///////////////////////////////////////////////////////////////////////////////
//
// Copyright (c) 2015 Microsoft Corporation. All rights reserved.
//
// This code is licensed under the MIT License (MIT).
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.
//
///////////////////////////////////////////////////////////////////////////////

#ifndef GSL_SPAN_H
#define GSL_SPAN_H

#include <xdiag/extern/gsl/assert> // for Expects
#include <xdiag/extern/gsl/byte>   // for byte
#include <xdiag/extern/gsl/util>   // for narrow_cast

#include <array>    // for array
#include <cstddef>  // for ptrdiff_t, size_t, nullptr_t
#include <iterator> // for reverse_iterator, distance, random_access_...
#include <xdiag/extern/gsl/span_ext> // for span specialization of gsl::at and other span-related extensions
#include <type_traits> // for enable_if_t, declval, is_convertible, inte...

#if defined(_MSC_VER) && !defined(__clang__)
#pragma warning(push)

// turn off some warnings that are noisy about our Expects statements
#pragma warning(disable : 4127) // conditional expression is constant
#pragma warning(disable : 4146) // unary minus operator applied to unsigned
                                // type, result still unsigned
#pragma warning(disable : 4702) // unreachable code

// Turn MSVC /analyze rules that generate too much noise. TODO: fix in the tool.
#pragma warning(                                                               \
    disable : 26495) // uninitalized member when constructor calls constructor
#pragma warning(                                                               \
    disable : 26446) // parser bug does not allow attributes on some templates

#endif // _MSC_VER

// See if we have enough C++17 power to use a static constexpr data member
// without needing an out-of-line definition
#if !(defined(__cplusplus) && (__cplusplus >= 201703L))
#define GSL_USE_STATIC_CONSTEXPR_WORKAROUND
#endif // !(defined(__cplusplus) && (__cplusplus >= 201703L))

// GCC 7 does not like the signed unsigned missmatch (size_t ptrdiff_t)
// While there is a conversion from signed to unsigned, it happens at
// compiletime, so the compiler wouldn't have to warn indiscriminately, but
// could check if the source value actually doesn't fit into the target type
// and only warn in those cases.
#if defined(__GNUC__) && __GNUC__ > 6
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wsign-conversion"
#endif

namespace gsl {

// implementation details
namespace details {
template <class T> struct is_span_oracle : std::false_type {};

template <class ElementType, std::size_t Extent>
struct is_span_oracle<gsl::span<ElementType, Extent>> : std::true_type {};

template <class T>
struct is_span : public is_span_oracle<std::remove_cv_t<T>> {};

template <class T> struct is_std_array_oracle : std::false_type {};

template <class ElementType, std::size_t Extent>
struct is_std_array_oracle<std::array<ElementType, Extent>> : std::true_type {};

template <class T>
struct is_std_array : is_std_array_oracle<std::remove_cv_t<T>> {};

template <std::size_t From, std::size_t To>
struct is_allowed_extent_conversion
    : std::integral_constant<bool, From == To || To == dynamic_extent> {};

template <class From, class To>
struct is_allowed_element_type_conversion
    : std::integral_constant<bool,
                             std::is_convertible<From (*)[], To (*)[]>::value> {
};

template <class Type> class span_iterator {
public:
  using iterator_category = std::random_access_iterator_tag;
  using value_type = std::remove_cv_t<Type>;
  using difference_type = std::ptrdiff_t;
  using pointer = Type *;
  using reference = Type &;

#ifdef _MSC_VER
  using _Unchecked_type = pointer;
#endif // _MSC_VER
  constexpr span_iterator() = default;

  constexpr span_iterator(pointer begin, pointer end, pointer current)
      : begin_(begin), end_(end), current_(current) {}

  constexpr operator span_iterator<const Type>() const noexcept {
    return {begin_, end_, current_};
  }

  constexpr reference operator*() const noexcept {
    Expects(begin_ && end_);
    Expects(begin_ <= current_ && current_ < end_);
    return *current_;
  }

  constexpr pointer operator->() const noexcept {
    Expects(begin_ && end_);
    Expects(begin_ <= current_ && current_ < end_);
    return current_;
  }
  constexpr span_iterator &operator++() noexcept {
    Expects(begin_ && current_ && end_);
    Expects(current_ < end_);
    // clang-format off
            GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
    // clang-format on
    ++current_;
    return *this;
  }

  constexpr span_iterator operator++(int) noexcept {
    span_iterator ret = *this;
    ++*this;
    return ret;
  }

  constexpr span_iterator &operator--() noexcept {
    Expects(begin_ && end_);
    Expects(begin_ < current_);
    --current_;
    return *this;
  }

  constexpr span_iterator operator--(int) noexcept {
    span_iterator ret = *this;
    --*this;
    return ret;
  }

  constexpr span_iterator &operator+=(const difference_type n) noexcept {
    if (n != 0)
      Expects(begin_ && current_ && end_);
    if (n > 0)
      Expects(end_ - current_ >= n);
    if (n < 0)
      Expects(current_ - begin_ >= -n);
    // clang-format off
            GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
    // clang-format on
    current_ += n;
    return *this;
  }

  constexpr span_iterator operator+(const difference_type n) const noexcept {
    span_iterator ret = *this;
    ret += n;
    return ret;
  }

  friend constexpr span_iterator operator+(const difference_type n,
                                           const span_iterator &rhs) noexcept {
    return rhs + n;
  }

  constexpr span_iterator &operator-=(const difference_type n) noexcept {
    if (n != 0)
      Expects(begin_ && current_ && end_);
    if (n > 0)
      Expects(current_ - begin_ >= n);
    if (n < 0)
      Expects(end_ - current_ >= -n);
    current_ -= n;
    return *this;
  }

  constexpr span_iterator operator-(const difference_type n) const noexcept {
    span_iterator ret = *this;
    ret -= n;
    return ret;
  }

  template <
      class Type2,
      std::enable_if_t<std::is_same<std::remove_cv_t<Type2>, value_type>::value,
                       int> = 0>
  constexpr difference_type
  operator-(const span_iterator<Type2> &rhs) const noexcept {
    Expects(begin_ == rhs.begin_ && end_ == rhs.end_);
    return current_ - rhs.current_;
  }

  constexpr reference operator[](const difference_type n) const noexcept {
    return *(*this + n);
  }

  template <
      class Type2,
      std::enable_if_t<std::is_same<std::remove_cv_t<Type2>, value_type>::value,
                       int> = 0>
  constexpr bool operator==(const span_iterator<Type2> &rhs) const noexcept {
    Expects(begin_ == rhs.begin_ && end_ == rhs.end_);
    return current_ == rhs.current_;
  }

  template <
      class Type2,
      std::enable_if_t<std::is_same<std::remove_cv_t<Type2>, value_type>::value,
                       int> = 0>
  constexpr bool operator!=(const span_iterator<Type2> &rhs) const noexcept {
    return !(*this == rhs);
  }

  template <
      class Type2,
      std::enable_if_t<std::is_same<std::remove_cv_t<Type2>, value_type>::value,
                       int> = 0>
  constexpr bool operator<(const span_iterator<Type2> &rhs) const noexcept {
    Expects(begin_ == rhs.begin_ && end_ == rhs.end_);
    return current_ < rhs.current_;
  }

  template <
      class Type2,
      std::enable_if_t<std::is_same<std::remove_cv_t<Type2>, value_type>::value,
                       int> = 0>
  constexpr bool operator>(const span_iterator<Type2> &rhs) const noexcept {
    return rhs < *this;
  }

  template <
      class Type2,
      std::enable_if_t<std::is_same<std::remove_cv_t<Type2>, value_type>::value,
                       int> = 0>
  constexpr bool operator<=(const span_iterator<Type2> &rhs) const noexcept {
    return !(rhs < *this);
  }

  template <
      class Type2,
      std::enable_if_t<std::is_same<std::remove_cv_t<Type2>, value_type>::value,
                       int> = 0>
  constexpr bool operator>=(const span_iterator<Type2> &rhs) const noexcept {
    return !(*this < rhs);
  }

#ifdef _MSC_VER
  // MSVC++ iterator debugging support; allows STL algorithms in 15.8+
  // to unwrap span_iterator to a pointer type after a range check in STL
  // algorithm calls
  friend constexpr void _Verify_range(
      span_iterator lhs,
      span_iterator rhs) noexcept {  // test that [lhs, rhs) forms a valid range
                                     // inside an STL algorithm
    Expects(lhs.begin_ == rhs.begin_ // range spans have to match
            && lhs.end_ == rhs.end_ &&
            lhs.current_ <= rhs.current_); // range must not be transposed
  }

  constexpr void _Verify_offset(const difference_type n)
      const noexcept { // test that *this + n is within the range of this call
    if (n != 0)
      Expects(begin_ && current_ && end_);
    if (n > 0)
      Expects(end_ - current_ >= n);
    if (n < 0)
      Expects(current_ - begin_ >= -n);
  }

  // clang-format off
        GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
  // clang-format on
  constexpr pointer
  _Unwrapped() const noexcept { // after seeking *this to a high water mark, or
                                // using one of the
    // _Verify_xxx functions above, unwrap this span_iterator to a raw
    // pointer
    return current_;
  }

  // Tell the STL that span_iterator should not be unwrapped if it can't
  // validate in advance, even in release / optimized builds:
#if defined(GSL_USE_STATIC_CONSTEXPR_WORKAROUND)
  static constexpr const bool _Unwrap_when_unverified = false;
#else
  static constexpr bool _Unwrap_when_unverified = false;
#endif
  // clang-format off
        GSL_SUPPRESS(con.3) // NO-FORMAT: attribute // TODO: false positive
  // clang-format on
  constexpr void
  _Seek_to(const pointer p) noexcept { // adjust the position of *this to
                                       // previously verified location p
    // after _Unwrapped
    current_ = p;
  }
#endif

  pointer begin_ = nullptr;
  pointer end_ = nullptr;
  pointer current_ = nullptr;
};

template <std::size_t Ext> class extent_type {
public:
  using size_type = std::size_t;

  constexpr extent_type() noexcept = default;

  constexpr explicit extent_type(extent_type<dynamic_extent>);

  constexpr explicit extent_type(size_type size) { Expects(size == Ext); }

  constexpr size_type size() const noexcept { return Ext; }

private:
#if defined(GSL_USE_STATIC_CONSTEXPR_WORKAROUND)
  static constexpr const size_type size_ = Ext; // static size equal to Ext
#else
  static constexpr size_type size_ = Ext; // static size equal to Ext
#endif
};

template <> class extent_type<dynamic_extent> {
public:
  using size_type = std::size_t;

  template <size_type Other>
  constexpr explicit extent_type(extent_type<Other> ext) : size_(ext.size()) {}

  constexpr explicit extent_type(size_type size) : size_(size) {
    Expects(size != dynamic_extent);
  }

  constexpr size_type size() const noexcept { return size_; }

private:
  size_type size_;
};

template <std::size_t Ext>
constexpr extent_type<Ext>::extent_type(extent_type<dynamic_extent> ext) {
  Expects(ext.size() == Ext);
}

template <class ElementType, std::size_t Extent, std::size_t Offset,
          std::size_t Count>
struct calculate_subspan_type {
  using type =
      span<ElementType,
           Count != dynamic_extent
               ? Count
               : (Extent != dynamic_extent ? Extent - Offset : Extent)>;
};
} // namespace details

// [span], class template span
template <class ElementType, std::size_t Extent> class span {
public:
  // constants and types
  using element_type = ElementType;
  using value_type = std::remove_cv_t<ElementType>;
  using size_type = std::size_t;
  using pointer = element_type *;
  using const_pointer = const element_type *;
  using reference = element_type &;
  using const_reference = const element_type &;
  using difference_type = std::ptrdiff_t;

  using iterator = details::span_iterator<ElementType>;
  using reverse_iterator = std::reverse_iterator<iterator>;

#if defined(GSL_USE_STATIC_CONSTEXPR_WORKAROUND)
  static constexpr const size_type extent{Extent};
#else
  static constexpr size_type extent{Extent};
#endif

  // [span.cons], span constructors, copy, assignment, and destructor
  template <bool Dependent = false,
            // "Dependent" is needed to make "std::enable_if_t<Dependent ||
            // Extent == 0 || Extent
            // == dynamic_extent>" SFINAE, since "std::enable_if_t<Extent == 0
            // || Extent == dynamic_extent>" is ill-formed when Extent is
            // greater than 0.
            class = std::enable_if_t<
                (Dependent ||
                 details::is_allowed_extent_conversion<0, Extent>::value)>>
  constexpr span() noexcept : storage_(nullptr, details::extent_type<0>()) {}

  template <std::size_t MyExtent = Extent,
            std::enable_if_t<MyExtent != dynamic_extent, int> = 0>
  constexpr explicit span(pointer ptr, size_type count) noexcept
      : storage_(ptr, count) {
    Expects(count == Extent);
  }

  template <std::size_t MyExtent = Extent,
            std::enable_if_t<MyExtent == dynamic_extent, int> = 0>
  constexpr span(pointer ptr, size_type count) noexcept
      : storage_(ptr, count) {}

  template <std::size_t MyExtent = Extent,
            std::enable_if_t<MyExtent != dynamic_extent, int> = 0>
  constexpr explicit span(pointer firstElem, pointer lastElem) noexcept
      : storage_(firstElem, narrow_cast<std::size_t>(lastElem - firstElem)) {
    Expects(lastElem - firstElem == static_cast<difference_type>(Extent));
  }

  template <std::size_t MyExtent = Extent,
            std::enable_if_t<MyExtent == dynamic_extent, int> = 0>
  constexpr span(pointer firstElem, pointer lastElem) noexcept
      : storage_(firstElem, narrow_cast<std::size_t>(lastElem - firstElem)) {}

  template <
      std::size_t N,
      std::enable_if_t<details::is_allowed_extent_conversion<N, Extent>::value,
                       int> = 0>
  constexpr span(element_type (&arr)[N]) noexcept
      : storage_(KnownNotNull{arr}, details::extent_type<N>()) {}

  template <
      class T, std::size_t N,
      std::enable_if_t<
          (details::is_allowed_extent_conversion<N, Extent>::value &&
           details::is_allowed_element_type_conversion<T, element_type>::value),
          int> = 0>
  constexpr span(std::array<T, N> &arr) noexcept
      : storage_(KnownNotNull{arr.data()}, details::extent_type<N>()) {}

  template <class T, std::size_t N,
            std::enable_if_t<
                (details::is_allowed_extent_conversion<N, Extent>::value &&
                 details::is_allowed_element_type_conversion<
                     const T, element_type>::value),
                int> = 0>
  constexpr span(const std::array<T, N> &arr) noexcept
      : storage_(KnownNotNull{arr.data()}, details::extent_type<N>()) {}

  // NB: the SFINAE on these constructors uses .data() as an
  // incomplete/imperfect proxy for the requirement on Container to be a
  // contiguous sequence container.
  template <
      std::size_t MyExtent = Extent, class Container,
      std::enable_if_t<MyExtent != dynamic_extent &&
                           !details::is_span<Container>::value &&
                           !details::is_std_array<Container>::value &&
                           std::is_pointer<decltype(
                               std::declval<Container &>().data())>::value &&
                           std::is_convertible<
                               std::remove_pointer_t<decltype(
                                   std::declval<Container &>().data())> (*)[],
                               element_type (*)[]>::value,
                       int> = 0>
  constexpr explicit span(Container &cont) noexcept
      : span(cont.data(), cont.size()) {}

  template <
      std::size_t MyExtent = Extent, class Container,
      std::enable_if_t<MyExtent == dynamic_extent &&
                           !details::is_span<Container>::value &&
                           !details::is_std_array<Container>::value &&
                           std::is_pointer<decltype(
                               std::declval<Container &>().data())>::value &&
                           std::is_convertible<
                               std::remove_pointer_t<decltype(
                                   std::declval<Container &>().data())> (*)[],
                               element_type (*)[]>::value,
                       int> = 0>
  constexpr span(Container &cont) noexcept : span(cont.data(), cont.size()) {}

  template <
      std::size_t MyExtent = Extent, class Container,
      std::enable_if_t<
          MyExtent != dynamic_extent && std::is_const<element_type>::value &&
              !details::is_span<Container>::value &&
              !details::is_std_array<Container>::value &&
              std::is_pointer<
                  decltype(std::declval<const Container &>().data())>::value &&
              std::is_convertible<
                  std::remove_pointer_t<
                      decltype(std::declval<const Container &>().data())> (*)[],
                  element_type (*)[]>::value,
          int> = 0>
  constexpr explicit span(const Container &cont) noexcept
      : span(cont.data(), cont.size()) {}

  template <
      std::size_t MyExtent = Extent, class Container,
      std::enable_if_t<
          MyExtent == dynamic_extent && std::is_const<element_type>::value &&
              !details::is_span<Container>::value &&
              !details::is_std_array<Container>::value &&
              std::is_pointer<
                  decltype(std::declval<const Container &>().data())>::value &&
              std::is_convertible<
                  std::remove_pointer_t<
                      decltype(std::declval<const Container &>().data())> (*)[],
                  element_type (*)[]>::value,
          int> = 0>
  constexpr span(const Container &cont) noexcept
      : span(cont.data(), cont.size()) {}

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

  template <class OtherElementType, std::size_t OtherExtent,
            std::size_t MyExtent = Extent,
            std::enable_if_t<(MyExtent == dynamic_extent ||
                              MyExtent == OtherExtent) &&
                                 details::is_allowed_element_type_conversion<
                                     OtherElementType, element_type>::value,
                             int> = 0>
  constexpr span(const span<OtherElementType, OtherExtent> &other) noexcept
      : storage_(other.data(),
                 details::extent_type<OtherExtent>(other.size())) {}

  template <class OtherElementType, std::size_t OtherExtent,
            std::size_t MyExtent = Extent,
            std::enable_if_t<MyExtent != dynamic_extent &&
                                 OtherExtent == dynamic_extent &&
                                 details::is_allowed_element_type_conversion<
                                     OtherElementType, element_type>::value,
                             int> = 0>
  constexpr explicit span(
      const span<OtherElementType, OtherExtent> &other) noexcept
      : storage_(other.data(),
                 details::extent_type<OtherExtent>(other.size())) {}

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

  // [span.sub], span subviews
  template <std::size_t Count>
  constexpr span<element_type, Count> first() const noexcept {
    Expects(Count <= size());
    return span<element_type, Count>{data(), Count};
  }

  template <std::size_t Count>
  // clang-format off
    GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
                          // clang-format on
      constexpr span<element_type, Count> last() const noexcept {
    Expects(Count <= size());
    return span<element_type, Count>{data() + (size() - Count), Count};
  }

  template <std::size_t Offset, std::size_t Count = dynamic_extent>
  // clang-format off
    GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
                          // clang-format on
      constexpr auto subspan() const noexcept ->
      typename details::calculate_subspan_type<ElementType, Extent, Offset,
                                               Count>::type {
    Expects((size() >= Offset) &&
            (Count == dynamic_extent || (Count <= size() - Offset)));
    using type = typename details::calculate_subspan_type<ElementType, Extent,
                                                          Offset, Count>::type;
    return type{data() + Offset,
                Count == dynamic_extent ? size() - Offset : Count};
  }

  constexpr span<element_type, dynamic_extent>
  first(size_type count) const noexcept {
    Expects(count <= size());
    return {data(), count};
  }

  constexpr span<element_type, dynamic_extent>
  last(size_type count) const noexcept {
    Expects(count <= size());
    return make_subspan(size() - count, dynamic_extent,
                        subspan_selector<Extent>{});
  }

  constexpr span<element_type, dynamic_extent>
  subspan(size_type offset, size_type count = dynamic_extent) const noexcept {
    return make_subspan(offset, count, subspan_selector<Extent>{});
  }

  // [span.obs], span observers
  constexpr size_type size() const noexcept { return storage_.size(); }

  constexpr size_type size_bytes() const noexcept {
    Expects(size() < dynamic_extent / sizeof(element_type));
    return size() * sizeof(element_type);
  }

  constexpr bool empty() const noexcept { return size() == 0; }

  // [span.elem], span element access
  // clang-format off
    GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
  // clang-format on
  constexpr reference operator[](size_type idx) const noexcept {
    Expects(idx < size());
    return data()[idx];
  }

  constexpr reference front() const noexcept {
    Expects(size() > 0);
    return data()[0];
  }

  constexpr reference back() const noexcept {
    Expects(size() > 0);
    return data()[size() - 1];
  }

  constexpr pointer data() const noexcept { return storage_.data(); }

  // [span.iter], span iterator support
  constexpr iterator begin() const noexcept {
    const auto data = storage_.data();
    // clang-format off
        GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
    // clang-format on
    return {data, data + size(), data};
  }

  constexpr iterator end() const noexcept {
    const auto data = storage_.data();
    // clang-format off
        GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
    // clang-format on
    const auto endData = data + storage_.size();
    return {data, endData, endData};
  }

  constexpr reverse_iterator rbegin() const noexcept {
    return reverse_iterator{end()};
  }
  constexpr reverse_iterator rend() const noexcept {
    return reverse_iterator{begin()};
  }

#ifdef _MSC_VER
  // Tell MSVC how to unwrap spans in range-based-for
  constexpr pointer _Unchecked_begin() const noexcept { return data(); }
  constexpr pointer _Unchecked_end() const noexcept {
    // clang-format off
        GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
    // clang-format on
    return data() + size();
  }
#endif // _MSC_VER

private:
  // Needed to remove unnecessary null check in subspans
  struct KnownNotNull {
    pointer p;
  };

  // this implementation detail class lets us take advantage of the
  // empty base class optimization to pay for only storage of a single
  // pointer in the case of fixed-size spans
  template <class ExtentType> class storage_type : public ExtentType {
  public:
    // KnownNotNull parameter is needed to remove unnecessary null check
    // in subspans and constructors from arrays
    template <class OtherExtentType>
    constexpr storage_type(KnownNotNull data, OtherExtentType ext)
        : ExtentType(ext), data_(data.p) {
      Expects(ExtentType::size() != dynamic_extent);
    }

    template <class OtherExtentType>
    constexpr storage_type(pointer data, OtherExtentType ext)
        : ExtentType(ext), data_(data) {
      Expects(ExtentType::size() != dynamic_extent);
      Expects(data || ExtentType::size() == 0);
    }

    constexpr pointer data() const noexcept { return data_; }

  private:
    pointer data_;
  };

  storage_type<details::extent_type<Extent>> storage_;

  // The rest is needed to remove unnecessary null check
  // in subspans and constructors from arrays
  constexpr span(KnownNotNull ptr, size_type count) noexcept
      : storage_(ptr, count) {}

  template <std::size_t CallerExtent> class subspan_selector {};

  template <std::size_t CallerExtent>
  constexpr span<element_type, dynamic_extent>
  make_subspan(size_type offset, size_type count,
               subspan_selector<CallerExtent>) const noexcept {
    const span<element_type, dynamic_extent> tmp(*this);
    return tmp.subspan(offset, count);
  }

  // clang-format off
    GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
  // clang-format on
  constexpr span<element_type, dynamic_extent>
  make_subspan(size_type offset, size_type count,
               subspan_selector<dynamic_extent>) const noexcept {
    Expects(size() >= offset);

    if (count == dynamic_extent) {
      return {KnownNotNull{data() + offset}, size() - offset};
    }

    Expects(size() - offset >= count);
    return {KnownNotNull{data() + offset}, count};
  }
};

#if (defined(__cpp_deduction_guides) && (__cpp_deduction_guides >= 201611L))

// Deduction Guides
template <class Type, std::size_t Extent>
span(Type (&)[Extent]) -> span<Type, Extent>;

template <class Type, std::size_t Size>
span(std::array<Type, Size> &) -> span<Type, Size>;

template <class Type, std::size_t Size>
span(const std::array<Type, Size> &) -> span<const Type, Size>;

template <class Container, class Element = std::remove_pointer_t<
                               decltype(std::declval<Container &>().data())>>
span(Container &) -> span<Element>;

template <class Container, class Element = std::remove_pointer_t<decltype(
                               std::declval<const Container &>().data())>>
span(const Container &) -> span<Element>;

#endif // ( defined(__cpp_deduction_guides) && (__cpp_deduction_guides >=
       // 201611L) )

#if defined(GSL_USE_STATIC_CONSTEXPR_WORKAROUND)
template <class ElementType, std::size_t Extent>
constexpr const typename span<ElementType, Extent>::size_type
    span<ElementType, Extent>::extent;
#endif

namespace details {
// if we only supported compilers with good constexpr support then
// this pair of classes could collapse down to a constexpr function

// we should use a narrow_cast<> to go to std::size_t, but older compilers may
// not see it as constexpr and so will fail compilation of the template
template <class ElementType, std::size_t Extent>
struct calculate_byte_size
    : std::integral_constant<std::size_t, sizeof(ElementType) * Extent> {
  static_assert(Extent < dynamic_extent / sizeof(ElementType),
                "Size is too big.");
};

template <class ElementType>
struct calculate_byte_size<ElementType, dynamic_extent>
    : std::integral_constant<std::size_t, dynamic_extent> {};
} // namespace details

// [span.objectrep], views of object representation
template <class ElementType, std::size_t Extent>
span<const byte, details::calculate_byte_size<ElementType, Extent>::value>
as_bytes(span<ElementType, Extent> s) noexcept {
  using type = span<const byte,
                    details::calculate_byte_size<ElementType, Extent>::value>;

  // clang-format off
    GSL_SUPPRESS(type.1) // NO-FORMAT: attribute
  // clang-format on
  return type{reinterpret_cast<const byte *>(s.data()), s.size_bytes()};
}

template <class ElementType, std::size_t Extent,
          std::enable_if_t<!std::is_const<ElementType>::value, int> = 0>
span<byte, details::calculate_byte_size<ElementType, Extent>::value>
as_writable_bytes(span<ElementType, Extent> s) noexcept {
  using type =
      span<byte, details::calculate_byte_size<ElementType, Extent>::value>;

  // clang-format off
    GSL_SUPPRESS(type.1) // NO-FORMAT: attribute
  // clang-format on
  return type{reinterpret_cast<byte *>(s.data()), s.size_bytes()};
}

} // namespace gsl

#if defined(_MSC_VER) && !defined(__clang__)

#pragma warning(pop)
#endif // _MSC_VER

#if defined(__GNUC__) && __GNUC__ > 6
#pragma GCC diagnostic pop
#endif // __GNUC__ > 6

#endif // GSL_SPAN_H
