/// <summary>
/// Lock free peek using ordered loads. As class name suggests access is limited to a single thread.
/// </summary>
/// <param name="item">The peeked item.</param>
/// <returns><c>true</c> if an item was retrieved, otherwise <c>false</c>.</returns>
/// <seealso cref="IQueue{T}.TryPeek"/>
public override bool TryPeek(out T item)
{
// Copy field to avoid re-reading after volatile load
T[] buffer = this.Buffer;
long consumerIndex = this.ConsumerIndex; // LoadLoad
long offset = this.CalcElementOffset(consumerIndex);
T e = RefArrayAccessUtil.LvElement(buffer, offset);
if (null == e)
{
// NOTE: Queue may not actually be empty in the case of a producer (P1) being interrupted after
// winning the CAS on offer but before storing the element in the queue. Other producers may go on
// to fill up the queue after this element.
if (consumerIndex != this.ProducerIndex)
{
do
{
e = RefArrayAccessUtil.LvElement(buffer, offset);
}while (e == null);
}
else
{
item = default(T);
return(false);
}
}
item = e;
return(true);
}
/// <summary>
/// Lock free poll using ordered loads/stores. As class name suggests, access is limited to a single thread.
/// </summary>
/// <param name="item">The dequeued item.</param>
/// <returns><c>true</c> if an item was retrieved, otherwise <c>false</c>.</returns>
/// <seealso cref="IQueue{T}.TryDequeue"/>
public override bool TryDequeue(out T item)
{
long consumerIndex = this.ConsumerIndex; // LoadLoad
long offset = this.CalcElementOffset(consumerIndex);
// Copy field to avoid re-reading after volatile load
T[] buffer = this.Buffer;
// If we can't see the next available element we can't poll
T e = RefArrayAccessUtil.LvElement(buffer, offset); // LoadLoad
if (e is null)
{
// NOTE: Queue may not actually be empty in the case of a producer (P1) being interrupted after
// winning the CAS on offer but before storing the element in the queue. Other producers may go on
// to fill up the queue after this element.
if (consumerIndex != this.ProducerIndex)
{
do
{
e = RefArrayAccessUtil.LvElement(buffer, offset);
}while (e is null);
}
else
{
item = default;
return(false);
}
}
RefArrayAccessUtil.SpElement(buffer, offset, default);
this.ConsumerIndex = consumerIndex + 1; // StoreStore
item = e;
return(true);
}
/// A volatile load (load + LoadLoad barrier) of an element from a given offset.
/// @param offset computed via {@link ConcurrentCircularArrayQueue#calcElementOffset(long)}
/// @return the element at the offset
protected T LvElement(long offset) => RefArrayAccessUtil.LvElement(this.Buffer, offset);