If outbox is empty, refill it by popping each element from inbox and pushing it onto outbox
Pop and return the top element from outbox
Using this method, each element will be in each stack exactly once - meaning each element will be pushed twice and popped twice, giving amortized constant time operations.
Here's an implementation in Java:
public class Queue<E>
{
private Stack<E> inbox = new Stack<E>();
private Stack<E> outbox = new Stack<E>();
public void queue(E item) {
inbox.push(item);
}
public E dequeue() {
if (outbox.isEmpty()) {
while (!inbox.isEmpty()) {
outbox.push(inbox.pop());
}
}
return outbox.pop();
}
}
That looks very unsafe (very little synchronization); how about something like:
class SizeQueue<T>
{
private readonly Queue<T> queue = new Queue<T>();
private readonly int maxSize;
public SizeQueue(int maxSize) { this.maxSize = maxSize; }
public void Enqueue(T item)
{
lock (queue)
{
while (queue.Count >= maxSize)
{
Monitor.Wait(queue);
}
queue.Enqueue(item);
if (queue.Count == 1)
{
// wake up any blocked dequeue
Monitor.PulseAll(queue);
}
}
}
public T Dequeue()
{
lock (queue)
{
while (queue.Count == 0)
{
Monitor.Wait(queue);
}
T item = queue.Dequeue();
if (queue.Count == maxSize - 1)
{
// wake up any blocked enqueue
Monitor.PulseAll(queue);
}
return item;
}
}
}
(edit)
In reality, you'd want a way to close the queue so that readers start exiting cleanly - perhaps something like a bool flag - if set, an empty queue just returns (rather than blocking):
bool closing;
public void Close()
{
lock(queue)
{
closing = true;
Monitor.PulseAll(queue);
}
}
public bool TryDequeue(out T value)
{
lock (queue)
{
while (queue.Count == 0)
{
if (closing)
{
value = default(T);
return false;
}
Monitor.Wait(queue);
}
value = queue.Dequeue();
if (queue.Count == maxSize - 1)
{
// wake up any blocked enqueue
Monitor.PulseAll(queue);
}
return true;
}
}
Best Answer
I don't know if this is canonical but, does this count?
Also here are examples of queues in SDL-88
http://www.sdl-forum.org/sdl88tutorial/4.ProcessCommunication/4.1_Signal_input_queue.htm