include/sdbusplus/async/stdexec/__detail/__intrusive_queue.hpp

/*
 * Copyright (c) 2021-2022 Facebook, Inc. and its affiliates
 * Copyright (c) 2021-2022 NVIDIA Corporation
 *
 * Licensed under the Apache License Version 2.0 with LLVM Exceptions
 * (the "License"); you may not use this file except in compliance with
 * the License. You may obtain a copy of the License at
 *
 *   https://llvm.org/LICENSE.txt
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
#pragma once

#include "__config.hpp"

#include <cassert>
#include <cstddef>
#include <utility>

namespace stdexec
{
namespace __queue
{
template <auto _Next>
class __intrusive_queue;

template <class _Item, _Item* _Item::*_Next>
class __intrusive_queue<_Next>
{
  public:
    __intrusive_queue() noexcept = default;

    __intrusive_queue(__intrusive_queue&& __other) noexcept :
        __head_(std::exchange(__other.__head_, nullptr)),
        __tail_(std::exchange(__other.__tail_, nullptr))
    {}

    __intrusive_queue(_Item* __head, _Item* __tail) noexcept :
        __head_(__head), __tail_(__tail)
    {}

    __intrusive_queue& operator=(__intrusive_queue __other) noexcept
    {
        std::swap(__head_, __other.__head_);
        std::swap(__tail_, __other.__tail_);
        return *this;
    }

    ~__intrusive_queue()
    {
        STDEXEC_ASSERT(empty());
    }

    static __intrusive_queue make_reversed(_Item* __list) noexcept
    {
        _Item* __new_head = nullptr;
        _Item* __new_tail = __list;
        while (__list != nullptr)
        {
            _Item* __next = __list->*_Next;
            __list->*_Next = __new_head;
            __new_head = __list;
            __list = __next;
        }

        __intrusive_queue __result;
        __result.__head_ = __new_head;
        __result.__tail_ = __new_tail;
        return __result;
    }

    static __intrusive_queue make(_Item* __list) noexcept
    {
        __intrusive_queue __result{};
        __result.__head_ = __list;
        __result.__tail_ = __list;
        if (__list == nullptr)
        {
            return __result;
        }
        while (__result.__tail_->*_Next != nullptr)
        {
            __result.__tail_ = __result.__tail_->*_Next;
        }
        return __result;
    }

    [[nodiscard]] bool empty() const noexcept
    {
        return __head_ == nullptr;
    }

    void clear() noexcept
    {
        __head_ = nullptr;
        __tail_ = nullptr;
    }

    [[nodiscard]] _Item* pop_front() noexcept
    {
        STDEXEC_ASSERT(!empty());
        _Item* __item = std::exchange(__head_, __head_->*_Next);
        // This should test if __head_ == nullptr, but due to a bug in
        // nvc++'s optimization, `__head_` isn't assigned until later.
        // Filed as NVBug#3952534.
        if (__item->*_Next == nullptr)
        {
            __tail_ = nullptr;
        }
        return __item;
    }

    void push_front(_Item* __item) noexcept
    {
        STDEXEC_ASSERT(__item != nullptr);
        __item->*_Next = __head_;
        __head_ = __item;
        if (__tail_ == nullptr)
        {
            __tail_ = __item;
        }
    }

    void push_back(_Item* __item) noexcept
    {
        STDEXEC_ASSERT(__item != nullptr);
        __item->*_Next = nullptr;
        if (__tail_ == nullptr)
        {
            __head_ = __item;
        }
        else
        {
            __tail_->*_Next = __item;
        }
        __tail_ = __item;
    }

    void append(__intrusive_queue __other) noexcept
    {
        if (__other.empty())
            return;
        auto* __other_head = std::exchange(__other.__head_, nullptr);
        if (empty())
        {
            __head_ = __other_head;
        }
        else
        {
            __tail_->*_Next = __other_head;
        }
        __tail_ = std::exchange(__other.__tail_, nullptr);
    }

    void prepend(__intrusive_queue __other) noexcept
    {
        if (__other.empty())
            return;

        __other.__tail_->*_Next = __head_;
        __head_ = __other.__head_;
        if (__tail_ == nullptr)
        {
            __tail_ = __other.__tail_;
        }

        __other.__tail_ = nullptr;
        __other.__head_ = nullptr;
    }

    struct iterator
    {
        using difference_type = std::ptrdiff_t;
        using value_type = _Item*;

        _Item* __predecessor_ = nullptr;
        _Item* __item_ = nullptr;

        iterator() noexcept = default;

        explicit iterator(_Item* __pred, _Item* __item) noexcept :
            __predecessor_(__pred), __item_(__item)
        {}

        [[nodiscard]] _Item* operator*() const noexcept
        {
            STDEXEC_ASSERT(__item_ != nullptr);
            return __item_;
        }

        [[nodiscard]] _Item** operator->() const noexcept
        {
            STDEXEC_ASSERT(__item_ != nullptr);
            return &__item_;
        }

        iterator& operator++() noexcept
        {
            __predecessor_ = __item_;
            if (__item_)
            {
                __item_ = __item_->*_Next;
            }
            return *this;
        }

        iterator operator++(int) noexcept
        {
            iterator __result = *this;
            ++*this;
            return __result;
        }

        friend bool operator==(const iterator&,
                               const iterator&) noexcept = default;
    };

    [[nodiscard]] iterator begin() const noexcept
    {
        return iterator(nullptr, __head_);
    }

    [[nodiscard]] iterator end() const noexcept
    {
        return iterator(__tail_, nullptr);
    }

    void splice(iterator pos, __intrusive_queue& other, iterator first,
                iterator last) noexcept
    {
        if (first == last)
        {
            return;
        }
        STDEXEC_ASSERT(first.__item_ != nullptr);
        STDEXEC_ASSERT(last.__predecessor_ != nullptr);
        if (other.__head_ == first.__item_)
        {
            other.__head_ = last.__item_;
            if (other.__head_ == nullptr)
            {
                other.__tail_ = nullptr;
            }
        }
        else
        {
            STDEXEC_ASSERT(first.__predecessor_ != nullptr);
            first.__predecessor_->*_Next = last.__item_;
            last.__predecessor_->*_Next = pos.__item_;
        }
        if (empty())
        {
            __head_ = first.__item_;
            __tail_ = last.__predecessor_;
        }
        else
        {
            pos.__predecessor_->*_Next = first.__item_;
            if (pos.__item_ == nullptr)
            {
                __tail_ = last.__predecessor_;
            }
        }
    }

    _Item* front() const noexcept
    {
        return __head_;
    }

    _Item* back() const noexcept
    {
        return __tail_;
    }

  private:
    _Item* __head_ = nullptr;
    _Item* __tail_ = nullptr;
};
} // namespace __queue

using __queue::__intrusive_queue;

} // namespace stdexec