /** \brief Get the number of valid \b indexed inputs.
   *
   * Returns the number of non-null indexed inputs. This method is
   * used to determine whether the correct number of inputs are set to
   * valid values.
   *
   * \sa GetNumberOfInputs
   * \sa GetNumberOfIndexedInputs
   */
  virtual DataObjectPointerArraySizeType GetNumberOfValidRequiredInputs() const;

  /** \brief Return an array with the indexed outputs. */
  DataObjectPointerArray GetIndexedOutputs();

  /** \brief The number of defined Indexed outputs. */
  DataObjectPointerArraySizeType GetNumberOfIndexedOutputs() const;

  /** \brief Make a DataObject of the correct type to used as the specified output.
   *
   * Every ProcessObject subclass must be able to create a
   * DataObject that can be used as a specified output. This method
   * is automatically called when DataObject::DisconnectPipeline() is
   * called.  DataObject::DisconnectPipeline, disconnects a data object
   * from being an output of its current source.  When the data object
   * is disconnected, the ProcessObject needs to construct a replacement
   * output data object so that the ProcessObject is in a valid state.
   * So DataObject::DisconnectPipeline eventually calls
   * ProcessObject::MakeOutput. Note that MakeOutput always returns a
   * itkSmartPointer to a DataObject. ImageSource and MeshSource override
   * this method to create the correct type of image and mesh respectively.
   * If a filter has multiple outputs of different types, then that
   * filter must provide an implementation of MakeOutput(). */
  virtual DataObjectPointer MakeOutput(DataObjectPointerArraySizeType idx);

  /** \brief Set the AbortGenerateData flag for the process object.
   *
   * Process objects may handle premature termination of execution in
   * different ways.
   */
  itkSetMacro(AbortGenerateData, bool);

  /** \brief Get the AbortGenerateData flag for the process object. */
  itkGetConstReferenceMacro(AbortGenerateData, bool);

  /** \brief Turn on and off the AbortGenerateData flag. */
  itkBooleanMacro(AbortGenerateData);

  /** \deprecated
   * Set the execution progress of a process object. The progress is
   * a floating number in [0,1] with 0 meaning no progress and 1 meaning
   * the filter has completed execution.  The ProgressEvent is NOT
   * invoked.
   * This method is deprecated because filters should not be calling
   * SetProgress directly but should be using UpdateProgress instead.
   * We avoid the use of the itkSetClampMacro because that macro calls
   * Modified on the filter, which will cause the filter to rerun even
   * if it doesn't need to.
   * Thus, we implement the SetClampMacro directly without the call to
   * Modified. */
#if ! defined ( ITK_LEGACY_REMOVE )
  void SetProgress(float progress)
  {
    // Clamp the value to be between 0 and 1.
    m_Progress = std::max(progress, 0.0f);
    m_Progress = std::min(m_Progress, 1.0f);
  }
#endif

  /** \brief Get the execution progress of a process object.
   *
   * The progress is a floating number in [0,1] with 0 meaning no
   * progress and 1 meaning the filter has completed execution.
   */
  itkGetConstReferenceMacro(Progress, float);

  /** \brief Update the progress of the process object.
   *
   * Sets the Progress ivar to amount and invokes any observers for
   * the ProgressEvent. The parameter amount should be in [0,1] and is
   * the cumulative (not incremental) progress.
    */
  void UpdateProgress(float progress);

  /** \brief Bring this filter up-to-date.
   *
   * Update() checks modified times against
   * last execution times, and re-executes objects if necessary. A side
   * effect of this method is that the whole pipeline may execute
   * in order to bring this filter up-to-date. This method updates the
   * currently prescribed requested region.  If no requested region has
   * been set on the output, then the requested region will be set to the
   * largest possible region. Once the requested region is set, Update()
   * will make sure the specified requested region is up-to-date. This
   * is a confusing side effect to users who are just calling Update() on
   * a filter.  A first call to Update() will cause the largest possible
   * region to be updated.  A second call to Update() will update that
   * same region.  If a modification to the upstream pipeline cause a
   * filter to have a different largest possible region, this second
   * call to Update() will not cause the output requested region to be
   * reset to the new largest possible region.  Instead, the output requested
   * region will be the same as the last time Update() was called. To have
   * a filter always to produce its largest possible region, users should
   * call UpdateLargestPossibleRegion() instead.
   */
  virtual void Update();

  /** \brief Sets the output requested region to the largest possible
   * region and updates.
   *
   * This is the method users
   * should call if they want the entire dataset to be processed.  If
   * a user wants to update the same output region as a previous call
   * to Update() or a previous call to UpdateLargestPossibleRegion(),
   * then they should call the method Update().
    */
  virtual void UpdateLargestPossibleRegion();

  /** \brief Update the information describing the output data.
   *
   * This method
   * traverses up the pipeline gathering modified time information.
   * On the way back down the pipeline, this method calls
   * GenerateOutputInformation() to set any necessary information
   * about the output data objects.  For instance, a filter that
   * shrinks an image will need to provide an implementation for
   * GenerateOutputInformation() that changes the spacing of the
   * pixels. Such filters should call their superclass' implementation
   * of GenerateOutputInformation prior to changing the information
   * values they need (i.e. GenerateOutputInformation() should call
   * Superclass::GenerateOutputInformation() prior to changing the
   * information.
   */
  virtual void UpdateOutputInformation();

  /** Send the requested region information back up the pipeline (to the
   * filters that precede this one). */
  virtual void PropagateRequestedRegion(DataObject *output);

  /** Actually generate new output  */
  virtual void UpdateOutputData(DataObject *output);

  /** Give the process object a chance to indictate that it will produce more
   * output than it was requested to produce. For example, many imaging
   * filters must compute the entire output at once or can only produce output
   * in complete slices. Such filters cannot handle smaller requested regions.
   * These filters must provide an implementation of this method, setting
   * the output requested region to the size they will produce.  By default,
   * a process object does not modify the size of the output requested
   * region.
    */
  virtual void EnlargeOutputRequestedRegion( DataObject *itkNotUsed(output) ){}

  /** \brief Reset the pipeline.
   *
   * If an exception is thrown during an Update(),
   * the pipeline may be in an inconsistent state.  This method clears
   * the internal state of the pipeline so Update() can be called.
    */
  virtual void ResetPipeline();

  /** \brief Make a DataObject of the correct type to used as the specified output.
   *
   *  Every ProcessObject subclass must be able to create a
   * DataObject that can be used as a specified output. This method
   * is automatically called when DataObject::DisconnectPipeline() is
   * called.  DataObject::DisconnectPipeline, disconnects a data object
   * from being an output of its current source.  When the data object
   * is disconnected, the ProcessObject needs to construct a replacement
   * output data object so that the ProcessObject is in a valid state.
   * So DataObject::DisconnectPipeline eventually calls
   * ProcessObject::MakeOutput. Note that MakeOutput always returns a
   * itkSmartPointer to a DataObject. ImageSource and MeshSource override
   * this method to create the correct type of image and mesh respectively.
   * If a filter has multiple outputs of different types, then that
   * filter must provide an implementation of MakeOutput().
   */
  virtual DataObjectPointer MakeOutput( const DataObjectIdentifierType & );

  /** Turn on/off the flags to control whether the bulk data belonging
   * to the outputs of this ProcessObject are released after being
   * used by a downstream ProcessObject. Default value is off. Another
   * options for controlling memory utilization is the
   * ReleaseDataBeforeUpdateFlag. */
  virtual void SetReleaseDataFlag(bool flag);
  virtual bool GetReleaseDataFlag() const;
  void ReleaseDataFlagOn() { this->SetReleaseDataFlag(true); }
  void ReleaseDataFlagOff() { this->SetReleaseDataFlag(false); }

  /** Turn on/off the flags to control whether the bulk data belonging
   * to the outputs of this ProcessObject are released/reallocated
   * during an Update().  In limited memory scenarios, a user may want
   * to force the elements of a pipeline to release any bulk data that
   * is going to be regenerated anyway during an Update() in order to
   * control peak memory allocation. Note that this flag is different
   * from the ReleaseDataFlag. ReleaseDataFlag manages the
   * deallocation of a ProcessObject's bulk output data once that data
   * has been consumed by a downstream ProcessObject.  The
   * ReleaseDataBeforeUpdateFlag manages the deallocation/reallocation
   * of bulk data during a pipeline update to control peak memory
   * utilization. Default value is on. */
  itkSetMacro(ReleaseDataBeforeUpdateFlag, bool);
  itkGetConstReferenceMacro(ReleaseDataBeforeUpdateFlag, bool);
  itkBooleanMacro(ReleaseDataBeforeUpdateFlag);

  /** Get/Set the number of work units to create when executing. */
  itkSetClampMacro(NumberOfWorkUnits, ThreadIdType, 1, ITK_MAX_THREADS);
  itkGetConstReferenceMacro(NumberOfWorkUnits, ThreadIdType);

#if !defined( ITK_LEGACY_REMOVE ) || defined( ITKV4_COMPATIBILITY )
  itkLegacyMacro(void SetNumberOfThreads(ThreadIdType count))
  {
    this->SetNumberOfWorkUnits(count);
  }

  itkLegacyMacro(ThreadIdType GetNumberOfThreads() const)
  {
    return this->GetNumberOfWorkUnits();
  }
#endif // !ITK_LEGACY_REMOVE

  /** Return the multithreader used by this class. */
  MultiThreaderType * GetMultiThreader() const
  { return m_MultiThreader; }

  /** Set the multithreader used by this class. */
  void SetMultiThreader(MultiThreaderType* threader);

  /** An opportunity to deallocate a ProcessObject's bulk data
   *  storage. Some filters may wish to reuse existing bulk data
   *  storage to avoid unnecessary deallocation/allocation
   *  sequences. The default implementation calls Initialize() on each
   *  output. DataObject::Initialize() frees its bulk data by default.
   */
  virtual void PrepareOutputs();

protected:
  ProcessObject();
  ~ProcessObject() override;

  void PrintSelf(std::ostream & os, Indent indent) const override;

  //
  // Input Methods
  //

  /** \brief Return an input.
   *
   * nullptr is returned if the name or indexed input is undefined.
   */
  DataObject * GetInput(const DataObjectIdentifierType & key);
  const DataObject * GetInput(const DataObjectIdentifierType & key) const;

  /** Method used internally for getting an indexed input.
   */
  DataObject * GetInput(DataObjectPointerArraySizeType idx)
  { return idx < m_IndexedInputs.size() ? m_IndexedInputs[idx]->second.GetPointer() : nullptr; }
  const DataObject * GetInput(DataObjectPointerArraySizeType idx) const
  { return idx < m_IndexedInputs.size() ? m_IndexedInputs[idx]->second.GetPointer() : nullptr; }

  /** \brief Protected method for setting indexed and named inputs.
   *
   * Subclasses make use of them for setting input. As this method
   * only used the base DataObject pointer, derived classes should
   * expose a type check methods.
    */
  virtual void SetInput(const DataObjectIdentifierType & key, DataObject *input);
  virtual void SetNthInput(DataObjectPointerArraySizeType num, DataObject *input);

  /** Sets first nullptr indexed input, appends to the end otherwise */
  virtual void AddInput(DataObject *input);

  /** \brief Push/Pop an indexed input of this process object.
   *
   * These methods allow a
   * filter to model its input vector as a queue or stack.  These
   * routines may not be appropriate for all filters, especially
   * filters with different types of inputs.  These routines follow
   * the semantics of STL.
   */
  virtual void PushBackInput(const DataObject *input);
  virtual void PopBackInput();
  virtual void PushFrontInput(const DataObject *input);
  virtual void PopFrontInput();

  /** \brief Remove an input.
   *
   *  If the input is the last indexed input the number of indexed
   *  inputs will be reduced by one. Otherwise, if the input is
   *  required or indexed it will be set to nullptr.
   */
  virtual void RemoveInput(const DataObjectIdentifierType & key);
  virtual void RemoveInput(DataObjectPointerArraySizeType);

  /** Return the main input */
  DataObject * GetPrimaryInput()
  { return m_IndexedInputs[0]->second; }
  const DataObject * GetPrimaryInput() const
  { return m_IndexedInputs[0]->second; }

  /** Set/Get the name associated with the Primary input.  Defaults to "Primary". */
  virtual void SetPrimaryInputName(const DataObjectIdentifierType & key);
  virtual const char *GetPrimaryInputName() const
  { return this->m_IndexedInputs[0]->first.c_str(); }

  /** Set the main input */
  virtual void SetPrimaryInput(DataObject *input);

  /** \brief Define the number of indexed inputs defined.
   *
   * The new indexed inputs' values are set to
   * nullptr. If the size is reduced then the input definition is
   * removed entirely from the named input entries and index inputs.
   */
  void SetNumberOfIndexedInputs(DataObjectPointerArraySizeType num);

  /** \brief Set the number of required \b indexed inputs.
   *
   * If an input is not associated with an index and only a named
   * entry, it is not considered in this count. In the method
   * VerifyPreconditions(), GetNumberOfValidRequiredInputs() is
   * checked to be at least this value.
   *
   * \note SetNumberOfRequiredIndexInputs maybe a better name.
   */
  virtual void SetNumberOfRequiredInputs(DataObjectPointerArraySizeType);
  itkGetConstReferenceMacro(NumberOfRequiredInputs, DataObjectPointerArraySizeType);


  /** \brief Remove the named input from the required inputs
   *
   * The named input remains defined or set afterwards.
   */
  bool RemoveRequiredInputName( const DataObjectIdentifierType & );

  /** \brief Query if the named input is required by name. */
  bool IsRequiredInputName( const DataObjectIdentifierType & ) const;

  /** \brief Set all required named inputs.
   *
   * All named inputs remain defined or set.
   */
  void SetRequiredInputNames( const NameArray & );

  /** \brief Define a required named input and optionally associate it
   * with a numbered index.
   *
   * The previous named input associated with idx will be removed from
   * the defined input names.
   *
   * If the specified named inputs already exists with a non-null
   * value, then that value will be used. Otherwise if the specified
   * index has a non-null value, then that will be the value set.
   */
  bool AddRequiredInputName( const DataObjectIdentifierType & );
  bool AddRequiredInputName( const DataObjectIdentifierType &, DataObjectPointerArraySizeType idx );

  /** \brief Define a named input that is not required  and optionally
   *  associate with a numbered index.
   *
   * The previous named input associated with idx will be removed from
   * the defined input names.
   *
   * If the specified named inputs already exists with a non-null
   * value, then that value will be used. Otherwise if the specified
   * index has a non-null value, then that will be the value set.
   */
  void AddOptionalInputName( const DataObjectIdentifierType & );
  void AddOptionalInputName( const DataObjectIdentifierType &, DataObjectPointerArraySizeType idx );


  //
  // Output Methods
  //

  /** Return an output */
  DataObject * GetOutput(const DataObjectIdentifierType & key);
  const DataObject * GetOutput(const DataObjectIdentifierType & key) const;

  /** Set/Get the name associated with the Primary output.  Defaults to "Primary". */
  virtual void SetPrimaryOutputName(const DataObjectIdentifierType & key);
  virtual const char *GetPrimaryOutputName() const
  { return this->m_IndexedOutputs[0]->first.c_str(); }

  /** Method used internally for getting an indexed output. */
  DataObject * GetOutput(DataObjectPointerArraySizeType idx);
  const DataObject * GetOutput(DataObjectPointerArraySizeType idx) const;

  /** Set an output */
  virtual void SetOutput(const DataObjectIdentifierType & key, DataObject *output);

  /** Remove an output */
  virtual void RemoveOutput(const DataObjectIdentifierType & key);

  /** Return the main output */
  DataObject * GetPrimaryOutput()
  { return m_IndexedOutputs[0]->second; }
  const DataObject * GetPrimaryOutput() const
  { return m_IndexedOutputs[0]->second; }

  /** Set the main output */
  virtual void SetPrimaryOutput(DataObject *output);

  /** Protected methods for setting outputs.
   * Subclasses make use of them for getting output. */
  virtual void SetNthOutput(DataObjectPointerArraySizeType num, DataObject *output);

  virtual void AddOutput(DataObject *output);

  virtual void RemoveOutput(DataObjectPointerArraySizeType idx);

  itkSetMacro(NumberOfRequiredOutputs, DataObjectPointerArraySizeType);
  itkGetConstReferenceMacro(NumberOfRequiredOutputs, DataObjectPointerArraySizeType);

  /** Called to allocate the output array.  Copies old outputs. */
  void SetNumberOfIndexedOutputs(DataObjectPointerArraySizeType num);


  DataObjectIdentifierType MakeNameFromInputIndex( DataObjectPointerArraySizeType idx ) const;
  DataObjectIdentifierType MakeNameFromOutputIndex( DataObjectPointerArraySizeType idx ) const;
  DataObjectPointerArraySizeType MakeIndexFromInputName( const DataObjectIdentifierType & name ) const;
  DataObjectPointerArraySizeType MakeIndexFromOutputName( const DataObjectIdentifierType & name ) const;
  bool IsIndexedInputName( const DataObjectIdentifierType & ) const;
  bool IsIndexedOutputName( const DataObjectIdentifierType & ) const;

  //
  // Pipeline Methods
  //

  /** \brief Verifies that the process object has been configured
   * correctly, that all required inputs are set, and needed parameters
   * are set appropriately. If not valid an exceptions will be thrown.
   *
   * This method is called before UpdateOutputInformation is
   * propagated to the inputs.
   *
   * The ProcessObject's implementation verifies that the
   * NumberOfRequiredInputs are set and not null.
   *
   */
  virtual void VerifyPreconditions() ITKv5_CONST;

  /** \brief Verifies that the inputs meta-data is consistent and valid
   * for continued execution of the pipeline, throws an exception if
   * not valid.
   *
   * This method is called immediately before GenerateOutputInformation().
   *
   * The ProcessObject implementation does nothing. Subclasses might
   * check if all the inputs are in the same coordinate frame.
   *
   */
  virtual void VerifyInputInformation() ITKv5_CONST;

  /** What is the input requested region that is required to produce the
   * output requested region? By default, the largest possible region is
   * always required but this is overridden in many subclasses. For instance,
   * for an image processing filter where an output pixel is a simple function
   * of an input pixel, the input requested region will be set to the output
   * requested region.  For an image processing filter where an output pixel
   * is a function of the pixels in a neighborhood of an input pixel, then
   * the input requested region will need to be larger than the output
   * requested region (to avoid introducing artificial boundary conditions).
   * This function should never request an input region that is outside the
   * the input largest possible region (i.e. implementations of this method
   * should crop the input requested region at the boundaries of the input
   * largest possible region). */
  virtual void GenerateInputRequestedRegion();

  /** Given one output whose requested region has been set, how should
   * the requested regions for the remaining outputs of the process object
   * be set?  By default, all the outputs are set to the same requested
   * region.  If a filter needs to produce different requested regions
   * for each output, for instance an image processing filter producing
   * several outputs at different resolutions, then that filter may
   * override this method and set the requested regions appropriatedly.
   *
   * Note that a filter producing multiple outputs of different types is
   * required to override this method.  The default implementation
   * can only correctly handle multiple outputs of the same type. */
  virtual void GenerateOutputRequestedRegion(DataObject *output);

  /** Generate the information describing the output data. The default
   * implementation of this method will copy information from the input to
   * the output.  A filter may override this method if its output will have
   * different information than its input.  For instance, a filter that
   * shrinks an image will need to provide an implementation for this
   * method that changes the spacing of the pixels. Such filters should call
   * their superclass' implementation of this method prior to changing the
   * information values they need (i.e. GenerateOutputInformation() should
   * call Superclass::GenerateOutputInformation() prior to changing the
   * information. */
  virtual void GenerateOutputInformation();

  /** This method causes the filter to generate its output. */
  virtual void GenerateData() {}

  /** Called to allocate the input array.  Copies old inputs. */
  /** Propagate a call to ResetPipeline() up the pipeline. Called only from
   * DataObject. */
  virtual void PropagateResetPipeline();

  /** A filter may need to release its input's bulk data after it has
   * finished calculating a new output. The filter may need to release
   * the inputs because the user has turned on the ReleaseDataFlag or
   * it may need to release the inputs because the filter is an "in
   * place" filter and it has overwritten its input with its output
   * data.  The implementation here simply checks the ReleaseDataFlag
   * of the inputs.  InPlaceImageFilter overrides this method so
   * release the input it has overwritten.
   *
   * \sa InPlaceImageFilter::ReleaseInputs()
   */
  virtual void ReleaseInputs();

  /**
   * Cache the state of any ReleaseDataFlag's on the inputs. While the
   * filter is executing, we need to set the ReleaseDataFlag's on the
   * inputs to false in case the current filter is implemented using a
   * mini-pipeline (which will try to release the inputs).  After the
   * filter finishes, we restore the state of the ReleaseDataFlag's
   * before the call to ReleaseInputs().
   */
  virtual void CacheInputReleaseDataFlags();

  /**
   * Restore the cached input ReleaseDataFlags.
   */
  virtual void RestoreInputReleaseDataFlags();

  /** These ivars are made protected so filters like itkStreamingImageFilter
   * can access them directly. */

  /** This flag indicates when the pipeline is executing.
   * It prevents infinite recursion when pipelines have loops. */
  bool m_Updating;

  /** Time when GenerateOutputInformation was last called. */
  TimeStamp m_OutputInformationMTime;

private:
  DataObjectIdentifierType MakeNameFromIndex( DataObjectPointerArraySizeType ) const;
  DataObjectPointerArraySizeType MakeIndexFromName( const DataObjectIdentifierType & ) const;

  /** STL map to store the named inputs and outputs */
  using DataObjectPointerMap = std::map< DataObjectIdentifierType, DataObjectPointer >;


  /** Named input and outputs containers */
  DataObjectPointerMap   m_Inputs;
  DataObjectPointerMap   m_Outputs;

  std::vector< DataObjectPointerMap::iterator > m_IndexedInputs;
  std::vector< DataObjectPointerMap::iterator > m_IndexedOutputs;

  /** An array that caches the ReleaseDataFlags of the inputs */
  std::map< DataObjectIdentifierType, bool > m_CachedInputReleaseDataFlags;

  DataObjectPointerArraySizeType  m_NumberOfRequiredInputs;
  DataObjectPointerArraySizeType  m_NumberOfRequiredOutputs;

  /** STL map to store the named inputs and outputs */
  using NameSet = std::set< DataObjectIdentifierType >;

  /** The required inputs */
  NameSet m_RequiredInputNames;

  /** These support the progress method and aborting filter execution. */
  bool  m_AbortGenerateData;
  float m_Progress;

  /** Support processing data in multiple threads. Used by subclasses
   * (e.g., ImageSource). */
  MultiThreaderType::Pointer m_MultiThreader;
  ThreadIdType               m_NumberOfWorkUnits;

  /** Memory management ivars */
  bool m_ReleaseDataBeforeUpdateFlag;

  /** Friends of ProcessObject */
  friend class DataObject;

  friend class DataObjectConstIterator;
  friend class InputDataObjectConstIterator;
  friend class OutputDataObjectConstIterator;

  friend class DataObjectIterator;
  friend class InputDataObjectIterator;
  friend class OutputDataObjectIterator;

  friend class TestProcessObject;
};
} // end namespace itk

#endif
