tpl_mpsc_queue.H

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/*
                          Aleph_w
 
  Data structures & Algorithms
  https://github.com/lrleon/Aleph-w
 
  This file is part of Aleph-w library
 
  Copyright (c) 2002-2026 Leandro Rabindranath Leon
 
  Permission is hereby granted, free of charge, to any person obtaining a copy
  of this software and associated documentation files (the "Software"), to deal
  in the Software without restriction, including without limitation the rights
  to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  copies of the Software, and to permit persons to whom the Software is
  furnished to do so, subject to the following conditions:
 
  The above copyright notice and this permission notice shall be included in all
  copies or substantial portions of the Software.
 
  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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*/
 
#ifndef TPL_MPSC_QUEUE_H
#define TPL_MPSC_QUEUE_H
 
#include <atomic>
#include <memory>
#include <optional>
#include <utility>
 
namespace Aleph
{
 
template <typename T>
class MpscQueue
{
  struct Node
  {
    std::atomic<Node *> next{nullptr};
    std::optional<T> value;
 
    Node() = default;
 
    template <typename... Args>
    explicit Node(std::in_place_t, Args &&... args)
      : value(std::in_place, std::forward<Args>(args)...)
    {}
  };
 
  alignas(64) std::atomic<Node *> head_;
  alignas(64) Node * tail_;
  Node stub_;
 
  void push_node(Node * n) noexcept
  {
    n->next.store(nullptr, std::memory_order_relaxed);
    Node * prev = head_.exchange(n, std::memory_order_acq_rel);
    prev->next.store(n, std::memory_order_release);
  }
 
  Node * pop_node() noexcept
  {
    Node * tail = tail_;
    Node * next = tail->next.load(std::memory_order_acquire);
 
    if (tail == &stub_)
      {
        if (next == nullptr)
          return nullptr;  // genuinely empty
        tail_ = next;
        tail = next;
        next = next->next.load(std::memory_order_acquire);
      }
 
    if (next != nullptr)
      {
        tail_ = next;
        return tail;
      }
 
    Node * head = head_.load(std::memory_order_acquire);
    if (tail != head)
      return nullptr;  // a producer is mid-push; try again later
 
    push_node(&stub_);  // re-anchor the queue so progress remains possible
    next = tail->next.load(std::memory_order_acquire);
    if (next != nullptr)
      {
        tail_ = next;
        return tail;
      }
 
    return nullptr;
  }
 
public:
  MpscQueue() noexcept : head_(&stub_), tail_(&stub_) {}
 
  ~MpscQueue()
  {
    // stub_ is a data member, not a heap node: it must never reach
    // `delete`. A racing push_node(&stub_) re-anchor (see pop_node()) can
    // leave stub_ reachable from tail_ without *being* tail_ (a real,
    // not-yet-popped node can end up between them), so every node in the
    // remaining chain -- not just the first -- must be checked before
    // deleting it.
    Node * n = tail_;
    while (n != nullptr)
      {
        Node * next = n->next.load(std::memory_order_relaxed);
        if (n != &stub_)
          delete n;
        n = next;
      }
  }
 
  MpscQueue(const MpscQueue &) = delete;
  MpscQueue & operator=(const MpscQueue &) = delete;
  MpscQueue(MpscQueue &&) = delete;
  MpscQueue & operator=(MpscQueue &&) = delete;
 
  void push(const T & value) { emplace(value); }
 
  void push(T && value) { emplace(std::move(value)); }
 
  template <typename... Args>
  void emplace(Args &&... args)
  {
    Node * n = new Node(std::in_place, std::forward<Args>(args)...);
    push_node(n);
  }
 
  bool try_pop(T & out)
  {
    Node * n = pop_node();
    if (n == nullptr)
      return false;
    // unique_ptr guarantees the node is freed even if the move below
    // throws (T's move assignment is not required to be noexcept).
    std::unique_ptr<Node> owned(n);
    out = std::move(*owned->value);
    return true;
  }
 
  [[nodiscard]] std::optional<T> try_pop()
  {
    Node * n = pop_node();
    if (n == nullptr)
      return std::nullopt;
    std::unique_ptr<Node> owned(n);
    return std::optional<T>(std::move(*owned->value));
  }
 
  [[nodiscard]] bool is_empty() const noexcept
  {
    return tail_ == &stub_ and
           tail_->next.load(std::memory_order_acquire) == nullptr;
  }
};
 
template <typename T>
using mpsc_queue = MpscQueue<T>;
 
}  // namespace Aleph
 
#endif  // TPL_MPSC_QUEUE_H