/*=========================================================================
 *
 *  Copyright NumFOCUS
 *
 *  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
 *
 *         https://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.
 *
 *=========================================================================*/
#ifndef itkImageConstIterator_h
#define itkImageConstIterator_h

#include "itkImage.h"
#include "itkIndex.h"
#include "itkNumericTraits.h"

namespace itk
{
/** \class ImageConstIterator
 * \brief A multi-dimensional image iterator templated over image type.
 *
 * ImageConstIterator is a templated class to represent a multi-dimensional
 * iterator. ImageConstIterator is templated over the type of
 * the image to be iterated over.
 *
 * ImageConstIterator is a base class for all the image iterators. It provides
 * the basic construction and comparison operations.  However, it does not
 * provide mechanisms for moving the iterator.  A subclass of ImageConstIterator
 * must be used to move the iterator.
 *
 * ImageConstIterator is a multi-dimensional iterator, requiring more information
 * be specified before the iterator can be used than conventional iterators.
 * Whereas the std::vector::iterator from the STL only needs to be passed
 * a pointer to establish the iterator, the multi-dimensional image iterator
 * needs a pointer, the size of the buffer, the size of the region, the
 * start index of the buffer, and the start index of the region. To gain
 * access to this information, ImageConstIterator holds a reference to the image
 * over which it is traversing.
 *
 * ImageConstIterator assumes a particular layout of the image data. In particular,
 * the data is arranged in a 1D array as if it were [][][][slice][row][col]
 * with Index[0] = col, Index[1] = row, Index[2] = slice, etc.
 *
 * \ingroup ImageConstIterators
 * \par MORE INFORMATION
 * For a complete description of the ITK Image Iterators and their API, please
 * see the Iterators chapter in the ITK Software Guide.  The ITK Software Guide
 * is available in print and as a free .pdf download from https://www.itk.org.
 *
 * \ingroup ImageIterators
 *
 * \sa ImageConstIterator \sa ConditionalConstIterator
 * \sa ConstNeighborhoodIterator \sa ConstShapedNeighborhoodIterator
 * \sa ConstSliceIterator  \sa CorrespondenceDataStructureIterator
 * \sa FloodFilledFunctionConditionalConstIterator
 * \sa FloodFilledImageFunctionConditionalConstIterator
 * \sa FloodFilledImageFunctionConditionalIterator
 * \sa FloodFilledSpatialFunctionConditionalConstIterator
 * \sa FloodFilledSpatialFunctionConditionalIterator
 * \sa ImageConstIterator \sa ImageConstIteratorWithIndex
 * \sa ImageIterator \sa ImageIteratorWithIndex
 * \sa ImageLinearConstIteratorWithIndex  \sa ImageLinearIteratorWithIndex
 * \sa ImageRandomConstIteratorWithIndex  \sa ImageRandomIteratorWithIndex
 * \sa ImageRegionConstIterator \sa ImageRegionConstIteratorWithIndex
 * \sa ImageRegionExclusionConstIteratorWithIndex
 * \sa ImageRegionExclusionIteratorWithIndex
 * \sa ImageRegionIterator  \sa ImageRegionIteratorWithIndex
 * \sa ImageRegionReverseConstIterator  \sa ImageRegionReverseIterator
 * \sa ImageReverseConstIterator  \sa ImageReverseIterator
 * \sa ImageSliceConstIteratorWithIndex  \sa ImageSliceIteratorWithIndex
 * \sa NeighborhoodIterator \sa PathConstIterator  \sa PathIterator
 * \sa ShapedNeighborhoodIterator  \sa SliceIterator
 * \ingroup ITKCommon
 */
template <typename TImage>
class ITK_TEMPLATE_EXPORT ImageConstIterator
{
public:
  /** Standard class type aliases. */
  using Self = ImageConstIterator;

  /** Dimension of the image the iterator walks.  This constant is needed so
   * functions that are templated over image iterator type (as opposed to
   * being templated over pixel type and dimension) can have compile time
   * access to the dimension of the image that the iterator walks. */
  static constexpr unsigned int ImageIteratorDimension = TImage::ImageDimension;

  /** \see LightObject::GetNameOfClass() */
  itkVirtualGetNameOfClassMacro(ImageConstIterator);

  /** Index type alias support */
  using IndexType = typename TImage::IndexType;

  /** Size type alias support */
  using SizeType = typename TImage::SizeType;

  /** Offset type alias support */
  using OffsetType = typename TImage::OffsetType;

  /** Region type alias support */
  using RegionType = typename TImage::RegionType;

  /** Image type alias support */
  using ImageType = TImage;

  /** PixelContainer type alias support. Used to refer to the container for
   * the pixel data. While this was already typedef'ed in the superclass
   * it needs to be redone here for this subclass to compile properly with gcc. */
  using PixelContainer = typename TImage::PixelContainer;
  using PixelContainerPointer = typename PixelContainer::Pointer;

  /** Internal Pixel Type */
  using InternalPixelType = typename TImage::InternalPixelType;

  /** External Pixel Type */
  using PixelType = typename TImage::PixelType;

  /**  Accessor type that convert data between internal and external
   *  representations. */
  using AccessorType = typename TImage::AccessorType;
  using AccessorFunctorType = typename TImage::AccessorFunctorType;

  /** Default Constructor. Need to provide a default constructor since we
   * provide a copy constructor. */
  ImageConstIterator()
    : m_Region()
    , m_PixelAccessor()
    , m_PixelAccessorFunctor()
  {
    m_Image = nullptr;
    m_Buffer = nullptr;
    m_Offset = 0;
    m_BeginOffset = 0;
    m_EndOffset = 0;
    m_PixelAccessorFunctor.SetBegin(m_Buffer);
  }

  /** Default Destructor. */
  virtual ~ImageConstIterator() = default;

  /** Copy Constructor. The copy constructor is provided to make sure the
   * handle to the image is properly reference counted. */
  ImageConstIterator(const Self & it)
  {
    m_Image = it.m_Image; // copy the smart pointer

    m_Region = it.m_Region;

    m_Buffer = it.m_Buffer;
    m_Offset = it.m_Offset;
    m_BeginOffset = it.m_BeginOffset;
    m_EndOffset = it.m_EndOffset;
    m_PixelAccessor = it.m_PixelAccessor;
    m_PixelAccessorFunctor = it.m_PixelAccessorFunctor;
    m_PixelAccessorFunctor.SetBegin(m_Buffer);
  }

  /** Constructor establishes an iterator to walk a particular image and a
   * particular region of that image. */
  ImageConstIterator(const ImageType * ptr, const RegionType & region)
  {
    m_Image = ptr;
    m_Buffer = m_Image->GetBufferPointer();

    SetRegion(region);

    m_PixelAccessor = ptr->GetPixelAccessor();
    m_PixelAccessorFunctor.SetPixelAccessor(m_PixelAccessor);
    m_PixelAccessorFunctor.SetBegin(m_Buffer);
  }

  /** operator= is provided to make sure the handle to the image is properly
   * reference counted. */
  Self &
  operator=(const Self & it)
  {
    if (this != &it)
    {
      m_Image = it.m_Image; // copy the smart pointer
      m_Region = it.m_Region;

      m_Buffer = it.m_Buffer;
      m_Offset = it.m_Offset;
      m_BeginOffset = it.m_BeginOffset;
      m_EndOffset = it.m_EndOffset;
      m_PixelAccessor = it.m_PixelAccessor;
      m_PixelAccessorFunctor = it.m_PixelAccessorFunctor;
      m_PixelAccessorFunctor.SetBegin(m_Buffer);
    }
    return *this;
  }

  /** Set the region of the image to iterate over. */
  virtual void
  SetRegion(const RegionType & region)
  {
    m_Region = region;

    if (region.GetNumberOfPixels() > 0) // If region is non-empty
    {
      const RegionType & bufferedRegion = m_Image->GetBufferedRegion();
      itkAssertOrThrowMacro((bufferedRegion.IsInside(m_Region)),
                            "Region " << m_Region << " is outside of buffered region " << bufferedRegion);
    }

    // Compute the start offset
    m_Offset = m_Image->ComputeOffset(m_Region.GetIndex());
    m_BeginOffset = m_Offset;

    // Compute the end offset. If any component of m_Region.GetSize()
    // is zero, the region is not valid and we set the EndOffset
    // to be same as BeginOffset so that iterator end condition is met
    // immediately.
    IndexType ind(m_Region.GetIndex());
    SizeType  size(m_Region.GetSize());
    if (m_Region.GetNumberOfPixels() == 0)
    {
      // region is empty, probably has a size of 0 along one dimension
      m_EndOffset = m_BeginOffset;
    }
    else
    {
      for (unsigned int i = 0; i < TImage::ImageDimension; ++i)
      {
        ind[i] += (static_cast<IndexValueType>(size[i]) - 1);
      }
      m_EndOffset = m_Image->ComputeOffset(ind);
      ++m_EndOffset;
    }
  }

  /** Get the dimension (size) of the index. */
  static unsigned int
  GetImageIteratorDimension()
  {
    return TImage::ImageDimension;
  }

  /** Comparison operator. Two iterators are the same if they "point to" the
   * same memory location */
  bool
  operator==(const Self & it) const
  {
    // two iterators are the same if they "point to" the same memory location
    return (m_Buffer + m_Offset) == (it.m_Buffer + it.m_Offset);
  }

  ITK_UNEQUAL_OPERATOR_MEMBER_FUNCTION(Self);

  /** Comparison operator. An iterator is "less than" another if it "points to"
   * a lower memory location. */
  bool
  operator<=(const Self & it) const
  {
    // an iterator is "less than" another if it "points to" a lower
    // memory location
    return (m_Buffer + m_Offset) <= (it.m_Buffer + it.m_Offset);
  }

  /** Comparison operator. An iterator is "less than" another if it "points to"
   * a lower memory location. */
  bool
  operator<(const Self & it) const
  {
    // an iterator is "less than" another if it "points to" a lower
    // memory location
    return (m_Buffer + m_Offset) < (it.m_Buffer + it.m_Offset);
  }

  /** Comparison operator. An iterator is "greater than" another if it
   * "points to" a higher location. */
  bool
  operator>=(const Self & it) const
  {
    // an iterator is "greater than" another if it "points to" a higher
    // memory location
    return (m_Buffer + m_Offset) >= (it.m_Buffer + it.m_Offset);
  }

  /** Comparison operator. An iterator is "greater than" another if it
   * "points to" a higher location. */
  bool
  operator>(const Self & it) const
  {
    // an iterator is "greater than" another if it "points to" a higher
    // memory location
    return (m_Buffer + m_Offset) > (it.m_Buffer + it.m_Offset);
  }

  /** Get the index. This provides a read only reference to the index.
   * This causes the index to be calculated from pointer arithmetic and is
   * therefore an expensive operation.
   * \sa SetIndex */
  const IndexType
  GetIndex() const
  {
    return m_Image->ComputeIndex(static_cast<OffsetValueType>(m_Offset));
  }

  /** Set the index. No bounds checking is performed.
   * \sa GetIndex */
  virtual void
  SetIndex(const IndexType & ind)
  {
    m_Offset = m_Image->ComputeOffset(ind);
  }

  /** Get the region that this iterator walks. ImageConstIterators know the
   * beginning and the end of the region of the image to iterate over. */
  const RegionType &
  GetRegion() const
  {
    return m_Region;
  }

  /** Get the image that this iterator walks. */
  const ImageType *
  GetImage() const
  {
    return m_Image.GetPointer();
  }

  /** Get the pixel value */
  PixelType
  Get() const
  {
    return m_PixelAccessorFunctor.Get(*(m_Buffer + m_Offset));
  }

  /** Return a const reference to the pixel
   * This method will provide the fastest access to pixel
   * data, but it will NOT support ImageAdaptors. */
  const PixelType &
  Value() const
  {
    return *(m_Buffer + m_Offset);
  }

  /** Move an iterator to the beginning of the region. "Begin" is
   * defined as the first pixel in the region. */
  void
  GoToBegin()
  {
    m_Offset = m_BeginOffset;
  }

  /** Move an iterator to the end of the region. "End" is defined as
   * one pixel past the last pixel of the region. */
  void
  GoToEnd()
  {
    m_Offset = m_EndOffset;
  }

  /** Is the iterator at the beginning of the region? "Begin" is defined
   * as the first pixel in the region. */
  bool
  IsAtBegin() const
  {
    return (m_Offset == m_BeginOffset);
  }

  /** Is the iterator at the end of the region? "End" is defined as one
   * pixel past the last pixel of the region. */
  bool
  IsAtEnd() const
  {
    return (m_Offset == m_EndOffset);
  }

protected: // made protected so other iterators can access
  typename TImage::ConstWeakPointer m_Image{};

  RegionType m_Region{}; // region to iterate over

  OffsetValueType m_Offset{};
  OffsetValueType m_BeginOffset{}; // offset to first pixel in region
  OffsetValueType m_EndOffset{};   // offset to one pixel past last pixel in region

  const InternalPixelType * m_Buffer{};

  AccessorType        m_PixelAccessor{};
  AccessorFunctorType m_PixelAccessorFunctor{};
};
} // end namespace itk

#endif
