[libcxx][views] Add drop_view.
The first view in the libc++ ranges library 🚀
Differential Revision: https://reviews.llvm.org/D102037
NOKEYCHECK=True
GitOrigin-RevId: 560170fa2de5ed3d8720bf6a88c7652ceb47ff8c
diff --git a/include/CMakeLists.txt b/include/CMakeLists.txt
index 75e0efc..6773ec3 100644
--- a/include/CMakeLists.txt
+++ b/include/CMakeLists.txt
@@ -141,6 +141,7 @@
__ranges/all.h
__ranges/concepts.h
__ranges/data.h
+ __ranges/drop_view.h
__ranges/empty.h
__ranges/empty_view.h
__ranges/enable_borrowed_range.h
diff --git a/include/__ranges/drop_view.h b/include/__ranges/drop_view.h
new file mode 100644
index 0000000..6103057
--- /dev/null
+++ b/include/__ranges/drop_view.h
@@ -0,0 +1,154 @@
+// -*- 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 <__config>
+#include <__iterator/iterator_traits.h>
+#include <__iterator/concepts.h>
+#include <__ranges/access.h>
+#include <__ranges/view_interface.h>
+#include <__ranges/all.h>
+#include <type_traits>
+#include <optional>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if !defined(_LIBCPP_HAS_NO_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 = optional<iterator_t<_View>>;
+ struct _Empty { };
+
+ // For forward ranges use std::optional to cache the begin iterator.
+ // No unique address + _Empty means we don't use any extra space when this
+ // is not a forward iterator.
+ [[no_unique_address]] conditional_t<_UseCache, _Cache, _Empty> __cached_begin_;
+ range_difference_t<_View> __count_ = 0;
+ _View __base_ = _View();
+
+public:
+ drop_view() requires default_initializable<_View> = default;
+
+ constexpr drop_view(_View __base, range_difference_t<_View> __count)
+ : __cached_begin_()
+ , __count_(__count)
+ , __base_(_VSTD::move(__base))
+ {
+ _LIBCPP_ASSERT(__count_ >= 0, "count must be greater than or equal to zero.");
+ }
+
+ constexpr drop_view(drop_view const& __other)
+ : __cached_begin_() // Intentionally not propagating the cached begin iterator.
+ , __count_(__other.__count_)
+ , __base_(__other.__base_)
+ { }
+
+ constexpr drop_view(drop_view&& __other)
+ : __cached_begin_() // Intentionally not propagating the cached begin iterator.
+ , __count_(_VSTD::move(__other.__count_))
+ , __base_(_VSTD::move(__other.__base_))
+ { }
+
+ constexpr drop_view& operator=(drop_view const& __other) {
+ if constexpr (_UseCache) {
+ __cached_begin_.reset();
+ }
+ __base_ = __other.__base_;
+ __count_ = __other.__count_;
+ return *this;
+ }
+
+ constexpr drop_view& operator=(drop_view&& __other) {
+ if constexpr (_UseCache) {
+ __cached_begin_.reset();
+ __other.__cached_begin_.reset();
+ }
+ __base_ = _VSTD::move(__other.__base_);
+ __count_ = _VSTD::move(__other.__count_);
+ return *this;
+ }
+
+ constexpr _View base() const& requires copy_constructible<_View> { return __base_; }
+ constexpr _View base() && { return _VSTD::move(__base_); }
+
+ constexpr auto begin()
+ requires (!(__simple_view<_View> &&
+ random_access_range<const _View> && sized_range<const _View>))
+ {
+ if constexpr (_UseCache)
+ if (__cached_begin_)
+ return *__cached_begin_;
+
+ auto __tmp = ranges::next(ranges::begin(__base_), __count_, ranges::end(__base_));
+ if constexpr (_UseCache)
+ __cached_begin_ = __tmp;
+ return __tmp;
+ }
+
+ constexpr auto begin() const
+ requires random_access_range<const _View> && sized_range<const _View>
+ {
+ return ranges::next(ranges::begin(__base_), __count_, ranges::end(__base_));
+ }
+
+ constexpr auto end()
+ requires (!__simple_view<_View>)
+ { return ranges::end(__base_); }
+
+ constexpr auto end() const
+ requires range<const _View>
+ { return ranges::end(__base_); }
+
+ 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;
+ }
+
+ constexpr auto size()
+ requires sized_range<_View>
+ { return __size(*this); }
+
+ constexpr auto size() const
+ requires sized_range<const _View>
+ { return __size(*this); }
+ };
+
+ template<class _Range>
+ drop_view(_Range&&, range_difference_t<_Range>)
+ // TODO: this is just recreating all_t.
+ -> drop_view<decltype(views::all(std::declval<_Range>()))>;
+
+} // namespace ranges
+
+#endif // !defined(_LIBCPP_HAS_NO_RANGES)
+
+_LIBCPP_END_NAMESPACE_STD
+
+_LIBCPP_POP_MACROS
+
+#endif // _LIBCPP___RANGES_DROP_VIEW_H
diff --git a/include/ranges b/include/ranges
index 70d0dd0..52c6912 100644
--- a/include/ranges
+++ b/include/ranges
@@ -97,6 +97,7 @@
#include <__ranges/all.h>
#include <__ranges/concepts.h>
#include <__ranges/data.h>
+#include <__ranges/drop_view.h>
#include <__ranges/empty.h>
#include <__ranges/empty_view.h>
#include <__ranges/enable_borrowed_range.h>