From df4316af2512ead61a6376ba875f244045249648 Mon Sep 17 00:00:00 2001
From: Michael Mueller <michael.mueller@uos.de>
Date: Wed, 23 Apr 2025 18:40:32 +0200
Subject: [PATCH] ealanos: Ported wait-free queue from MxTasking.

---
 .../ealanos/include/ealanos/util/mpsc_queue.h | 124 ++++++++++++++++++
 1 file changed, 124 insertions(+)
 create mode 100644 repos/ealanos/include/ealanos/util/mpsc_queue.h

diff --git a/repos/ealanos/include/ealanos/util/mpsc_queue.h b/repos/ealanos/include/ealanos/util/mpsc_queue.h
new file mode 100644
index 0000000000..eee0d10f52
--- /dev/null
+++ b/repos/ealanos/include/ealanos/util/mpsc_queue.h
@@ -0,0 +1,124 @@
+
+
+#ifndef __EALANOS__INCLUDE__UTIL__MPSC_QUEUE_H_
+#define __EALANOS__INCLUDE__UTIL__MPSC_QUEUE_H_
+
+namespace Ealan::util {
+/**
+ * Multi producer, single consumer queue with unlimited slots.
+ * Every thread can push values into the queue without using latches.
+ *
+ * Inspired by http://www.1024cores.net/home/lock-free-algorithms/queues/intrusive-mpsc-node-based-queue
+ */
+template <class T> class MPSCQueue
+{
+public:
+    constexpr MPSCQueue() noexcept
+        : _head(reinterpret_cast<T *>(&_stub)), _tail(reinterpret_cast<T *>(&_stub)),
+          _end(reinterpret_cast<T *>(&_stub))
+    {
+        reinterpret_cast<T &>(_stub).next(nullptr);
+    }
+    ~MPSCQueue() noexcept = default;
+
+    /**
+     * Inserts the given item into the queue.
+     * @param item Item to insert.
+     */
+    void push_back(T *item) noexcept
+    {
+        item->next(nullptr);
+        auto *prev = __atomic_exchange_n(&_head, item, __ATOMIC_RELAXED);
+        prev->next(item);
+    }
+
+    /**
+     * Inserts all items between begin and end into the queue.
+     * Items must be linked among themselves.
+     * @param begin First item to insert.
+     * @param end Last item to insert.
+     */
+    void push_back(T *begin, T *end) noexcept
+    {
+        end->next(nullptr);
+        auto *old_head = __atomic_exchange_n(&_head, end, __ATOMIC_RELAXED);
+        old_head->next(begin);
+    }
+
+    /**
+     * @return End of the queue.
+     */
+    [[nodiscard]] const T *end() const noexcept { return _end; }
+
+    [[nodiscard]] T *head() noexcept { return _head; }
+
+    /**
+     * @return True, when the queue is empty.
+     */
+    [[nodiscard]] bool empty() const noexcept
+    {
+        return _tail == _end && reinterpret_cast<T const &>(_stub).next() == nullptr;
+    }
+
+    /**
+     * @return Takes and removes the first item from the queue.
+     */
+    T *pop_front() noexcept;
+
+private:
+    // Head of the queue (accessed by every producer).
+    alignas(64) T *_head;
+
+    // Tail of the queue (accessed by the consumer and producers if queue is empty)-
+    alignas(64) T *_tail;
+
+    // Pointer to the end.
+    alignas(16) T *const _end;
+
+    // Dummy item for empty queue.
+    alignas(64) char _stub[sizeof(T)];
+};
+
+template <class T> T *MPSCQueue<T>::pop_front() noexcept
+{
+    auto *tail = this->_tail;
+    auto *next = tail->next();
+
+    if (tail == this->_end)
+    {
+        if (next == nullptr)
+        {
+            return nullptr;
+        }
+
+        this->_tail = next;
+        tail = next;
+        next = next->next();
+    }
+
+    if (next != nullptr)
+    {
+        this->_tail = next;
+        return tail;
+    }
+
+    const auto *head = this->_head;
+    if (tail != head)
+    {
+        return nullptr;
+    }
+
+    this->push_back(this->_end);
+
+    next = tail->next();
+    if (next != nullptr)
+    {
+        this->_tail = next;
+        return tail;
+    }
+
+    return nullptr;
+}
+} // namespace Ealan::util
+
+#endif /* __EALANOS__INCLUDE__UTIL__MPSC_QUEUE_H_ */
\ No newline at end of file