/// <summary>
/// Adds an object to the priority queue.
/// </summary>
internal void Push(T value, double priority)
{
// Increase the size of the array if necessary.
if (_count == _heap.Length)
{
Array.Resize <PriorityQueueItem <T> >(ref _heap, _count * 2);
}
PriorityQueueItem <T> queueItem = new PriorityQueueItem <T>(value, priority);
// A common usage is to Push N items, then Pop them. Optimize for that
// case by treating Push as a simple append until the first Top or Pop,
// which establishes the heap property. After that, Push needs
// to maintain the heap property.
if (_isHeap)
{
SiftUp(_count, ref queueItem, 0);
}
else
{
_heap[_count] = queueItem;
}
_count++;
}
/// <summary>
/// Removes the first node (i.e., the logical root) from the heap.
/// </summary>
internal void Pop()
{
Debug.Assert(_count != 0);
if (!_isHeap)
{
Heapify();
}
if (_count > 0)
{
--_count;
// discarding the root creates a gap at position 0. We fill the
// gap with the item x from the last position, after first sifting
// the gap to a position where inserting x will maintain the
// heap property. This is done in two phases - SiftDown and SiftUp.
//
// The one-phase method found in many textbooks does 2 comparisons
// per level, while this method does only 1. The one-phase method
// examines fewer levels than the two-phase method, but it does
// more comparisons unless x ends up in the top 2/3 of the tree.
// That accounts for only n^(2/3) items, and x is even more likely
// to end up near the bottom since it came from the bottom in the
// first place. Overall, the two-phase method is noticeably better.
PriorityQueueItem <T> x = _heap[_count]; // lift item x out from the last position
int index = SiftDown(0); // sift the gap at the root down to the bottom
SiftUp(index, ref x, 0); // sift the gap up, and insert x in its rightful position
_heap[_count] = default(PriorityQueueItem <T>); // don't leak x
}
}
// Establish the heap property: _heap[k] >= _heap[HeapParent(k)], for 0<k<_count
// Do this "bottom up", by iterating backwards. At each iteration, the
// property inductively holds for k >= HeapLeftChild(i)+2; the body of
// the loop extends the property to the children of position i (namely
// k=HLC(i) and k=HLC(i)+1) by lifting item x out from position i, sifting
// the resulting gap down to the bottom, then sifting it back up (within
// the subtree under i) until finding x's rightful position.
//
// Iteration i does work proportional to the height (distance to leaf)
// of the node at position i. Half the nodes are leaves with height 0;
// there's nothing to do for these nodes, so we skip them by initializing
// i to the last non-leaf position. A quarter of the nodes have height 1,
// an eigth have height 2, etc. so the total work is ~ 1*n/4 + 2*n/8 +
// 3*n/16 + ... = O(n). This is much cheaper than maintaining the
// heap incrementally during the "Push" phase, which would cost O(n*log n).
private void Heapify()
{
if (!_isHeap)
{
for (int i = _count / 2 - 1; i >= 0; --i)
{
// we use a two-phase method for the same reason Pop does
PriorityQueueItem <T> x = _heap[i];
int index = SiftDown(i);
SiftUp(index, ref x, i);
}
_isHeap = true;
}
}
// sift a gap at index up until it reaches the correct position for x,
// or reaches the given boundary. Place x in the resulting position.
private void SiftUp(int index, ref PriorityQueueItem <T> x, int boundary)
{
while (index > boundary)
{
int parent = HeapParent(index);
if (_comparer.Compare(_heap[parent], x) > 0)
{
_heap[index] = _heap[parent];
index = parent;
}
else
{
break;
}
}
_heap[index] = x;
}
//
// The _heap array represents a binary tree with the "shape" property.
// If we number the nodes of a binary tree from left-to-right and top-
// to-bottom as shown,
//
// 0
// / \
// / \
// 1 2
// / \ / \
// 3 4 5 6
// /\ /
// 7 8 9
//
// The shape property means that there are no gaps in the sequence of
// numbered nodes, i.e., for all N > 0, if node N exists then node N-1
// also exists. For example, the next node added to the above tree would
// be node 10, the right child of node 4.
//
// Because of this constraint, we can easily represent the "tree" as an
// array, where node number == array index, and parent/child relationships
// can be calculated instead of maintained explicitly. For example, for
// any node N > 0, the parent of N is at array index (N - 1) / 2.
//
// In addition to the above, the first _count members of the _heap array
// compose a "heap", meaning each child node is greater than or equal to
// its parent node; thus, the root node is always the minimum (i.e., the
// best match for the specified style, weight, and stretch) of the nodes
// in the heap.
//
// Initially _count < 0, which means we have not yet constructed the heap.
// On the first call to MoveNext, we construct the heap by "pushing" all
// the nodes into it. Each successive call "pops" a node off the heap
// until the heap is empty (_count == 0), at which time we've reached the
// end of the sequence.
//
#region constructors
internal PriorityQueue(int capacity = 0)
{
_heap = new PriorityQueueItem <T> [capacity > 0 ? capacity: DefaultCapacity];
_count = 0;
_comparer = new PriorityQueueComparer <T>();
}
public int Compare(PriorityQueueItem <T> x, PriorityQueueItem <T> y)
{
return(x.Priority.CompareTo(y.Priority));
}