text: // Copyright (c) 2018 The Bitcoin Core developers
  // Distributed under the MIT software license, see the accompanying
  // file COPYING or http://www.opensource.org/licenses/mit-license.php.
  
  #ifndef BITCOINRUSH_SPAN_H
  #define BITCOINRUSH_SPAN_H
  
  #include <type_traits>
  #include <cstddef>
  #include <algorithm>
  
  /** A Span is an object that can refer to a contiguous sequence of objects.
   *
   * It implements a subset of C++20's std::span.
   */
  template<typename C>
  class Span
  {
      C* m_data;
      std::ptrdiff_t m_size;
  
  public:
      constexpr Span() noexcept : m_data(nullptr), m_size(0) {}
      constexpr Span(C* data, std::ptrdiff_t size) noexcept : m_data(data), m_size(size) {}
      constexpr Span(C* data, C* end) noexcept : m_data(data), m_size(end - data) {}
  
      constexpr C* data() const noexcept { return m_data; }
      constexpr C* begin() const noexcept { return m_data; }
      constexpr C* end() const noexcept { return m_data + m_size; }
      constexpr std::ptrdiff_t size() const noexcept { return m_size; }
      constexpr C& operator[](std::ptrdiff_t pos) const noexcept { return m_data[pos]; }
  
      constexpr Span<C> subspan(std::ptrdiff_t offset) const noexcept { return Span<C>(m_data + offset, m_size - offset); }
      constexpr Span<C> subspan(std::ptrdiff_t offset, std::ptrdiff_t count) const noexcept { return Span<C>(m_data + offset, count); }
      constexpr Span<C> first(std::ptrdiff_t count) const noexcept { return Span<C>(m_data, count); }
      constexpr Span<C> last(std::ptrdiff_t count) const noexcept { return Span<C>(m_data + m_size - count, count); }
  
      friend constexpr bool operator==(const Span& a, const Span& b) noexcept { return a.size() == b.size() && std::equal(a.begin(), a.end(), b.begin()); }
      friend constexpr bool operator!=(const Span& a, const Span& b) noexcept { return !(a == b); }
      friend constexpr bool operator<(const Span& a, const Span& b) noexcept { return std::lexicographical_compare(a.begin(), a.end(), b.begin(), b.end()); }
      friend constexpr bool operator<=(const Span& a, const Span& b) noexcept { return !(b < a); }
      friend constexpr bool operator>(const Span& a, const Span& b) noexcept { return (b < a); }
      friend constexpr bool operator>=(const Span& a, const Span& b) noexcept { return !(a < b); }
  };
  
  /** Create a span to a container exposing data() and size().
   *
   * This correctly deals with constness: the returned Span's element type will be
   * whatever data() returns a pointer to. If either the passed container is const,
   * or its element type is const, the resulting span will have a const element type.
   *
   * std::span will have a constructor that implements this functionality directly.
   */
  template<typename A, int N>
  constexpr Span<A> MakeSpan(A (&a)[N]) { return Span<A>(a, N); }
  
  template<typename V>
  constexpr Span<typename std::remove_pointer<decltype(std::declval<V>().data())>::type> MakeSpan(V& v) { return Span<typename std::remove_pointer<decltype(std::declval<V>().data())>::type>(v.data(), v.size()); }
  
  #endif
  