private void InsertItemInPriorityChain(PriorityItem <T> item, PriorityChain <T> chain)
{
// Scan along the sequential chain, in the previous direction,
// looking for an item that is already in the new chain. We will
// insert ourselves after the item we found. We can short-circuit
// this search if the new chain is empty.
if (chain.Head == null)
{
Debug.Assert(chain.Tail == null, "PriorityQueue.InsertItemInPriorityChain: both the head and the tail should be null.");
InsertItemInPriorityChain(item, chain, null);
}
else
{
Debug.Assert(chain.Tail != null, "PriorityQueue.InsertItemInPriorityChain: both the head and the tail should not be null.");
PriorityItem <T> after = null;
// Search backwards along the sequential chain looking for an
// item already in this list.
for (after = item.SequentialPrev; after != null; after = after.SequentialPrev)
{
if (after.Chain == chain)
{
break;
}
}
InsertItemInPriorityChain(item, chain, after);
}
}
private void RemoveItemFromPriorityChain(PriorityItem <T> item)
{
Debug.Assert(item != null, "PriorityQueue.RemoveItemFromPriorityChain: invalid item.");
Debug.Assert(item.Chain != null, "PriorityQueue.RemoveItemFromPriorityChain: a chain should exist.");
// Step 1: Fix up the previous link
if (item.PriorityPrev != null)
{
Debug.Assert(item.Chain.Head != item, "PriorityQueue.RemoveItemFromPriorityChain: the head should not point to this item.");
item.PriorityPrev.PriorityNext = item.PriorityNext;
}
else
{
Debug.Assert(item.Chain.Head == item, "PriorityQueue.RemoveItemFromPriorityChain: the head should point to this item.");
item.Chain.Head = item.PriorityNext;
}
// Step 2: Fix up the next link
if (item.PriorityNext != null)
{
Debug.Assert(item.Chain.Tail != item, "PriorityQueue.RemoveItemFromPriorityChain: the tail should not point to this item.");
item.PriorityNext.PriorityPrev = item.PriorityPrev;
}
else
{
Debug.Assert(item.Chain.Tail == item, "PriorityQueue.RemoveItemFromPriorityChain: the tail should point to this item.");
item.Chain.Tail = item.PriorityPrev;
}
// Step 3: cleanup
item.PriorityPrev = item.PriorityNext = null;
item.Chain.Count--;
if (item.Chain.Count == 0)
{
if (item.Chain.Priority == (int)_priorityChains.Keys[_priorityChains.Count - 1])
{
_priorityChains.RemoveAt(_priorityChains.Count - 1);
}
else
{
_priorityChains.Remove((int)item.Chain.Priority);
}
if (_cacheReusableChains.Count < 10)
{
item.Chain.Priority = int.MaxValue; // NOTE: should be Priority.Invalid
_cacheReusableChains.Push(item.Chain);
}
}
item.Chain = null;
}
public void RemoveItem(PriorityItem <T> item)
{
Debug.Assert(item != null, "PriorityQueue.RemoveItem: invalid item.");
Debug.Assert(item.Chain != null, "PriorityQueue.RemoveItem: a chain should exist.");
PriorityChain <T> chain = item.Chain;
// Step 1: Remove the item from its priority chain.
RemoveItemFromPriorityChain(item);
// Step 2: Remove the item from the sequential chain.
RemoveItemFromSequentialChain(item);
// Note: we do not clean up empty chains on purpose to reduce churn.
}
public void ChangeItemPriority(PriorityItem <T> item, int priority) // NOTE: should be Priority
{
// Remove the item from its current priority and insert it into
// the new priority chain. Note that this does not change the
// sequential ordering.
// Step 1: Remove the item from the priority chain.
RemoveItemFromPriorityChain(item);
// Step 2: Insert the item into the new priority chain.
// Find the existing chain for this priority, or create a new one
// if one does not exist.
PriorityChain <T> chain = GetChain(priority);
InsertItemInPriorityChain(item, chain);
}
internal void InsertItemInPriorityChain(PriorityItem <T> item, PriorityChain <T> chain, PriorityItem <T> after)
{
Debug.Assert(chain != null, "PriorityQueue.InsertItemInPriorityChain: a chain must be provided.");
Debug.Assert(item.Chain == null && item.PriorityPrev == null && item.PriorityNext == null, "PriorityQueue.InsertItemInPriorityChain: item must not already be in a priority chain.");
item.Chain = chain;
if (after == null)
{
// Note: passing null for after means insert at the head.
if (chain.Head != null)
{
Debug.Assert(chain.Tail != null, "PriorityQueue.InsertItemInPriorityChain: both the head and the tail should not be null.");
chain.Head.PriorityPrev = item;
item.PriorityNext = chain.Head;
chain.Head = item;
}
else
{
Debug.Assert(chain.Tail == null, "PriorityQueue.InsertItemInPriorityChain: both the head and the tail should be null.");
chain.Head = chain.Tail = item;
}
}
else
{
item.PriorityPrev = after;
if (after.PriorityNext != null)
{
item.PriorityNext = after.PriorityNext;
after.PriorityNext.PriorityPrev = item;
after.PriorityNext = item;
}
else
{
Debug.Assert(item.Chain.Tail == after, "PriorityQueue.InsertItemInPriorityChain: the chain's tail should be the item we are inserting after.");
after.PriorityNext = item;
chain.Tail = item;
}
}
chain.Count++;
}
public PriorityItem <T> Enqueue(int priority, T data) // NOTE: should be Priority
{
// Find the existing chain for this priority, or create a new one
// if one does not exist.
PriorityChain <T> chain = GetChain(priority);
// Wrap the item in a PriorityItem so we can put it in our
// linked list.
PriorityItem <T> priorityItem = new PriorityItem <T>(data);
// Step 1: Append this to the end of the "sequential" linked list.
InsertItemInSequentialChain(priorityItem, _tail);
// Step 2: Append the item into the priority chain.
InsertItemInPriorityChain(priorityItem, chain, chain.Tail);
return(priorityItem);
}
internal void InsertItemInSequentialChain(PriorityItem <T> item, PriorityItem <T> after)
{
Debug.Assert(item.SequentialPrev == null && item.SequentialNext == null, "PriorityQueue.InsertItemInSequentialChain: item must not already be in the sequential chain.");
if (after == null)
{
// Note: passing null for after means insert at the head.
if (_head != null)
{
Debug.Assert(_tail != null, "PriorityQueue.InsertItemInSequentialChain: both the head and the tail should not be null.");
_head.SequentialPrev = item;
item.SequentialNext = _head;
_head = item;
}
else
{
Debug.Assert(_tail == null, "PriorityQueue.InsertItemInSequentialChain: both the head and the tail should be null.");
_head = _tail = item;
}
}
else
{
item.SequentialPrev = after;
if (after.SequentialNext != null)
{
item.SequentialNext = after.SequentialNext;
after.SequentialNext.SequentialPrev = item;
after.SequentialNext = item;
}
else
{
Debug.Assert(_tail == after, "PriorityQueue.InsertItemInSequentialChain: the tail should be the item we are inserting after.");
after.SequentialNext = item;
_tail = item;
}
}
_count++;
}
public T Peek()
{
T data = default(T);
// Get the max-priority chain.
int count = _priorityChains.Count;
if (count > 0)
{
PriorityChain <T> chain = _priorityChains.Values[count - 1];
Debug.Assert(chain != null, "PriorityQueue.Peek: a chain should exist.");
PriorityItem <T> item = chain.Head;
Debug.Assert(item != null, "PriorityQueue.Peek: a priority item should exist.");
data = item.Data;
}
return(data);
}
public T Dequeue()
{
// Get the max-priority chain.
int count = _priorityChains.Count;
if (count > 0)
{
PriorityChain <T> chain = _priorityChains.Values[count - 1];
Debug.Assert(chain != null, "PriorityQueue.Dequeue: a chain should exist.");
PriorityItem <T> item = chain.Head;
Debug.Assert(item != null, "PriorityQueue.Dequeue: a priority item should exist.");
RemoveItem(item);
return(item.Data);
}
else
{
throw new InvalidOperationException();
}
}
private void RemoveItemFromSequentialChain(PriorityItem <T> item)
{
Debug.Assert(item != null, "PriorityQueue.RemoveItemFromSequentialChain: invalid item.");
// Step 1: Fix up the previous link
if (item.SequentialPrev != null)
{
Debug.Assert(_head != item, "PriorityQueue.RemoveItemFromSequentialChain: the head should not point to this item.");
item.SequentialPrev.SequentialNext = item.SequentialNext;
}
else
{
Debug.Assert(_head == item, "PriorityQueue.RemoveItemFromSequentialChain: the head should point to this item.");
_head = item.SequentialNext;
}
// Step 2: Fix up the next link
if (item.SequentialNext != null)
{
Debug.Assert(_tail != item, "PriorityQueue.RemoveItemFromSequentialChain: the tail should not point to this item.");
item.SequentialNext.SequentialPrev = item.SequentialPrev;
}
else
{
Debug.Assert(_tail == item, "PriorityQueue.RemoveItemFromSequentialChain: the tail should point to this item.");
_tail = item.SequentialPrev;
}
// Step 3: cleanup
item.SequentialPrev = item.SequentialNext = null;
_count--;
}