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#pragma once

#include <cstdint>
#include <memory>
#include <type_traits>

namespace epee
{
  /*!
    \brief Non-owning sequence of data. Does not deep copy

    Inspired by `gsl::span` and/or `boost::iterator_range`. This class is
    intended to be used as a parameter type for functions that need to take a
    writable or read-only sequence of data. Most common cases are `span<char>`
    and `span<std::uint8_t>`. Using as a class member is only recommended if
    clearly documented as not doing a deep-copy. C-arrays are easily convertible
    to this type.

    \note Conversion from C string literal to `span<const char>` will include
      the NULL-terminator.
    \note Never allows derived-to-base pointer conversion; an array of derived
      types is not an array of base types.
   */
  template<typename T>
  class span
  {
    /* Supporting class types is tricky - the {ptr,len} constructor will allow
       derived-to-base conversions. This is NOT desireable because an array of
       derived types is not an array of base types. It is possible to handle
       this case, implement when/if needed. */
    static_assert(!std::is_class<T>(), "no class types are currently allowed");
  public:
    using value_type = T;
    using size_type = std::size_t;
    using difference_type = std::ptrdiff_t;
    using pointer = T*;
    using const_pointer = const T*;
    using reference = T&;
    using const_reference = const T&;
    using iterator = pointer;
    using const_iterator = const_pointer;

    constexpr span() noexcept : ptr(nullptr), len(0) {}
    constexpr span(std::nullptr_t) noexcept : span() {}

    constexpr span(T* const src_ptr, const std::size_t count) noexcept
      : ptr(src_ptr), len(count) {}

    //! Conversion from C-array. Prevents common bugs with sizeof + arrays.
    template<std::size_t N>
    constexpr span(T (&src)[N]) noexcept : span(src, N) {}

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

    constexpr iterator begin() const noexcept { return ptr; }
    constexpr const_iterator cbegin() const noexcept { return ptr; }

    constexpr iterator end() const noexcept { return begin() + size(); }
    constexpr const_iterator cend() const noexcept { return cbegin() + size(); }

    constexpr bool empty() const noexcept { return size() == 0; }
    constexpr pointer data() const noexcept { return ptr; }
    constexpr std::size_t size() const noexcept { return len; }
    constexpr std::size_t size_bytes() const noexcept { return size() * sizeof(value_type); }

  private:
    T* ptr;
    std::size_t len;
  };

  //! \return `span<const T::value_type>` from a STL compatible `src`.
  template<typename T>
  constexpr span<const typename T::value_type> to_span(const T& src)
  {
    // compiler provides diagnostic if size() is not size_t.
    return {src.data(), src.size()};
  }

  template<typename T>
  constexpr bool has_padding() noexcept
  {
    return !std::is_standard_layout<T>() || alignof(T) != 1;
  }

  //! \return Cast data from `src` as `span<const std::uint8_t>`.
  template<typename T>
  span<const std::uint8_t> to_byte_span(const span<const T> src) noexcept
  {
    static_assert(!has_padding<T>(), "source type may have padding");
    return {reinterpret_cast<const std::uint8_t*>(src.data()), src.size_bytes()}; 
  }

  //! \return `span<const std::uint8_t>` which represents the bytes at `&src`.
  template<typename T>
  span<const std::uint8_t> as_byte_span(const T& src) noexcept
  {
    static_assert(!std::is_empty<T>(), "empty types will not work -> sizeof == 1");
    static_assert(!has_padding<T>(), "source type may have padding");
    return {reinterpret_cast<const std::uint8_t*>(std::addressof(src)), sizeof(T)};
  }
}