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 //
 // Copyright 2017 The Abseil Authors.
 //
 // 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
 //
 // 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.
 //
 // -----------------------------------------------------------------------------
 // span.h
 // -----------------------------------------------------------------------------
 //
 // This header file defines a `Span<T>` type for holding a reference to existing
 // array data. The `Span` object, much like the `absl::string_view` object,
 // does not own such data itself, and the data being referenced by the span must
 // outlive the span itself. Unlike `view` type references, a span can hold a
 // reference to mutable data (and can mutate it for underlying types of
 // non-const T.) A span provides a lightweight way to pass a reference to such
 // data.
 //
 // Additionally, this header file defines `MakeSpan()` and `MakeConstSpan()`
 // factory functions, for clearly creating spans of type `Span<T>` or read-only
 // `Span<const T>` when such types may be difficult to identify due to issues
 // with implicit conversion.
 //
 // The C++20 draft standard includes a `std::span` type. As of June 2020, the
 // differences between `absl::Span` and `std::span` are:
 //    * `absl::Span` has `operator==` (which is likely a design bug,
 //       per https://abseil.io/blog/20180531-regular-types)
 //    * `absl::Span` has the factory functions `MakeSpan()` and
 //      `MakeConstSpan()`
 //    * bounds-checked access to `absl::Span` is accomplished with `at()`
 //    * `absl::Span` has compiler-provided move and copy constructors and
 //      assignment. This is due to them being specified as `constexpr`, but that
 //      implies const in C++11.
 //    * A read-only `absl::Span<const T>` can be implicitly constructed from an
 //      initializer list.
 //    * `absl::Span` has no `bytes()`, `size_bytes()`, `as_bytes()`, or
 //      `as_writable_bytes()` methods
 //    * `absl::Span` has no static extent template parameter, nor constructors
 //      which exist only because of the static extent parameter.
 //    * `absl::Span` has an explicit mutable-reference constructor
 //
 // For more information, see the class comments below.
 #ifndef ABSL_TYPES_SPAN_H_
 #define ABSL_TYPES_SPAN_H_
 
 #include <algorithm>
 #include <cassert>
 #include <cstddef>
 #include <initializer_list>
 #include <iterator>
 #include <type_traits>
 #include <utility>
 
 #include "absl/base/attributes.h"
 #include "absl/base/internal/throw_delegate.h"
 #include "absl/base/macros.h"
 #include "absl/base/nullability.h"
 #include "absl/base/optimization.h"
 #include "absl/base/port.h"    // TODO(strel): remove this include
 #include "absl/meta/type_traits.h"
 #include "absl/types/internal/span.h"
 
 namespace absl {
 ABSL_NAMESPACE_BEGIN
 
 //------------------------------------------------------------------------------
 // Span
 //------------------------------------------------------------------------------
 //
 // A `Span` is an "array reference" type for holding a reference of contiguous
 // array data; the `Span` object does not and cannot own such data itself. A
 // span provides an easy way to provide overloads for anything operating on
 // contiguous sequences without needing to manage pointers and array lengths
 // manually.
 
 // A span is conceptually a pointer (ptr) and a length (size) into an already
 // existing array of contiguous memory; the array it represents references the
 // elements "ptr[0] .. ptr[size-1]". Passing a properly-constructed `Span`
 // instead of raw pointers avoids many issues related to index out of bounds
 // errors.
 //
 // Spans may also be constructed from containers holding contiguous sequences.
 // Such containers must supply `data()` and `size() const` methods (e.g
 // `std::vector<T>`, `absl::InlinedVector<T, N>`). All implicit conversions to
 // `absl::Span` from such containers will create spans of type `const T`;
 // spans which can mutate their values (of type `T`) must use explicit
 // constructors.
 //
 // A `Span<T>` is somewhat analogous to an `absl::string_view`, but for an array
 // of elements of type `T`, and unlike an `absl::string_view`, a span can hold a
 // reference to mutable data. A user of `Span` must ensure that the data being
 // pointed to outlives the `Span` itself.
 //
 // You can construct a `Span<T>` in several ways:
 //
 //   * Explicitly from a reference to a container type
 //   * Explicitly from a pointer and size
 //   * Implicitly from a container type (but only for spans of type `const T`)
 //   * Using the `MakeSpan()` or `MakeConstSpan()` factory functions.
 //
 // Examples:
 //
 //   // Construct a Span explicitly from a container:
 //   std::vector<int> v = {1, 2, 3, 4, 5};
 //   auto span = absl::Span<const int>(v);
 //
 //   // Construct a Span explicitly from a C-style array:
 //   int a[5] =  {1, 2, 3, 4, 5};
 //   auto span = absl::Span<const int>(a);
 //
 //   // Construct a Span implicitly from a container
 //   void MyRoutine(absl::Span<const int> a) {
 //     ...
 //   }
 //   std::vector v = {1,2,3,4,5};
 //   MyRoutine(v)                     // convert to Span<const T>
 //
 // Note that `Span` objects, in addition to requiring that the memory they
 // point to remains alive, must also ensure that such memory does not get
 // reallocated. Therefore, to avoid undefined behavior, containers with
 // associated spans should not invoke operations that may reallocate memory
 // (such as resizing) or invalidate iterators into the container.
 //
 // One common use for a `Span` is when passing arguments to a routine that can
 // accept a variety of array types (e.g. a `std::vector`, `absl::InlinedVector`,
 // a C-style array, etc.). Instead of creating overloads for each case, you
 // can simply specify a `Span` as the argument to such a routine.
 //
 // Example:
 //
 //   void MyRoutine(absl::Span<const int> a) {
 //     ...
 //   }
 //
 //   std::vector v = {1,2,3,4,5};
 //   MyRoutine(v);
 //
 //   absl::InlinedVector<int, 4> my_inline_vector;
 //   MyRoutine(my_inline_vector);
 //
 //   // Explicit constructor from pointer,size
 //   int* my_array = new int[10];
 //   MyRoutine(absl::Span<const int>(my_array, 10));
 template <typename T>
 class ABSL_INTERNAL_ATTRIBUTE_VIEW Span {
  private:
   // Used to determine whether a Span can be constructed from a container of
   // type C.
   template <typename C>
   using EnableIfConvertibleFrom =
       typename std::enable_if<span_internal::HasData<T, C>::value &&
                               span_internal::HasSize<C>::value>::type;
 
   // Used to SFINAE-enable a function when the slice elements are const.
   template <typename U>
   using EnableIfValueIsConst =
       typename std::enable_if<std::is_const<T>::value, U>::type;
 
   // Used to SFINAE-enable a function when the slice elements are mutable.
   template <typename U>
   using EnableIfValueIsMutable =
       typename std::enable_if<!std::is_const<T>::value, U>::type;
 
  public:
   using element_type = T;
   using value_type = absl::remove_cv_t<T>;
   // TODO(b/316099902) - pointer should be Nullable<T*>, but this makes it hard
   // to recognize foreach loops as safe.
   using pointer = T*;
   using const_pointer = const T*;
   using reference = T&;
   using const_reference = const T&;
   using iterator = pointer;
   using const_iterator = const_pointer;
   using reverse_iterator = std::reverse_iterator<iterator>;
   using const_reverse_iterator = std::reverse_iterator<const_iterator>;
   using size_type = size_t;
   using difference_type = ptrdiff_t;
   using absl_internal_is_view = std::true_type;
 
   static const size_type npos = ~(size_type(0));
 
   constexpr Span() noexcept : Span(nullptr, 0) {}
   constexpr Span(pointer array, size_type length) noexcept
       : ptr_(array), len_(length) {}
 
   // Implicit conversion constructors
   template <size_t N>
   constexpr Span(T (&a)[N]) noexcept  // NOLINT(runtime/explicit)
       : Span(a, N) {}
 
   // Explicit reference constructor for a mutable `Span<T>` type. Can be
   // replaced with MakeSpan() to infer the type parameter.
   template <typename V, typename = EnableIfConvertibleFrom<V>,
             typename = EnableIfValueIsMutable<V>,
             typename = span_internal::EnableIfNotIsView<V>>
   explicit Span(
       V& v
           ABSL_ATTRIBUTE_LIFETIME_BOUND) noexcept  // NOLINT(runtime/references)
       : Span(span_internal::GetData(v), v.size()) {}
 
   // Implicit reference constructor for a read-only `Span<const T>` type
   template <typename V, typename = EnableIfConvertibleFrom<V>,
             typename = EnableIfValueIsConst<V>,
             typename = span_internal::EnableIfNotIsView<V>>
   constexpr Span(
       const V& v
           ABSL_ATTRIBUTE_LIFETIME_BOUND) noexcept  // NOLINT(runtime/explicit)
       : Span(span_internal::GetData(v), v.size()) {}
 
   // Overloads of the above two functions that are only enabled for view types.
   // This is so we can drop the ABSL_ATTRIBUTE_LIFETIME_BOUND annotation. These
   // overloads must be made unique by using a different template parameter list
   // (hence the = 0 for the IsView enabler).
   template <typename V, typename = EnableIfConvertibleFrom<V>,
             typename = EnableIfValueIsMutable<V>,
             span_internal::EnableIfIsView<V> = 0>
   explicit Span(V& v) noexcept  // NOLINT(runtime/references)
       : Span(span_internal::GetData(v), v.size()) {}
   template <typename V, typename = EnableIfConvertibleFrom<V>,
             typename = EnableIfValueIsConst<V>,
             span_internal::EnableIfIsView<V> = 0>
   constexpr Span(const V& v) noexcept  // NOLINT(runtime/explicit)
       : Span(span_internal::GetData(v), v.size()) {}
 
   // Implicit constructor from an initializer list, making it possible to pass a
   // brace-enclosed initializer list to a function expecting a `Span`. Such
   // spans constructed from an initializer list must be of type `Span<const T>`.
   //
   //   void Process(absl::Span<const int> x);
   //   Process({1, 2, 3});
   //
   // Note that as always the array referenced by the span must outlive the span.
   // Since an initializer list constructor acts as if it is fed a temporary
   // array (cf. C++ standard [dcl.init.list]/5), it's safe to use this
   // constructor only when the `std::initializer_list` itself outlives the span.
   // In order to meet this requirement it's sufficient to ensure that neither
   // the span nor a copy of it is used outside of the expression in which it's
   // created:
   //
   //   // Assume that this function uses the array directly, not retaining any
   //   // copy of the span or pointer to any of its elements.
   //   void Process(absl::Span<const int> ints);
   //
   //   // Okay: the std::initializer_list<int> will reference a temporary array
   //   // that isn't destroyed until after the call to Process returns.
   //   Process({ 17, 19 });
   //
   //   // Not okay: the storage used by the std::initializer_list<int> is not
   //   // allowed to be referenced after the first line.
   //   absl::Span<const int> ints = { 17, 19 };
   //   Process(ints);
   //
   //   // Not okay for the same reason as above: even when the elements of the
   //   // initializer list expression are not temporaries the underlying array
   //   // is, so the initializer list must still outlive the span.
   //   const int foo = 17;
   //   absl::Span<const int> ints = { foo };
   //   Process(ints);
   //
   template <typename LazyT = T,
             typename = EnableIfValueIsConst<LazyT>>
   Span(std::initializer_list<value_type> v
            ABSL_ATTRIBUTE_LIFETIME_BOUND) noexcept  // NOLINT(runtime/explicit)
       : Span(v.begin(), v.size()) {}
 
   // Accessors
 
   // Span::data()
   //
   // Returns a pointer to the span's underlying array of data (which is held
   // outside the span).
   constexpr pointer data() const noexcept { return ptr_; }
 
   // Span::size()
   //
   // Returns the size of this span.
   constexpr size_type size() const noexcept { return len_; }
 
   // Span::length()
   //
   // Returns the length (size) of this span.
   constexpr size_type length() const noexcept { return size(); }
 
   // Span::empty()
   //
   // Returns a boolean indicating whether or not this span is considered empty.
   constexpr bool empty() const noexcept { return size() == 0; }
 
   // Span::operator[]
   //
   // Returns a reference to the i'th element of this span.
   constexpr reference operator[](size_type i) const noexcept {
     return ABSL_HARDENING_ASSERT(i < size()), ptr_[i];
   }
 
   // Span::at()
   //
   // Returns a reference to the i'th element of this span.
   constexpr reference at(size_type i) const {
     return ABSL_PREDICT_TRUE(i < size())  //
                ? *(data() + i)
                : (base_internal::ThrowStdOutOfRange(
                       "Span::at failed bounds check"),
                   *(data() + i));
   }
 
   // Span::front()
   //
   // Returns a reference to the first element of this span. The span must not
   // be empty.
   constexpr reference front() const noexcept {
     return ABSL_HARDENING_ASSERT(size() > 0), *data();
   }
 
   // Span::back()
   //
   // Returns a reference to the last element of this span. The span must not
   // be empty.
   constexpr reference back() const noexcept {
     return ABSL_HARDENING_ASSERT(size() > 0), *(data() + size() - 1);
   }
 
   // Span::begin()
   //
   // Returns an iterator pointing to the first element of this span, or `end()`
   // if the span is empty.
   constexpr iterator begin() const noexcept { return data(); }
 
   // Span::cbegin()
   //
   // Returns a const iterator pointing to the first element of this span, or
   // `end()` if the span is empty.
   constexpr const_iterator cbegin() const noexcept { return begin(); }
 
   // Span::end()
   //
   // Returns an iterator pointing just beyond the last element at the
   // end of this span. This iterator acts as a placeholder; attempting to
   // access it results in undefined behavior.
   constexpr iterator end() const noexcept { return data() + size(); }
 
   // Span::cend()
   //
   // Returns a const iterator pointing just beyond the last element at the
   // end of this span. This iterator acts as a placeholder; attempting to
   // access it results in undefined behavior.
   constexpr const_iterator cend() const noexcept { return end(); }
 
   // Span::rbegin()
   //
   // Returns a reverse iterator pointing to the last element at the end of this
   // span, or `rend()` if the span is empty.
   constexpr reverse_iterator rbegin() const noexcept {
     return reverse_iterator(end());
   }
 
   // Span::crbegin()
   //
   // Returns a const reverse iterator pointing to the last element at the end of
   // this span, or `crend()` if the span is empty.
   constexpr const_reverse_iterator crbegin() const noexcept { return rbegin(); }
 
   // Span::rend()
   //
   // Returns a reverse iterator pointing just before the first element
   // at the beginning of this span. This pointer acts as a placeholder;
   // attempting to access its element results in undefined behavior.
   constexpr reverse_iterator rend() const noexcept {
     return reverse_iterator(begin());
   }
 
   // Span::crend()
   //
   // Returns a reverse const iterator pointing just before the first element
   // at the beginning of this span. This pointer acts as a placeholder;
   // attempting to access its element results in undefined behavior.
   constexpr const_reverse_iterator crend() const noexcept { return rend(); }
 
   // Span mutations
 
   // Span::remove_prefix()
   //
   // Removes the first `n` elements from the span.
   void remove_prefix(size_type n) noexcept {
     ABSL_HARDENING_ASSERT(size() >= n);
     ptr_ += n;
     len_ -= n;
   }
 
   // Span::remove_suffix()
   //
   // Removes the last `n` elements from the span.
   void remove_suffix(size_type n) noexcept {
     ABSL_HARDENING_ASSERT(size() >= n);
     len_ -= n;
   }
 
   // Span::subspan()
   //
   // Returns a `Span` starting at element `pos` and of length `len`. Both `pos`
   // and `len` are of type `size_type` and thus non-negative. Parameter `pos`
   // must be <= size(). Any `len` value that points past the end of the span
   // will be trimmed to at most size() - `pos`. A default `len` value of `npos`
   // ensures the returned subspan continues until the end of the span.
   //
   // Examples:
   //
   //   std::vector<int> vec = {10, 11, 12, 13};
   //   absl::MakeSpan(vec).subspan(1, 2);  // {11, 12}
   //   absl::MakeSpan(vec).subspan(2, 8);  // {12, 13}
   //   absl::MakeSpan(vec).subspan(1);     // {11, 12, 13}
   //   absl::MakeSpan(vec).subspan(4);     // {}
   //   absl::MakeSpan(vec).subspan(5);     // throws std::out_of_range
   constexpr Span subspan(size_type pos = 0, size_type len = npos) const {
     return (pos <= size())
                ? Span(data() + pos, (std::min)(size() - pos, len))
                : (base_internal::ThrowStdOutOfRange("pos > size()"), Span());
   }
 
   // Span::first()
   //
   // Returns a `Span` containing first `len` elements. Parameter `len` is of
   // type `size_type` and thus non-negative. `len` value must be <= size().
   //
   // Examples:
   //
   //   std::vector<int> vec = {10, 11, 12, 13};
   //   absl::MakeSpan(vec).first(1);  // {10}
   //   absl::MakeSpan(vec).first(3);  // {10, 11, 12}
   //   absl::MakeSpan(vec).first(5);  // throws std::out_of_range
   constexpr Span first(size_type len) const {
     return (len <= size())
                ? Span(data(), len)
                : (base_internal::ThrowStdOutOfRange("len > size()"), Span());
   }
 
   // Span::last()
   //
   // Returns a `Span` containing last `len` elements. Parameter `len` is of
   // type `size_type` and thus non-negative. `len` value must be <= size().
   //
   // Examples:
   //
   //   std::vector<int> vec = {10, 11, 12, 13};
   //   absl::MakeSpan(vec).last(1);  // {13}
   //   absl::MakeSpan(vec).last(3);  // {11, 12, 13}
   //   absl::MakeSpan(vec).last(5);  // throws std::out_of_range
   constexpr Span last(size_type len) const {
     return (len <= size())
                ? Span(size() - len + data(), len)
                : (base_internal::ThrowStdOutOfRange("len > size()"), Span());
   }
 
   // Support for absl::Hash.
   template <typename H>
   friend H AbslHashValue(H h, Span v) {
     return H::combine(H::combine_contiguous(std::move(h), v.data(), v.size()),
                       v.size());
   }
 
  private:
   pointer ptr_;
   size_type len_;
 };
 
 template <typename T>
 const typename Span<T>::size_type Span<T>::npos;
 
 // Span relationals
 
 // Equality is compared element-by-element, while ordering is lexicographical.
 // We provide three overloads for each operator to cover any combination on the
 // left or right hand side of mutable Span<T>, read-only Span<const T>, and
 // convertible-to-read-only Span<T>.
 // TODO(zhangxy): Due to MSVC overload resolution bug with partial ordering
 // template functions, 5 overloads per operator is needed as a workaround. We
 // should update them to 3 overloads per operator using non-deduced context like
 // string_view, i.e.
 // - (Span<T>, Span<T>)
 // - (Span<T>, non_deduced<Span<const T>>)
 // - (non_deduced<Span<const T>>, Span<T>)
 
 // operator==
 template <typename T>
 bool operator==(Span<T> a, Span<T> b) {
   return span_internal::EqualImpl<Span, const T>(a, b);
 }
 template <typename T>
 bool operator==(Span<const T> a, Span<T> b) {
   return span_internal::EqualImpl<Span, const T>(a, b);
 }
 template <typename T>
 bool operator==(Span<T> a, Span<const T> b) {
   return span_internal::EqualImpl<Span, const T>(a, b);
 }
 template <
     typename T, typename U,
     typename = span_internal::EnableIfConvertibleTo<U, absl::Span<const T>>>
 bool operator==(const U& a, Span<T> b) {
   return span_internal::EqualImpl<Span, const T>(a, b);
 }
 template <
     typename T, typename U,
     typename = span_internal::EnableIfConvertibleTo<U, absl::Span<const T>>>
 bool operator==(Span<T> a, const U& b) {
   return span_internal::EqualImpl<Span, const T>(a, b);
 }
 
 // operator!=
 template <typename T>
 bool operator!=(Span<T> a, Span<T> b) {
   return !(a == b);
 }
 template <typename T>
 bool operator!=(Span<const T> a, Span<T> b) {
   return !(a == b);
 }
 template <typename T>
 bool operator!=(Span<T> a, Span<const T> b) {
   return !(a == b);
 }
 template <
     typename T, typename U,
     typename = span_internal::EnableIfConvertibleTo<U, absl::Span<const T>>>
 bool operator!=(const U& a, Span<T> b) {
   return !(a == b);
 }
 template <
     typename T, typename U,
     typename = span_internal::EnableIfConvertibleTo<U, absl::Span<const T>>>
 bool operator!=(Span<T> a, const U& b) {
   return !(a == b);
 }
 
 // operator<
 template <typename T>
 bool operator<(Span<T> a, Span<T> b) {
   return span_internal::LessThanImpl<Span, const T>(a, b);
 }
 template <typename T>
 bool operator<(Span<const T> a, Span<T> b) {
   return span_internal::LessThanImpl<Span, const T>(a, b);
 }
 template <typename T>
 bool operator<(Span<T> a, Span<const T> b) {
   return span_internal::LessThanImpl<Span, const T>(a, b);
 }
 template <
     typename T, typename U,
     typename = span_internal::EnableIfConvertibleTo<U, absl::Span<const T>>>
 bool operator<(const U& a, Span<T> b) {
   return span_internal::LessThanImpl<Span, const T>(a, b);
 }
 template <
     typename T, typename U,
     typename = span_internal::EnableIfConvertibleTo<U, absl::Span<const T>>>
 bool operator<(Span<T> a, const U& b) {
   return span_internal::LessThanImpl<Span, const T>(a, b);
 }
 
 // operator>
 template <typename T>
 bool operator>(Span<T> a, Span<T> b) {
   return b < a;
 }
 template <typename T>
 bool operator>(Span<const T> a, Span<T> b) {
   return b < a;
 }
 template <typename T>
 bool operator>(Span<T> a, Span<const T> b) {
   return b < a;
 }
 template <
     typename T, typename U,
     typename = span_internal::EnableIfConvertibleTo<U, absl::Span<const T>>>
 bool operator>(const U& a, Span<T> b) {
   return b < a;
 }
 template <
     typename T, typename U,
     typename = span_internal::EnableIfConvertibleTo<U, absl::Span<const T>>>
 bool operator>(Span<T> a, const U& b) {
   return b < a;
 }
 
 // operator<=
 template <typename T>
 bool operator<=(Span<T> a, Span<T> b) {
   return !(b < a);
 }
 template <typename T>
 bool operator<=(Span<const T> a, Span<T> b) {
   return !(b < a);
 }
 template <typename T>
 bool operator<=(Span<T> a, Span<const T> b) {
   return !(b < a);
 }
 template <
     typename T, typename U,
     typename = span_internal::EnableIfConvertibleTo<U, absl::Span<const T>>>
 bool operator<=(const U& a, Span<T> b) {
   return !(b < a);
 }
 template <
     typename T, typename U,
     typename = span_internal::EnableIfConvertibleTo<U, absl::Span<const T>>>
 bool operator<=(Span<T> a, const U& b) {
   return !(b < a);
 }
 
 // operator>=
 template <typename T>
 bool operator>=(Span<T> a, Span<T> b) {
   return !(a < b);
 }
 template <typename T>
 bool operator>=(Span<const T> a, Span<T> b) {
   return !(a < b);
 }
 template <typename T>
 bool operator>=(Span<T> a, Span<const T> b) {
   return !(a < b);
 }
 template <
     typename T, typename U,
     typename = span_internal::EnableIfConvertibleTo<U, absl::Span<const T>>>
 bool operator>=(const U& a, Span<T> b) {
   return !(a < b);
 }
 template <
     typename T, typename U,
     typename = span_internal::EnableIfConvertibleTo<U, absl::Span<const T>>>
 bool operator>=(Span<T> a, const U& b) {
   return !(a < b);
 }
 
 // MakeSpan()
 //
 // Constructs a mutable `Span<T>`, deducing `T` automatically from either a
 // container or pointer+size.
 //
 // Because a read-only `Span<const T>` is implicitly constructed from container
 // types regardless of whether the container itself is a const container,
 // constructing mutable spans of type `Span<T>` from containers requires
 // explicit constructors. The container-accepting version of `MakeSpan()`
 // deduces the type of `T` by the constness of the pointer received from the
 // container's `data()` member. Similarly, the pointer-accepting version returns
 // a `Span<const T>` if `T` is `const`, and a `Span<T>` otherwise.
 //
 // Examples:
 //
 //   void MyRoutine(absl::Span<MyComplicatedType> a) {
 //     ...
 //   };
 //   // my_vector is a container of non-const types
 //   std::vector<MyComplicatedType> my_vector;
 //
 //   // Constructing a Span implicitly attempts to create a Span of type
 //   // `Span<const T>`
 //   MyRoutine(my_vector);                // error, type mismatch
 //
 //   // Explicitly constructing the Span is verbose
 //   MyRoutine(absl::Span<MyComplicatedType>(my_vector));
 //
 //   // Use MakeSpan() to make an absl::Span<T>
 //   MyRoutine(absl::MakeSpan(my_vector));
 //
 //   // Construct a span from an array ptr+size
 //   absl::Span<T> my_span() {
 //     return absl::MakeSpan(&array[0], num_elements_);
 //   }
 //
 template <int&... ExplicitArgumentBarrier, typename T>
 constexpr Span<T> MakeSpan(absl::Nullable<T*> ptr, size_t size) noexcept {
   return Span<T>(ptr, size);
 }
 
 template <int&... ExplicitArgumentBarrier, typename T>
 Span<T> MakeSpan(absl::Nullable<T*> begin, absl::Nullable<T*> end) noexcept {
   return ABSL_HARDENING_ASSERT(begin <= end),
          Span<T>(begin, static_cast<size_t>(end - begin));
 }
 
 template <int&... ExplicitArgumentBarrier, typename C>
 constexpr auto MakeSpan(C& c) noexcept  // NOLINT(runtime/references)
     -> decltype(absl::MakeSpan(span_internal::GetData(c), c.size())) {
   return MakeSpan(span_internal::GetData(c), c.size());
 }
 
 template <int&... ExplicitArgumentBarrier, typename T, size_t N>
 constexpr Span<T> MakeSpan(T (&array)[N]) noexcept {
   return Span<T>(array, N);
 }
 
 // MakeConstSpan()
 //
 // Constructs a `Span<const T>` as with `MakeSpan`, deducing `T` automatically,
 // but always returning a `Span<const T>`.
 //
 // Examples:
 //
 //   void ProcessInts(absl::Span<const int> some_ints);
 //
 //   // Call with a pointer and size.
 //   int array[3] = { 0, 0, 0 };
 //   ProcessInts(absl::MakeConstSpan(&array[0], 3));
 //
 //   // Call with a [begin, end) pair.
 //   ProcessInts(absl::MakeConstSpan(&array[0], &array[3]));
 //
 //   // Call directly with an array.
 //   ProcessInts(absl::MakeConstSpan(array));
 //
 //   // Call with a contiguous container.
 //   std::vector<int> some_ints = ...;
 //   ProcessInts(absl::MakeConstSpan(some_ints));
 //   ProcessInts(absl::MakeConstSpan(std::vector<int>{ 0, 0, 0 }));
 //
 template <int&... ExplicitArgumentBarrier, typename T>
 constexpr Span<const T> MakeConstSpan(absl::Nullable<T*> ptr,
                                       size_t size) noexcept {
   return Span<const T>(ptr, size);
 }
 
 template <int&... ExplicitArgumentBarrier, typename T>
 Span<const T> MakeConstSpan(absl::Nullable<T*> begin,
                             absl::Nullable<T*> end) noexcept {
   return ABSL_HARDENING_ASSERT(begin <= end), Span<const T>(begin, end - begin);
 }
 
 template <int&... ExplicitArgumentBarrier, typename C>
 constexpr auto MakeConstSpan(const C& c) noexcept -> decltype(MakeSpan(c)) {
   return MakeSpan(c);
 }
 
 template <int&... ExplicitArgumentBarrier, typename T, size_t N>
 constexpr Span<const T> MakeConstSpan(const T (&array)[N]) noexcept {
   return Span<const T>(array, N);
 }
 ABSL_NAMESPACE_END
 }  // namespace absl
 #endif  // ABSL_TYPES_SPAN_H_

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