public PriorityQueueNode <T> Enqueue(T item)
{
var result = new PriorityQueueNode <T>(item);
Enqueue(result);
return(result);
}
/// <summary>Add an element to the priority queue - O(log(n)) time operation.</summary>
/// <param name="item">The item to be added to the queue</param>
public void Enqueue([NotNull] PriorityQueueNode <T> item)
{
if (item.Index != -1)
{
throw new InvalidOperationException("Item belongs to another PriorityNodeQueue.");
}
// We add the item to the end of the list (at the bottom of the
// tree). Then, the heap-property could be violated between this element
// and it's parent. If this is the case, we swap this element with the
// parent (a safe operation to do since the element is known to be less
// than it's parent). Now the element move one level up the tree. We repeat
// this test with the element and it's new parent. The element, if lesser
// than everybody else in the tree will eventually bubble all the way up
// to the root of the tree (or the head of the list). It is easy to see
// this will take log(N) time, since we are working with a balanced binary
// tree.
var n = items.Count;
items.Add(item);
item.Index = n;
while (n != 0)
{
var p = n / 2; // This is the 'parent' of this item
if (comparer.Compare(items[n].Value, items[p].Value) >= 0)
{
break; // Item >= parent
}
// Swap item and parent
var tmp = items[n];
var tmp2 = items[p];
tmp2.Index = n;
tmp.Index = p;
items[n] = tmp2;
items[p] = tmp;
n = p; // And continue
}
}
/// <summary>
/// Removes the specified item.
/// </summary>
/// <param name="item">The item to remove.</param>
public void Remove([NotNull] PriorityQueueNode <T> item)
{
var index = item.Index;
if (index == -1)
{
return;
}
// The element to return is of course the first element in the array,
// or the root of the tree. However, this will leave a 'hole' there. We
// fill up this hole with the last element from the array. This will
// break the heap property. So we bubble the element downwards by swapping
// it with it's lower child until it reaches it's correct level. The lower
// child (one of the orignal elements with index 1 or 2) will now be at the
// head of the queue (root of the tree).
var nMax = items.Count - 1;
var itemMax = items[nMax];
itemMax.Index = index;
var itemToRemove = items[index];
itemToRemove.Index = -1;
items[index] = itemMax;
items.RemoveAt(nMax); // Move the last element to the top
var p = index;
while (true)
{
// c is the child we want to swap with. If there
// is no child at all, then the heap is balanced
var c = p * 2; if (c >= nMax)
{
break;
}
// If the second child is smaller than the first, that's the one
// we want to swap with this parent.
if (c + 1 < nMax && comparer.Compare(items[c + 1].Value, items[c].Value) < 0)
{
c++;
}
// If the parent is already smaller than this smaller child, then
// we are done
if (comparer.Compare(items[p].Value, items[c].Value) <= 0)
{
break;
}
// Othewise, swap parent and child, and follow down the parent
var tmp = items[p];
var tmp2 = items[c];
tmp.Index = c;
tmp2.Index = p;
items[p] = tmp2;
items[c] = tmp;
p = c;
}
item.Index = -1;
}
