[libcxx-commits] [libcxx] [libc++][ranges] implement `std::ranges::zip_transform_view` (PR #79605)

[Louis Dionne via libcxx-commits]

================
@@ -0,0 +1,359 @@
+// -*- 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_ZIP_TRANSFORM_VIEW_H
+#define _LIBCPP___RANGES_ZIP_TRANSFORM_VIEW_H
+
+#include <__config>
+
+#include <__concepts/constructible.h>
+#include <__concepts/convertible_to.h>
+#include <__concepts/derived_from.h>
+#include <__concepts/equality_comparable.h>
+#include <__concepts/invocable.h>
+#include <__functional/invoke.h>
+#include <__iterator/concepts.h>
+#include <__iterator/incrementable_traits.h>
+#include <__iterator/iterator_traits.h>
+#include <__memory/addressof.h>
+#include <__ranges/access.h>
+#include <__ranges/all.h>
+#include <__ranges/concepts.h>
+#include <__ranges/empty_view.h>
+#include <__ranges/movable_box.h>
+#include <__ranges/view_interface.h>
+#include <__ranges/zip_view.h>
+#include <__type_traits/decay.h>
+#include <__type_traits/invoke.h>
+#include <__type_traits/is_object.h>
+#include <__type_traits/is_reference.h>
+#include <__type_traits/is_referenceable.h>
+#include <__type_traits/maybe_const.h>
+#include <__type_traits/remove_cvref.h>
+#include <__utility/forward.h>
+#include <__utility/in_place.h>
+#include <__utility/move.h>
+#include <tuple> // for std::apply
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER >= 23
+
+namespace ranges {
+
+template <move_constructible _Fn, input_range... _Views>
+  requires(view<_Views> && ...) &&
+          (sizeof...(_Views) > 0) && is_object_v<_Fn> && regular_invocable<_Fn&, range_reference_t<_Views>...> &&
+          __referenceable<invoke_result_t<_Fn&, range_reference_t<_Views>...>>
+class zip_transform_view : public view_interface<zip_transform_view<_Fn, _Views...>> {
+  _LIBCPP_NO_UNIQUE_ADDRESS zip_view<_Views...> __zip_;
+  _LIBCPP_NO_UNIQUE_ADDRESS __movable_box<_Fn> __fun_;
+
+  using _InnerView _LIBCPP_NODEBUG = zip_view<_Views...>;
+  template <bool _Const>
+  using __ziperator _LIBCPP_NODEBUG = iterator_t<__maybe_const<_Const, _InnerView>>;
+  template <bool _Const>
+  using __zentinel _LIBCPP_NODEBUG = sentinel_t<__maybe_const<_Const, _InnerView>>;
+
+  template <bool>
+  class __iterator;
+
+  template <bool>
+  class __sentinel;
+
+public:
+  _LIBCPP_HIDE_FROM_ABI zip_transform_view() = default;
+
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit zip_transform_view(_Fn __fun, _Views... __views)
+      : __zip_(std::move(__views)...), __fun_(in_place, std::move(__fun)) {}
+
+  _LIBCPP_HIDE_FROM_ABI constexpr auto begin() { return __iterator<false>(*this, __zip_.begin()); }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr auto begin() const
+    requires range<const _InnerView> && regular_invocable<const _Fn&, range_reference_t<const _Views>...>
+  {
+    return __iterator<true>(*this, __zip_.begin());
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr auto end() {
+    if constexpr (common_range<_InnerView>) {
+      return __iterator<false>(*this, __zip_.end());
+    } else {
+      return __sentinel<false>(__zip_.end());
+    }
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr auto end() const
+    requires range<const _InnerView> && regular_invocable<const _Fn&, range_reference_t<const _Views>...>
+  {
+    if constexpr (common_range<const _InnerView>) {
+      return __iterator<true>(*this, __zip_.end());
+    } else {
+      return __sentinel<true>(__zip_.end());
+    }
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr auto size()
+    requires sized_range<_InnerView>
+  {
+    return __zip_.size();
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr auto size() const
+    requires sized_range<const _InnerView>
+  {
+    return __zip_.size();
+  }
+};
+
+template <class _Fn, class... _Ranges>
+zip_transform_view(_Fn, _Ranges&&...) -> zip_transform_view<_Fn, views::all_t<_Ranges>...>;
+
+template <bool _Const, class _Fn, class... _Views>
+struct __zip_transform_iterator_category_base {};
+
+template <bool _Const, class _Fn, class... _Views>
+  requires forward_range<__maybe_const<_Const, zip_view<_Views...>>>
+struct __zip_transform_iterator_category_base<_Const, _Fn, _Views...> {
+private:
+  template <class _View>
+  using __tag _LIBCPP_NODEBUG = typename iterator_traits<iterator_t<__maybe_const<_Const, _View>>>::iterator_category;
+
+  static consteval auto __get_iterator_category() {
+    if constexpr (!is_reference_v<invoke_result_t<__maybe_const<_Const, _Fn>&,
+                                                  range_reference_t<__maybe_const<_Const, _Views>>...>>) {
+      return input_iterator_tag();
+    } else if constexpr ((derived_from<__tag<_Views>, random_access_iterator_tag> && ...)) {
+      return random_access_iterator_tag();
+    } else if constexpr ((derived_from<__tag<_Views>, bidirectional_iterator_tag> && ...)) {
+      return bidirectional_iterator_tag();
+    } else if constexpr ((derived_from<__tag<_Views>, forward_iterator_tag> && ...)) {
+      return forward_iterator_tag();
+    } else {
+      return input_iterator_tag();
+    }
+  }
+
+public:
+  using iterator_category = decltype(__get_iterator_category());
+};
+
+template <move_constructible _Fn, input_range... _Views>
+  requires(view<_Views> && ...) &&
+          (sizeof...(_Views) > 0) && is_object_v<_Fn> && regular_invocable<_Fn&, range_reference_t<_Views>...> &&
+          __referenceable<invoke_result_t<_Fn&, range_reference_t<_Views>...>>
+template <bool _Const>
+class zip_transform_view<_Fn, _Views...>::__iterator
+    : public __zip_transform_iterator_category_base<_Const, _Fn, _Views...> {
+  using _Parent _LIBCPP_NODEBUG = __maybe_const<_Const, zip_transform_view>;
+  using _Base _LIBCPP_NODEBUG   = __maybe_const<_Const, _InnerView>;
+
+  friend zip_transform_view<_Fn, _Views...>;
+
+  _Parent* __parent_ = nullptr;
+  __ziperator<_Const> __inner_;
+
+  _LIBCPP_HIDE_FROM_ABI constexpr __iterator(_Parent& __parent, __ziperator<_Const> __inner)
+      : __parent_(std::addressof(__parent)), __inner_(std::move(__inner)) {}
+
+  _LIBCPP_HIDE_FROM_ABI constexpr auto __get_deref_and_invoke() const noexcept {
+    return [this](const auto&... __iters) noexcept(
----------------
ldionne wrote:

I would suggest capturing the function object directly, it seems easier to follow from a "is it dangling?" perspective.

```
[&__fun = *__parent_->__fun_](const auto&... __iters) { ... }
```

That way you end up with a lambda that captures only a function object that lives inside the parent view -- seems more direct.

https://github.com/llvm/llvm-project/pull/79605