multiplexer.hpp

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/* Copyright 2025 Kevin Exton
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * 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
#ifndef IO_MULTIPLEXER_HPP
#define IO_MULTIPLEXER_HPP
#include "detail/immovable.hpp"
#include "io/detail/concepts.hpp"
 
#include <variant>
namespace io::execution {
 
template <MuxTag Tag> struct basic_multiplexer : public Tag {
  using multiplexer_type = Tag;
 
  class intrusive_task_queue {
  public:
    struct task : immovable {
      task *next = this;
 
      std::variant<task *, void (*)(task *) noexcept> tail;
 
      auto execute() noexcept -> void { std::get<1>(tail)(this); }
    };
 
    [[nodiscard]] auto is_empty() const noexcept -> bool
    {
      return std::get<0>(head_.tail) == &head_;
    }
 
    auto push(task *task) noexcept -> void
    {
      task->next = &head_;
      head_.tail = std::get<0>(head_.tail)->next = task;
    }
 
    // NOLINTNEXTLINE(cppcoreguidelines-rvalue-reference-param-not-moved)
    auto move_back(intrusive_task_queue &&other) noexcept -> void
    {
      if (!other.is_empty())
      {
        std::get<0>(other.head_.tail)->next = std::get<0>(head_.tail)->next;
        std::get<0>(head_.tail)->next = other.head_.next;
        head_.tail = other.head_.tail;
 
        other.head_.next = std::get<0>(other.head_.tail) = &other.head_;
      }
    }
 
    auto pop() noexcept -> task *
    {
      if (std::get<0>(head_.tail) == head_.next)
        head_.tail = &head_;
      return std::exchange(head_.next, head_.next->next);
    }
 
  private:
    task head_{.next = &head_, .tail = &head_};
  };
};
} // namespace io::execution
#endif // IO_MULTIPLEXER_HPP