// -*- 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___RANGES_DROP_VIEW_H
#define _LIBCPP___RANGES_DROP_VIEW_H

#include <__algorithm/min.h>
#include <__assert>
#include <__config>
#include <__functional/bind_back.h>
#include <__fwd/span.h>
#include <__fwd/string_view.h>
#include <__iterator/concepts.h>
#include <__iterator/distance.h>
#include <__iterator/iterator_traits.h>
#include <__iterator/next.h>
#include <__ranges/access.h>
#include <__ranges/all.h>
#include <__ranges/concepts.h>
#include <__ranges/empty_view.h>
#include <__ranges/enable_borrowed_range.h>
#include <__ranges/iota_view.h>
#include <__ranges/non_propagating_cache.h>
#include <__ranges/range_adaptor.h>
#include <__ranges/size.h>
#include <__ranges/subrange.h>
#include <__ranges/view_interface.h>
#include <__utility/auto_cast.h>
#include <__utility/forward.h>
#include <__utility/move.h>
#include <concepts>
#include <type_traits>

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

_LIBCPP_BEGIN_NAMESPACE_STD

#if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)

namespace ranges {
  template<view _View>
  class drop_view
    : public view_interface<drop_view<_View>>
  {
    // We cache begin() whenever ranges::next is not guaranteed O(1) to provide an
    // amortized O(1) begin() method. If this is an input_range, then we cannot cache
    // begin because begin is not equality preserving.
    // Note: drop_view<input-range>::begin() is still trivially amortized O(1) because
    // one can't call begin() on it more than once.
    static constexpr bool _UseCache = forward_range<_View> && !(random_access_range<_View> && sized_range<_View>);
    using _Cache = _If<_UseCache, __non_propagating_cache<iterator_t<_View>>, __empty_cache>;
    _LIBCPP_NO_UNIQUE_ADDRESS _Cache __cached_begin_ = _Cache();
    range_difference_t<_View> __count_ = 0;
    _View __base_ = _View();

public:
    drop_view() requires default_initializable<_View> = default;

    _LIBCPP_HIDE_FROM_ABI
    constexpr drop_view(_View __base, range_difference_t<_View> __count)
      : __count_(__count)
      , __base_(std::move(__base))
    {
      _LIBCPP_ASSERT(__count_ >= 0, "count must be greater than or equal to zero.");
    }

    _LIBCPP_HIDE_FROM_ABI constexpr _View base() const& requires copy_constructible<_View> { return __base_; }
    _LIBCPP_HIDE_FROM_ABI constexpr _View base() && { return std::move(__base_); }

    _LIBCPP_HIDE_FROM_ABI
    constexpr auto begin()
      requires (!(__simple_view<_View> &&
                  random_access_range<const _View> && sized_range<const _View>))
    {
      if constexpr (_UseCache)
        if (__cached_begin_.__has_value())
          return *__cached_begin_;

      auto __tmp = ranges::next(ranges::begin(__base_), __count_, ranges::end(__base_));
      if constexpr (_UseCache)
        __cached_begin_.__emplace(__tmp);
      return __tmp;
    }

    _LIBCPP_HIDE_FROM_ABI
    constexpr auto begin() const
      requires random_access_range<const _View> && sized_range<const _View>
    {
      return ranges::next(ranges::begin(__base_), __count_, ranges::end(__base_));
    }

    _LIBCPP_HIDE_FROM_ABI
    constexpr auto end()
      requires (!__simple_view<_View>)
    { return ranges::end(__base_); }

    _LIBCPP_HIDE_FROM_ABI
    constexpr auto end() const
      requires range<const _View>
    { return ranges::end(__base_); }

    _LIBCPP_HIDE_FROM_ABI
    static constexpr auto __size(auto& __self) {
      const auto __s = ranges::size(__self.__base_);
      const auto __c = static_cast<decltype(__s)>(__self.__count_);
      return __s < __c ? 0 : __s - __c;
    }

    _LIBCPP_HIDE_FROM_ABI
    constexpr auto size()
      requires sized_range<_View>
    { return __size(*this); }

    _LIBCPP_HIDE_FROM_ABI
    constexpr auto size() const
      requires sized_range<const _View>
    { return __size(*this); }
  };

template<class _Range>
drop_view(_Range&&, range_difference_t<_Range>) -> drop_view<views::all_t<_Range>>;

template<class _Tp>
inline constexpr bool enable_borrowed_range<drop_view<_Tp>> = enable_borrowed_range<_Tp>;

namespace views {
namespace __drop {

template <class _Tp>
inline constexpr bool __is_empty_view = false;

template <class _Tp>
inline constexpr bool __is_empty_view<empty_view<_Tp>> = true;

template <class _Tp>
inline constexpr bool __is_passthrough_specialization = false;

template <class _Tp, size_t _Extent>
inline constexpr bool __is_passthrough_specialization<span<_Tp, _Extent>> = true;

template <class _CharT, class _Traits>
inline constexpr bool __is_passthrough_specialization<basic_string_view<_CharT, _Traits>> = true;

template <class _Np, class _Bound>
inline constexpr bool __is_passthrough_specialization<iota_view<_Np, _Bound>> = true;

template <class _Iter, class _Sent, subrange_kind _Kind>
inline constexpr bool __is_passthrough_specialization<subrange<_Iter, _Sent, _Kind>> =
    !subrange<_Iter, _Sent, _Kind>::_StoreSize;

template <class _Tp>
inline constexpr bool __is_subrange_specialization_with_store_size = false;

template <class _Iter, class _Sent, subrange_kind _Kind>
inline constexpr bool __is_subrange_specialization_with_store_size<subrange<_Iter, _Sent, _Kind>> =
    subrange<_Iter, _Sent, _Kind>::_StoreSize;

template <class _Tp>
struct __passthrough_type;

template <class _Tp, size_t _Extent>
struct __passthrough_type<span<_Tp, _Extent>> {
  using type = span<_Tp>;
};

template <class _CharT, class _Traits>
struct __passthrough_type<basic_string_view<_CharT, _Traits>> {
  using type = basic_string_view<_CharT, _Traits>;
};

template <class _Np, class _Bound>
struct __passthrough_type<iota_view<_Np, _Bound>> {
  using type = iota_view<_Np, _Bound>;
};

template <class _Iter, class _Sent, subrange_kind _Kind>
struct __passthrough_type<subrange<_Iter, _Sent, _Kind>> {
  using type = subrange<_Iter, _Sent, _Kind>;
};

template <class _Tp>
using __passthrough_type_t = typename __passthrough_type<_Tp>::type;

struct __fn {
  // [range.drop.overview]: the `empty_view` case.
  template <class _Range, convertible_to<range_difference_t<_Range>> _Np>
    requires __is_empty_view<remove_cvref_t<_Range>>
  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI
  constexpr auto operator()(_Range&& __range, _Np&&) const
    noexcept(noexcept(_LIBCPP_AUTO_CAST(std::forward<_Range>(__range))))
    -> decltype(      _LIBCPP_AUTO_CAST(std::forward<_Range>(__range)))
    { return          _LIBCPP_AUTO_CAST(std::forward<_Range>(__range)); }

  // [range.drop.overview]: the `span | basic_string_view | iota_view | subrange (StoreSize == false)` case.
  template <class _Range,
            convertible_to<range_difference_t<_Range>> _Np,
            class _RawRange = remove_cvref_t<_Range>,
            class _Dist = range_difference_t<_Range>>
    requires (!__is_empty_view<_RawRange> &&
              random_access_range<_RawRange> &&
              sized_range<_RawRange> &&
              __is_passthrough_specialization<_RawRange>)
  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI
  constexpr auto operator()(_Range&& __rng, _Np&& __n) const
    noexcept(noexcept(__passthrough_type_t<_RawRange>(
                              ranges::begin(__rng) + std::min<_Dist>(ranges::distance(__rng), std::forward<_Np>(__n)),
                              ranges::end(__rng)
                              )))
    -> decltype(      __passthrough_type_t<_RawRange>(
                              // Note: deliberately not forwarding `__rng` to guard against double moves.
                              ranges::begin(__rng) + std::min<_Dist>(ranges::distance(__rng), std::forward<_Np>(__n)),
                              ranges::end(__rng)
                              ))
    { return          __passthrough_type_t<_RawRange>(
                              ranges::begin(__rng) + std::min<_Dist>(ranges::distance(__rng), std::forward<_Np>(__n)),
                              ranges::end(__rng)
                              ); }

  // [range.drop.overview]: the `subrange (StoreSize == true)` case.
  template <class _Range,
            convertible_to<range_difference_t<_Range>> _Np,
            class _RawRange = remove_cvref_t<_Range>,
            class _Dist = range_difference_t<_Range>>
    requires (!__is_empty_view<_RawRange> &&
              random_access_range<_RawRange> &&
              sized_range<_RawRange> &&
              __is_subrange_specialization_with_store_size<_RawRange>)
  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI
  constexpr auto operator()(_Range&& __rng, _Np&& __n) const
    noexcept(noexcept(_RawRange(
                              ranges::begin(__rng) + std::min<_Dist>(ranges::distance(__rng), std::forward<_Np>(__n)),
                              ranges::end(__rng),
                              std::__to_unsigned_like(ranges::distance(__rng) -
                                  std::min<_Dist>(ranges::distance(__rng), std::forward<_Np>(__n)))
                              )))
    -> decltype(      _RawRange(
                              // Note: deliberately not forwarding `__rng` to guard against double moves.
                              ranges::begin(__rng) + std::min<_Dist>(ranges::distance(__rng), std::forward<_Np>(__n)),
                              ranges::end(__rng),
                              std::__to_unsigned_like(ranges::distance(__rng) -
                                  std::min<_Dist>(ranges::distance(__rng), std::forward<_Np>(__n)))
                              ))
    {
      // Introducing local variables avoids calculating `min` and `distance` twice (at the cost of diverging from the
      // expression used in the `noexcept` clause and the return statement).
      auto dist = ranges::distance(__rng);
      auto clamped = std::min<_Dist>(dist, std::forward<_Np>(__n));
      return          _RawRange(
                              ranges::begin(__rng) + clamped,
                              ranges::end(__rng),
                              std::__to_unsigned_like(dist - clamped)
                              );}

  // [range.drop.overview]: the "otherwise" case.
  template <class _Range, convertible_to<range_difference_t<_Range>> _Np,
            class _RawRange = remove_cvref_t<_Range>>
    // Note: without specifically excluding the other cases, GCC sees this overload as ambiguous with the other
    // overloads.
    requires (!(__is_empty_view<_RawRange> ||
               (__is_subrange_specialization_with_store_size<_RawRange> &&
               sized_range<_RawRange> &&
                random_access_range<_RawRange>) ||
               (__is_passthrough_specialization<_RawRange> &&
                sized_range<_RawRange> &&
                random_access_range<_RawRange>)
             ))
  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI
  constexpr auto operator()(_Range&& __range, _Np&& __n) const
    noexcept(noexcept(drop_view(std::forward<_Range>(__range), std::forward<_Np>(__n))))
    -> decltype(      drop_view(std::forward<_Range>(__range), std::forward<_Np>(__n)))
    { return          drop_view(std::forward<_Range>(__range), std::forward<_Np>(__n)); }

  template <class _Np>
    requires constructible_from<decay_t<_Np>, _Np>
  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI
  constexpr auto operator()(_Np&& __n) const
    noexcept(is_nothrow_constructible_v<decay_t<_Np>, _Np>)
  { return __range_adaptor_closure_t(std::__bind_back(*this, std::forward<_Np>(__n))); }
};

} // namespace __drop

inline namespace __cpo {
  inline constexpr auto drop = __drop::__fn{};
} // namespace __cpo
} // namespace views

} // namespace ranges

#endif // _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)

_LIBCPP_END_NAMESPACE_STD

#endif // _LIBCPP___RANGES_DROP_VIEW_H