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/*! \file
 * \brief Declares gmx::ArrayRefWithPadding that refers to memory whose
 * size includes padding for SIMD operations.
 *
 * \author Mark Abraham <mark.j.abraham@gmail.com>
 *
 * \inpublicapi
 * \ingroup module_math
 */
#ifndef GMX_MATH_ARRAYREFWITHPADDING_H
#define GMX_MATH_ARRAYREFWITHPADDING_H

#include <cstddef>

#include "gromacs/utility/arrayref.h"
#include "gromacs/utility/basedefinitions.h"
#include "gromacs/utility/gmxassert.h"

namespace gmx
{

/*! \brief Interface to a C array of T (or part of a std container of
 * T), that includes padding that is suitable for the kinds of SIMD
 * operations GROMACS uses.
 *
 * High level code e.g. in the force calculation routines need to hold
 * a non-owning view of memory and be able to create ArrayRef objects
 * that view padded or unpadded memory, suitable for the various
 * component routines. This class implements that non-owning view,
 * and the only available functionality refers to its size, and the
 * methods to create such ArrayRef objects.
 *
 * \copydoc ArrayRef
 * \inlibraryapi
 * \ingroup module_math
 */
template <typename T>
class ArrayRefWithPadding
{
    public:
        //! Type of values stored in the reference.
        using value_type = T;
        //! Type for representing size of the reference.
        using size_type = index;
        //! Const pointer to an element.
        using const_pointer = const T*;
        //! Const iterator type to an element.
        using const_iterator = const T*;
        //! Pointer to an element.
        using pointer = T*;
        //! Iterator type to an element.
        using iterator = T*;

        /*! \brief
         * Constructs an empty reference.
         */
        ArrayRefWithPadding()
            : begin_(nullptr), end_(nullptr), paddedEnd_(nullptr) {}
        /*! \brief
         * Constructs an empty reference.
         *
         * This is provided for convenience, such that EmptyArrayRef can be
         * used to initialize any ArrayRefWithPadding, without specifying the template
         * type.  It is not explicit to enable that usage.
         */
        ArrayRefWithPadding(const EmptyArrayRef & /*unused*/)
            : begin_(nullptr), end_(nullptr), paddedEnd_(nullptr) {}
        /*! \brief
         * Constructs a reference to a particular range.
         *
         * \param[in] begin        Pointer to the beginning of a range.
         * \param[in] end          Pointer to the end of a range without padding.
         * \param[in] paddedEnd    Pointer to the end of a range including padding.
         *
         * Passed pointers must remain valid for the lifetime of this object.
         */
        ArrayRefWithPadding(pointer begin, pointer end, pointer paddedEnd)
            : begin_(begin), end_(end), paddedEnd_(paddedEnd)
        {
            GMX_ASSERT(end >= begin, "Invalid range");
            GMX_ASSERT(paddedEnd >= end, "Invalid range");
        }
        //! Copy constructor
        ArrayRefWithPadding(const ArrayRefWithPadding &o)
            : begin_(o.begin_), end_(o.end_), paddedEnd_(o.paddedEnd_) {}
        //! Move constructor
        ArrayRefWithPadding(ArrayRefWithPadding &&o) noexcept
            : begin_(std::move(o.begin_)), end_(std::move(o.end_)), paddedEnd_(std::move(o.paddedEnd_)) {}
        //! Copy assignment operator
        ArrayRefWithPadding &operator=(ArrayRefWithPadding const &o)
        {
            if (&o != this)
            {
                begin_     = o.begin_;
                end_       = o.end_;
                paddedEnd_ = o.paddedEnd_;
            }
            return *this;
        }
        //! Move assignment operator
        ArrayRefWithPadding &operator=(ArrayRefWithPadding &&o) noexcept
        {
            if (&o != this)
            {
                begin_     = std::move(o.begin_);
                end_       = std::move(o.end_);
                paddedEnd_ = std::move(o.paddedEnd_);
            }
            return *this;
        }

        //! Return the size of the view, i.e with the padding.
        size_type size() const { return paddedEnd_ - begin_; }
        //! Whether the reference refers to no memory.
        bool empty() const { return begin_ == end_; }

        //! Returns an ArrayRef of elements that does not include the padding region.
        ArrayRef<T> unpaddedArrayRef()
        {
            return {begin_, end_};
        }
        //! Returns an ArrayRef of const elements that does not include the padding region.
        ArrayRef<const T> unpaddedConstArrayRef() const
        {
            return {begin_, end_};
        }
        //! Returns an ArrayRef of elements that does include the padding region.
        ArrayRef<T> paddedArrayRef()
        {
            return {begin_, paddedEnd_};
        }
        //! Returns an ArrayRef of const elements that does include the padding region.
        ArrayRef<const T> paddedConstArrayRef() const
        {
            return {begin_, paddedEnd_};
        }
        /*! \brief
         * Swaps referenced memory with the other object.
         *
         * The actual memory areas are not modified, only the references are
         * swapped.
         */
        void swap(ArrayRefWithPadding<T> &other)
        {
            std::swap(begin_, other.begin_);
            std::swap(end_, other.end_);
        }

    private:
        pointer begin_;
        pointer end_;
        pointer paddedEnd_;
};

} // namespace gmx

#endif
