/// A wait free alternative to offer which fails on CAS failure.
/// @param e new element, not null
/// @return 1 if next element cannot be filled, -1 if CAS failed, 0 if successful
public int WeakEnqueue(T e)
{
Contract.Requires(e != null);
long mask = this.Mask;
long capacity = mask + 1;
long currentTail = this.ProducerIndex; // LoadLoad
long consumerIndexCache = this.ConsumerIndexCache; // LoadLoad
long wrapPoint = currentTail - capacity;
if (consumerIndexCache <= wrapPoint)
{
long currHead = this.ConsumerIndex; // LoadLoad
if (currHead <= wrapPoint)
{
return(1); // FULL :(
}
else
{
this.ConsumerIndexCache = currHead; // StoreLoad
}
}
// look Ma, no loop!
if (!this.TrySetProducerIndex(currentTail, currentTail + 1))
{
return(-1); // CAS FAIL :(
}
// Won CAS, move on to storing
long offset = RefArrayAccessUtil.CalcElementOffset(currentTail, mask);
this.SoElement(offset, e);
return(0); // AWESOME :)
}
/// {@inheritDoc}
/// <p />
/// IMPLEMENTATION NOTES:
/// <br />
/// Lock free peek using ordered loads. As class name suggests access is limited to a single thread.
/// @see java.util.Queue#poll()
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);
}
/// {@inheritDoc}
/// <br />
/// IMPLEMENTATION NOTES:
/// <br />
/// Lock free offer using a single CAS. As class name suggests access is permitted to many threads
/// concurrently.
/// @see java.util.Queue#offer(java.lang.Object)
public override bool TryEnqueue(T e)
{
Contract.Requires(e != null);
// use a cached view on consumer index (potentially updated in loop)
long mask = this.Mask;
long capacity = mask + 1;
long consumerIndexCache = this.ConsumerIndexCache; // LoadLoad
long currentProducerIndex;
do
{
currentProducerIndex = this.ProducerIndex; // LoadLoad
long wrapPoint = currentProducerIndex - capacity;
if (consumerIndexCache <= wrapPoint)
{
long currHead = this.ConsumerIndex; // LoadLoad
if (currHead <= wrapPoint)
{
return(false); // FULL :(
}
else
{
// update shared cached value of the consumerIndex
this.ConsumerIndexCache = currHead; // StoreLoad
// update on stack copy, we might need this value again if we lose the CAS.
consumerIndexCache = currHead;
}
}
}while (!this.TrySetProducerIndex(currentProducerIndex, currentProducerIndex + 1));
// NOTE: the new producer index value is made visible BEFORE the element in the array. If we relied on
// the index visibility to poll() we would need to handle the case where the element is not visible.
// Won CAS, move on to storing
long offset = RefArrayAccessUtil.CalcElementOffset(currentProducerIndex, mask);
this.SoElement(offset, e); // StoreStore
return(true); // AWESOME :)
}
/// 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);
/// An ordered store(store + StoreStore barrier) of an element to a given offset
/// @param offset computed via {@link ConcurrentCircularArrayQueue#calcElementOffset(long)}
/// @param e an orderly kitty
protected void SoElement(long offset, T e) => RefArrayAccessUtil.SoElement(this.Buffer, offset, e);
/// @param index desirable element index /// @return the offset in bytes within the array for a given index. protected long CalcElementOffset(long index) => RefArrayAccessUtil.CalcElementOffset(index, this.Mask);