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package org.pkl.thirdparty.graalvm.collections;
import java.util.concurrent.atomic.AtomicReference;
/**
* Concurrent lock-free pool data structure.
*
* Supports two operations -- adding an element, and removing an element. The
* {@link LockFreePool#add(Object)} operation adds an element into the pool. The
* {@link LockFreePool#get()} operation returns one of the elements previously added to the pool.
* There is no guarantee about the order in which the elements are returned by
* {@link LockFreePool#get()}. The guarantee is that an element will only be returned by
* {@link LockFreePool#get()} as many times as it was previously added to the pool by calling
* {@link LockFreePool#add(Object)}. If there are no more elements to return, it will return
* {@code null}. Both operations are lock-free and linearizable -- this data structure is intended
* for use by multiple threads.
*
* The internal implementation is a simple Treiber stack.
*
* @param Type of the elements in this pool.
*
* @since 23.0
*/
public class LockFreePool {
/**
* The top-of-the-Treiber-stack pointer.
*/
private final AtomicReference> head;
public LockFreePool() {
this.head = new AtomicReference<>();
}
/**
* Returns a previously added element.
*
* This method returns a previously added element only once to some caller. If the element was
* added multiple times, calling this method will return that element as many times before
* returning {@code null}.
*
* This method does not do any object allocations.
*
* @return A previously added element, or {@code null} if there are no previously added elements
* that have not been already returned.
*
* @since 23.0
*/
public T get() {
while (true) {
Node oldHead = head.get();
if (oldHead == null) {
return null;
}
Node newHead = oldHead.tail;
if (head.compareAndSet(oldHead, newHead)) {
return oldHead.element;
}
}
}
/**
* Adds an element to this pool.
*
* An element can be added multiple times to the pool, but in this case may be returned as many
* times by {@link LockFreePool#get()}.
*
* This method internally allocates objects on the heap.
*
* @param element An element to add to the pool.
*
* @since 23.0
*/
public void add(T element) {
while (true) {
Node oldHead = head.get();
Node newHead = new Node<>(element, oldHead);
if (head.compareAndSet(oldHead, newHead)) {
return;
}
}
}
/**
* Internal wrapper node used to wrap the element and the {@code tail} pointer.
*/
private static final class Node {
/**
* Element stored in this node.
*/
final E element;
/**
* Pointer to the tail of the linked list, or {@code null} if the end of the list.
*/
final Node tail;
private Node(E element, Node tail) {
this.element = element;
this.tail = tail;
}
}
}