// -*- C++ -*-
//===----------------------------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===---------------------------------------------------------------------===//

#ifndef _LIBCPP_SPAN
#define _LIBCPP_SPAN

/*
    span synopsis

namespace std {

// constants
inline constexpr size_t dynamic_extent = numeric_limits<size_t>::max();

// [views.span], class template span
template <class ElementType, size_t Extent = dynamic_extent>
    class span;

template<class ElementType, size_t Extent>
  inline constexpr bool ranges::enable_view<span<ElementType, Extent>> = true;

template<class ElementType, size_t Extent>
    inline constexpr bool ranges::enable_borrowed_range<span<ElementType, Extent>> = true;

// [span.objectrep], views of object representation
template <class ElementType, size_t Extent>
    span<const byte, ((Extent == dynamic_extent) ? dynamic_extent :
        (sizeof(ElementType) * Extent))> as_bytes(span<ElementType, Extent> s) noexcept;

template <class ElementType, size_t Extent>
    span<      byte, ((Extent == dynamic_extent) ? dynamic_extent :
        (sizeof(ElementType) * Extent))> as_writable_bytes(span<ElementType, Extent> s) noexcept;


template <class ElementType, size_t Extent = dynamic_extent>
class span {
public:
    // constants and types
    using element_type = ElementType;
    using value_type = remove_cv_t<ElementType>;
    using size_type = size_t;
    using difference_type = ptrdiff_t;
    using pointer = element_type*;
    using const_pointer = const element_type*;
    using reference = element_type&;
    using const_reference = const element_type&;
    using iterator = implementation-defined;
    using reverse_iterator = std::reverse_iterator<iterator>;
    static constexpr size_type extent = Extent;

    // [span.cons], span constructors, copy, assignment, and destructor
    constexpr span() noexcept;
    template <class It>
    constexpr explicit(Extent != dynamic_extent) span(It first, size_type count);
    template <class It, class End>
    constexpr explicit(Extent != dynamic_extent) span(It first, End last);
    template <size_t N>
        constexpr span(type_identity_t<element_type> (&arr)[N]) noexcept;
    template <size_t N>
        constexpr span(array<value_type, N>& arr) noexcept;
    template <size_t N>
        constexpr span(const array<value_type, N>& arr) noexcept;
    template<class R>
      constexpr explicit(Extent != dynamic_extent) span(R&& r);
    constexpr span(const span& other) noexcept = default;
    template <class OtherElementType, size_t OtherExtent>
        constexpr explicit(Extent != dynamic_extent) span(const span<OtherElementType, OtherExtent>& s) noexcept;
    ~span() noexcept = default;
    constexpr span& operator=(const span& other) noexcept = default;

    // [span.sub], span subviews
    template <size_t Count>
        constexpr span<element_type, Count> first() const;
    template <size_t Count>
        constexpr span<element_type, Count> last() const;
    template <size_t Offset, size_t Count = dynamic_extent>
        constexpr span<element_type, see below> subspan() const;

    constexpr span<element_type, dynamic_extent> first(size_type count) const;
    constexpr span<element_type, dynamic_extent> last(size_type count) const;
    constexpr span<element_type, dynamic_extent> subspan(size_type offset, size_type count = dynamic_extent) const;

    // [span.obs], span observers
    constexpr size_type size() const noexcept;
    constexpr size_type size_bytes() const noexcept;
    [[nodiscard]] constexpr bool empty() const noexcept;

    // [span.elem], span element access
    constexpr reference operator[](size_type idx) const;
    constexpr reference front() const;
    constexpr reference back() const;
    constexpr pointer data() const noexcept;

    // [span.iterators], span iterator support
    constexpr iterator begin() const noexcept;
    constexpr iterator end() const noexcept;
    constexpr reverse_iterator rbegin() const noexcept;
    constexpr reverse_iterator rend() const noexcept;

private:
    pointer data_;    // exposition only
    size_type size_;  // exposition only
};

template<class It, class EndOrSize>
    span(It, EndOrSize) -> span<remove_reference_t<iter_reference_t<_It>>>;

template<class T, size_t N>
    span(T (&)[N]) -> span<T, N>;

template<class T, size_t N>
    span(array<T, N>&) -> span<T, N>;

template<class T, size_t N>
    span(const array<T, N>&) -> span<const T, N>;

template<class R>
    span(R&&) -> span<remove_reference_t<ranges::range_reference_t<R>>>;

} // namespace std

*/

#include <__config>
#include <__debug>
#include <__iterator/concepts.h>
#include <__iterator/wrap_iter.h>
#include <__ranges/concepts.h>
#include <__ranges/data.h>
#include <__ranges/enable_borrowed_range.h>
#include <__ranges/enable_view.h>
#include <__ranges/size.h>
#include <array>        // for array
#include <cstddef>      // for byte
#include <iterator>     // for iterators
#include <limits>
#include <type_traits>  // for remove_cv, etc
#include <version>

#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
#pragma GCC system_header
#endif

_LIBCPP_PUSH_MACROS
#include <__undef_macros>

_LIBCPP_BEGIN_NAMESPACE_STD

#if _LIBCPP_STD_VER > 17

inline constexpr size_t dynamic_extent = numeric_limits<size_t>::max();
template <typename _Tp, size_t _Extent = dynamic_extent> class span;


template <class _Tp>
struct __is_std_array : false_type {};

template <class _Tp, size_t _Sz>
struct __is_std_array<array<_Tp, _Sz>> : true_type {};

template <class _Tp>
struct __is_std_span : false_type {};

template <class _Tp, size_t _Sz>
struct __is_std_span<span<_Tp, _Sz>> : true_type {};

#if defined(_LIBCPP_HAS_NO_CONCEPTS) || defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
// This is a temporary workaround until we ship <ranges> -- we've unfortunately been
// shipping <span> before its API was finalized, and we used to provide a constructor
// from container types that had the requirements below. To avoid breaking code that
// has started relying on the range-based constructor until we ship all of <ranges>,
// we emulate the constructor requirements like this.
template <class _Range, class _ElementType, class = void>
struct __span_compatible_range : false_type { };

template <class _Range, class _ElementType>
struct __span_compatible_range<_Range, _ElementType, void_t<
    enable_if_t<!__is_std_span<remove_cvref_t<_Range>>::value>,
    enable_if_t<!__is_std_array<remove_cvref_t<_Range>>::value>,
    enable_if_t<!is_array_v<remove_cvref_t<_Range>>>,
    decltype(data(declval<_Range>())),
    decltype(size(declval<_Range>())),
    enable_if_t<is_convertible_v<remove_pointer_t<decltype(data(declval<_Range&>()))>(*)[], _ElementType(*)[]>>
>> : true_type { };
#else
template <class _Range, class _ElementType>
concept __span_compatible_range =
  ranges::contiguous_range<_Range> &&
  ranges::sized_range<_Range> &&
  (ranges::borrowed_range<_Range> || is_const_v<_ElementType>) &&
  !__is_std_span<remove_cvref_t<_Range>>::value  &&
  !__is_std_array<remove_cvref_t<_Range>>::value &&
  !is_array_v<remove_cvref_t<_Range>> &&
  is_convertible_v<remove_reference_t<ranges::range_reference_t<_Range>>(*)[], _ElementType(*)[]>;
#endif

template <typename _Tp, size_t _Extent>
class _LIBCPP_TEMPLATE_VIS span {
public:
//  constants and types
    using element_type           = _Tp;
    using value_type             = remove_cv_t<_Tp>;
    using size_type              = size_t;
    using difference_type        = ptrdiff_t;
    using pointer                = _Tp *;
    using const_pointer          = const _Tp *;
    using reference              = _Tp &;
    using const_reference        = const _Tp &;
#if (_LIBCPP_DEBUG_LEVEL == 2) || defined(_LIBCPP_ABI_SPAN_POINTER_ITERATORS)
    using iterator               = pointer;
#else
    using iterator               = __wrap_iter<pointer>;
#endif
    using reverse_iterator       = _VSTD::reverse_iterator<iterator>;

    static constexpr size_type extent = _Extent;

// [span.cons], span constructors, copy, assignment, and destructor
    template <size_t _Sz = _Extent, enable_if_t<_Sz == 0, nullptr_t> = nullptr>
    _LIBCPP_INLINE_VISIBILITY constexpr span() noexcept : __data{nullptr} {}

    constexpr span           (const span&) noexcept = default;
    constexpr span& operator=(const span&) noexcept = default;

#if !defined(_LIBCPP_HAS_NO_CONCEPTS)
    template <class _It,
              enable_if_t<contiguous_iterator<_It> &&
                              is_convertible_v<remove_reference_t<iter_reference_t<_It>>(*)[], element_type (*)[]>,
                          nullptr_t> = nullptr>
    _LIBCPP_INLINE_VISIBILITY
    constexpr explicit span(_It __first, size_type __count)
        : __data{_VSTD::to_address(__first)} {
      (void)__count;
      _LIBCPP_ASSERT(_Extent == __count, "size mismatch in span's constructor (iterator, len)");
    }

    template <
        class _It, class _End,
        enable_if_t<is_convertible_v<remove_reference_t<iter_reference_t<_It> > (*)[], element_type (*)[]> &&
                        contiguous_iterator<_It> && sized_sentinel_for<_End, _It> && !is_convertible_v<_End, size_t>,
                    nullptr_t> = nullptr>
    _LIBCPP_INLINE_VISIBILITY
    constexpr explicit span(_It __first, _End __last) : __data{_VSTD::to_address(__first)} {
      (void)__last;
      _LIBCPP_ASSERT((__last - __first >= 0), "invalid range in span's constructor (iterator, sentinel)");
      _LIBCPP_ASSERT(__last - __first == _Extent,
                     "invalid range in span's constructor (iterator, sentinel): last - first != extent");
    }
#endif // !defined(_LIBCPP_HAS_NO_CONCEPTS)

    _LIBCPP_INLINE_VISIBILITY constexpr span(type_identity_t<element_type> (&__arr)[_Extent]) noexcept : __data{__arr} {}

    template <class _OtherElementType,
              enable_if_t<is_convertible_v<_OtherElementType(*)[], element_type (*)[]>, nullptr_t> = nullptr>
    _LIBCPP_INLINE_VISIBILITY
    constexpr span(array<_OtherElementType, _Extent>& __arr) noexcept : __data{__arr.data()} {}

    template <class _OtherElementType,
              enable_if_t<is_convertible_v<const _OtherElementType(*)[], element_type (*)[]>, nullptr_t> = nullptr>
    _LIBCPP_INLINE_VISIBILITY
    constexpr span(const array<_OtherElementType, _Extent>& __arr) noexcept : __data{__arr.data()} {}

#if defined(_LIBCPP_HAS_NO_CONCEPTS) || defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
    template <class _Container, class = enable_if_t<
        __span_compatible_range<_Container, element_type>::value
    >>
    _LIBCPP_INLINE_VISIBILITY
    constexpr explicit span(_Container& __c) : __data{std::data(__c)} {
      _LIBCPP_ASSERT(std::size(__c) == _Extent, "size mismatch in span's constructor (range)");
    }
    template <class _Container, class = enable_if_t<
        __span_compatible_range<const _Container, element_type>::value
    >>
    _LIBCPP_INLINE_VISIBILITY
    constexpr explicit span(const _Container& __c) : __data{std::data(__c)} {
      _LIBCPP_ASSERT(std::size(__c) == _Extent, "size mismatch in span's constructor (range)");
    }
#else
    template <__span_compatible_range<element_type> _Range>
    _LIBCPP_INLINE_VISIBILITY
    constexpr explicit span(_Range&& __r) : __data{ranges::data(__r)} {
      _LIBCPP_ASSERT(ranges::size(__r) == _Extent, "size mismatch in span's constructor (range)");
    }
#endif // !defined(_LIBCPP_HAS_NO_CONCEPTS) && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)

    template <class _OtherElementType>
    _LIBCPP_INLINE_VISIBILITY
        constexpr span(const span<_OtherElementType, _Extent>& __other,
                       enable_if_t<
                          is_convertible_v<_OtherElementType(*)[], element_type (*)[]>,
                          nullptr_t> = nullptr)
        : __data{__other.data()} {}

    template <class _OtherElementType>
    _LIBCPP_INLINE_VISIBILITY
        constexpr explicit span(const span<_OtherElementType, dynamic_extent>& __other,
                       enable_if_t<
                          is_convertible_v<_OtherElementType(*)[], element_type (*)[]>,
                          nullptr_t> = nullptr) noexcept
        : __data{__other.data()} { _LIBCPP_ASSERT(_Extent == __other.size(), "size mismatch in span's constructor (other span)"); }


//  ~span() noexcept = default;

    template <size_t _Count>
    _LIBCPP_INLINE_VISIBILITY
    constexpr span<element_type, _Count> first() const noexcept
    {
        static_assert(_Count <= _Extent, "Count out of range in span::first()");
        return span<element_type, _Count>{data(), _Count};
    }

    template <size_t _Count>
    _LIBCPP_INLINE_VISIBILITY
    constexpr span<element_type, _Count> last() const noexcept
    {
        static_assert(_Count <= _Extent, "Count out of range in span::last()");
        return span<element_type, _Count>{data() + size() - _Count, _Count};
    }

    _LIBCPP_INLINE_VISIBILITY
    constexpr span<element_type, dynamic_extent> first(size_type __count) const noexcept
    {
        _LIBCPP_ASSERT(__count <= size(), "Count out of range in span::first(count)");
        return {data(), __count};
    }

    _LIBCPP_INLINE_VISIBILITY
    constexpr span<element_type, dynamic_extent> last(size_type __count) const noexcept
    {
        _LIBCPP_ASSERT(__count <= size(), "Count out of range in span::last(count)");
        return {data() + size() - __count, __count};
    }

    template <size_t _Offset, size_t _Count = dynamic_extent>
    _LIBCPP_INLINE_VISIBILITY
    constexpr auto subspan() const noexcept
        -> span<element_type, _Count != dynamic_extent ? _Count : _Extent - _Offset>
    {
        static_assert(_Offset <= _Extent, "Offset out of range in span::subspan()");
        static_assert(_Count == dynamic_extent || _Count <= _Extent - _Offset, "Offset + count out of range in span::subspan()");

        using _ReturnType = span<element_type, _Count != dynamic_extent ? _Count : _Extent - _Offset>;
        return _ReturnType{data() + _Offset, _Count == dynamic_extent ? size() - _Offset : _Count};
    }


    _LIBCPP_INLINE_VISIBILITY
    constexpr span<element_type, dynamic_extent>
       subspan(size_type __offset, size_type __count = dynamic_extent) const noexcept
    {
        _LIBCPP_ASSERT(__offset <= size(), "Offset out of range in span::subspan(offset, count)");
        _LIBCPP_ASSERT(__count  <= size() || __count == dynamic_extent, "Count out of range in span::subspan(offset, count)");
        if (__count == dynamic_extent)
            return {data() + __offset, size() - __offset};
        _LIBCPP_ASSERT(__count <= size() - __offset, "Offset + count out of range in span::subspan(offset, count)");
        return {data() + __offset, __count};
    }

    _LIBCPP_INLINE_VISIBILITY constexpr size_type size()           const noexcept { return _Extent; }
    _LIBCPP_INLINE_VISIBILITY constexpr size_type size_bytes()     const noexcept { return _Extent * sizeof(element_type); }
    [[nodiscard]] _LIBCPP_INLINE_VISIBILITY constexpr bool empty() const noexcept { return _Extent == 0; }

    _LIBCPP_INLINE_VISIBILITY constexpr reference operator[](size_type __idx) const noexcept
    {
        _LIBCPP_ASSERT(__idx < size(), "span<T,N>[] index out of bounds");
        return __data[__idx];
    }

    _LIBCPP_INLINE_VISIBILITY constexpr reference front() const noexcept
    {
        _LIBCPP_ASSERT(!empty(), "span<T, N>::front() on empty span");
        return __data[0];
    }

    _LIBCPP_INLINE_VISIBILITY constexpr reference back() const noexcept
    {
        _LIBCPP_ASSERT(!empty(), "span<T, N>::back() on empty span");
        return __data[size()-1];
    }

    _LIBCPP_INLINE_VISIBILITY constexpr pointer data()                         const noexcept { return __data; }

// [span.iter], span iterator support
    _LIBCPP_INLINE_VISIBILITY constexpr iterator                 begin() const noexcept { return iterator(data()); }
    _LIBCPP_INLINE_VISIBILITY constexpr iterator                   end() const noexcept { return iterator(data() + size()); }
    _LIBCPP_INLINE_VISIBILITY constexpr reverse_iterator        rbegin() const noexcept { return reverse_iterator(end()); }
    _LIBCPP_INLINE_VISIBILITY constexpr reverse_iterator          rend() const noexcept { return reverse_iterator(begin()); }

    _LIBCPP_INLINE_VISIBILITY span<const byte, _Extent * sizeof(element_type)> __as_bytes() const noexcept
    { return span<const byte, _Extent * sizeof(element_type)>{reinterpret_cast<const byte *>(data()), size_bytes()}; }

    _LIBCPP_INLINE_VISIBILITY span<byte, _Extent * sizeof(element_type)> __as_writable_bytes() const noexcept
    { return span<byte, _Extent * sizeof(element_type)>{reinterpret_cast<byte *>(data()), size_bytes()}; }

private:
    pointer    __data;

};


template <typename _Tp>
class _LIBCPP_TEMPLATE_VIS span<_Tp, dynamic_extent> {
private:

public:
//  constants and types
    using element_type           = _Tp;
    using value_type             = remove_cv_t<_Tp>;
    using size_type              = size_t;
    using difference_type        = ptrdiff_t;
    using pointer                = _Tp *;
    using const_pointer          = const _Tp *;
    using reference              = _Tp &;
    using const_reference        = const _Tp &;
#if (_LIBCPP_DEBUG_LEVEL == 2) || defined(_LIBCPP_ABI_SPAN_POINTER_ITERATORS)
    using iterator               = pointer;
#else
    using iterator               = __wrap_iter<pointer>;
#endif
    using reverse_iterator       = _VSTD::reverse_iterator<iterator>;

    static constexpr size_type extent = dynamic_extent;

// [span.cons], span constructors, copy, assignment, and destructor
    _LIBCPP_INLINE_VISIBILITY constexpr span() noexcept : __data{nullptr}, __size{0} {}

    constexpr span           (const span&) noexcept = default;
    constexpr span& operator=(const span&) noexcept = default;

#if !defined(_LIBCPP_HAS_NO_CONCEPTS)
    template <class _It,
              enable_if_t<contiguous_iterator<_It> &&
                              is_convertible_v<remove_reference_t<iter_reference_t<_It> > (*)[], element_type (*)[]>,
                          nullptr_t> = nullptr>
    _LIBCPP_INLINE_VISIBILITY
    constexpr span(_It __first, size_type __count)
        : __data{_VSTD::to_address(__first)}, __size{__count} {}

    template <
        class _It, class _End,
        enable_if_t<is_convertible_v<remove_reference_t<iter_reference_t<_It> > (*)[], element_type (*)[]> &&
                        contiguous_iterator<_It> && sized_sentinel_for<_End, _It> && !is_convertible_v<_End, size_t>,
                    nullptr_t> = nullptr>
    _LIBCPP_INLINE_VISIBILITY
    constexpr span(_It __first, _End __last)
        : __data(_VSTD::to_address(__first)), __size(__last - __first) {}
#endif // !defined(_LIBCPP_HAS_NO_CONCEPTS)

    template <size_t _Sz>
    _LIBCPP_INLINE_VISIBILITY
    constexpr span(type_identity_t<element_type> (&__arr)[_Sz]) noexcept : __data{__arr}, __size{_Sz} {}

    template <class _OtherElementType, size_t _Sz,
              enable_if_t<is_convertible_v<_OtherElementType(*)[], element_type (*)[]>, nullptr_t> = nullptr>
    _LIBCPP_INLINE_VISIBILITY
    constexpr span(array<_OtherElementType, _Sz>& __arr) noexcept : __data{__arr.data()}, __size{_Sz} {}

    template <class _OtherElementType, size_t _Sz,
              enable_if_t<is_convertible_v<const _OtherElementType(*)[], element_type (*)[]>, nullptr_t> = nullptr>
    _LIBCPP_INLINE_VISIBILITY
    constexpr span(const array<_OtherElementType, _Sz>& __arr) noexcept : __data{__arr.data()}, __size{_Sz} {}

#if defined(_LIBCPP_HAS_NO_CONCEPTS) || defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
    template <class _Container, class = enable_if_t<
        __span_compatible_range<_Container, element_type>::value
    >>
    _LIBCPP_INLINE_VISIBILITY
    constexpr span(_Container& __c) : __data(std::data(__c)), __size{std::size(__c)} {}
    template <class _Container, class = enable_if_t<
        __span_compatible_range<const _Container, element_type>::value
    >>
    _LIBCPP_INLINE_VISIBILITY
    constexpr span(const _Container& __c) : __data(std::data(__c)), __size{std::size(__c)} {}
#else
    template <__span_compatible_range<element_type> _Range>
    _LIBCPP_INLINE_VISIBILITY
    constexpr span(_Range&& __r) : __data(ranges::data(__r)), __size{ranges::size(__r)} {}
#endif // !defined(_LIBCPP_HAS_NO_CONCEPTS) && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)

    template <class _OtherElementType, size_t _OtherExtent>
    _LIBCPP_INLINE_VISIBILITY
        constexpr span(const span<_OtherElementType, _OtherExtent>& __other,
                       enable_if_t<
                          is_convertible_v<_OtherElementType(*)[], element_type (*)[]>,
                          nullptr_t> = nullptr) noexcept
        : __data{__other.data()}, __size{__other.size()} {}

//    ~span() noexcept = default;

    template <size_t _Count>
    _LIBCPP_INLINE_VISIBILITY
    constexpr span<element_type, _Count> first() const noexcept
    {
        _LIBCPP_ASSERT(_Count <= size(), "Count out of range in span::first()");
        return span<element_type, _Count>{data(), _Count};
    }

    template <size_t _Count>
    _LIBCPP_INLINE_VISIBILITY
    constexpr span<element_type, _Count> last() const noexcept
    {
        _LIBCPP_ASSERT(_Count <= size(), "Count out of range in span::last()");
        return span<element_type, _Count>{data() + size() - _Count, _Count};
    }

    _LIBCPP_INLINE_VISIBILITY
    constexpr span<element_type, dynamic_extent> first(size_type __count) const noexcept
    {
        _LIBCPP_ASSERT(__count <= size(), "Count out of range in span::first(count)");
        return {data(), __count};
    }

    _LIBCPP_INLINE_VISIBILITY
    constexpr span<element_type, dynamic_extent> last (size_type __count) const noexcept
    {
        _LIBCPP_ASSERT(__count <= size(), "Count out of range in span::last(count)");
        return {data() + size() - __count, __count};
    }

    template <size_t _Offset, size_t _Count = dynamic_extent>
    _LIBCPP_INLINE_VISIBILITY
    constexpr span<element_type, _Count> subspan() const noexcept
    {
        _LIBCPP_ASSERT(_Offset <= size(), "Offset out of range in span::subspan()");
        _LIBCPP_ASSERT(_Count == dynamic_extent || _Count <= size() - _Offset, "Offset + count out of range in span::subspan()");
        return span<element_type, _Count>{data() + _Offset, _Count == dynamic_extent ? size() - _Offset : _Count};
    }

    constexpr span<element_type, dynamic_extent>
    _LIBCPP_INLINE_VISIBILITY
    subspan(size_type __offset, size_type __count = dynamic_extent) const noexcept
    {
        _LIBCPP_ASSERT(__offset <= size(), "Offset out of range in span::subspan(offset, count)");
        _LIBCPP_ASSERT(__count  <= size() || __count == dynamic_extent, "count out of range in span::subspan(offset, count)");
        if (__count == dynamic_extent)
            return {data() + __offset, size() - __offset};
        _LIBCPP_ASSERT(__count <= size() - __offset, "Offset + count out of range in span::subspan(offset, count)");
        return {data() + __offset, __count};
    }

    _LIBCPP_INLINE_VISIBILITY constexpr size_type size()           const noexcept { return __size; }
    _LIBCPP_INLINE_VISIBILITY constexpr size_type size_bytes()     const noexcept { return __size * sizeof(element_type); }
    [[nodiscard]] _LIBCPP_INLINE_VISIBILITY constexpr bool empty() const noexcept { return __size == 0; }

    _LIBCPP_INLINE_VISIBILITY constexpr reference operator[](size_type __idx) const noexcept
    {
        _LIBCPP_ASSERT(__idx < size(), "span<T>[] index out of bounds");
        return __data[__idx];
    }

    _LIBCPP_INLINE_VISIBILITY constexpr reference front() const noexcept
    {
        _LIBCPP_ASSERT(!empty(), "span<T>[].front() on empty span");
        return __data[0];
    }

    _LIBCPP_INLINE_VISIBILITY constexpr reference back() const noexcept
    {
        _LIBCPP_ASSERT(!empty(), "span<T>[].back() on empty span");
        return __data[size()-1];
    }


    _LIBCPP_INLINE_VISIBILITY constexpr pointer data()                         const noexcept { return __data; }

// [span.iter], span iterator support
    _LIBCPP_INLINE_VISIBILITY constexpr iterator                 begin() const noexcept { return iterator(data()); }
    _LIBCPP_INLINE_VISIBILITY constexpr iterator                   end() const noexcept { return iterator(data() + size()); }
    _LIBCPP_INLINE_VISIBILITY constexpr reverse_iterator        rbegin() const noexcept { return reverse_iterator(end()); }
    _LIBCPP_INLINE_VISIBILITY constexpr reverse_iterator          rend() const noexcept { return reverse_iterator(begin()); }

    _LIBCPP_INLINE_VISIBILITY span<const byte, dynamic_extent> __as_bytes() const noexcept
    { return {reinterpret_cast<const byte *>(data()), size_bytes()}; }

    _LIBCPP_INLINE_VISIBILITY span<byte, dynamic_extent> __as_writable_bytes() const noexcept
    { return {reinterpret_cast<byte *>(data()), size_bytes()}; }

private:
    pointer   __data;
    size_type __size;
};

#if !defined(_LIBCPP_HAS_NO_CONCEPTS)
template <class _Tp, size_t _Extent>
inline constexpr bool ranges::enable_borrowed_range<span<_Tp, _Extent> > = true;

template <class _ElementType, size_t _Extent>
inline constexpr bool ranges::enable_view<span<_ElementType, _Extent>> = true;
#endif // !defined(_LIBCPP_HAS_NO_CONCEPTS)

//  as_bytes & as_writable_bytes
template <class _Tp, size_t _Extent>
_LIBCPP_INLINE_VISIBILITY
auto as_bytes(span<_Tp, _Extent> __s) noexcept
-> decltype(__s.__as_bytes())
{ return    __s.__as_bytes(); }

template <class _Tp, size_t _Extent>
_LIBCPP_INLINE_VISIBILITY
auto as_writable_bytes(span<_Tp, _Extent> __s) noexcept
-> enable_if_t<!is_const_v<_Tp>, decltype(__s.__as_writable_bytes())>
{ return __s.__as_writable_bytes(); }

#if !defined(_LIBCPP_HAS_NO_CONCEPTS)
template<contiguous_iterator _It, class _EndOrSize>
    span(_It, _EndOrSize) -> span<remove_reference_t<iter_reference_t<_It>>>;
#endif // !defined(_LIBCPP_HAS_NO_CONCEPTS)

template<class _Tp, size_t _Sz>
    span(_Tp (&)[_Sz]) -> span<_Tp, _Sz>;

template<class _Tp, size_t _Sz>
    span(array<_Tp, _Sz>&) -> span<_Tp, _Sz>;

template<class _Tp, size_t _Sz>
    span(const array<_Tp, _Sz>&) -> span<const _Tp, _Sz>;

#if defined(_LIBCPP_HAS_NO_CONCEPTS) || defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
template<class _Container>
    span(_Container&) -> span<typename _Container::value_type>;

template<class _Container>
    span(const _Container&) -> span<const typename _Container::value_type>;
#else
template<ranges::contiguous_range _Range>
    span(_Range&&) -> span<remove_reference_t<ranges::range_reference_t<_Range>>>;
#endif

#endif // _LIBCPP_STD_VER > 17

_LIBCPP_END_NAMESPACE_STD

_LIBCPP_POP_MACROS

#endif // _LIBCPP_SPAN