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// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.
//-----------------------------------------------------------------------
//
// Microsoft Windows Client Platform
//
//
// File: PriorityQueue.cs
//
// Contents: Implementation of PriorityQueue class.
//
// Created: 2-14-2005 Niklas Borson (niklasb)
//
//------------------------------------------------------------------------
// !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
//
// Do not use this class; it has a serious bug.
//
// A correct implementation is available in PriorityQueue.cs. That implementation
// also includes a performance optimization for the frequent case where the client
// does all the Push operations before doing any Top or Pop operations.
//
// .Net Shiproom insists on keeping the old (buggy) code for existing uses,
// citing concerns about compatibility. And they won't allow the new (fixed and
// perf-improved) code into the build if it's not being used. If you have a
// new use for PriorityQueue, add the new class to the build and use it instead
// of this class.
//
// The bug is that the output is out of order, in the sense that Top can return
// an item that is not the lowest-priority of all items in the queue. This only
// happens for sufficiently large inputs, and only for some sequences of operations.
// The smallest example involves 6 items; when Pushed in a certain order, a
// sequence of 3 Pops can leave the queue in a state where Top returns an item
// that is not the lowest-priority of the 3 remaining items.
//
// In terms of current use, the bug would arise when trying to find a physical
// font to display a particular character. Font files are assigned priority
// based on how closely they match the desired size, weight, and stretch.
// If there are (at least) 6 candidate font files, discovered in a certain order,
// and if the first 3 do not contain a glyph for the character, WPF could choose
// a font file that is not the best match among the remaining candidates.
// In other words, WPF might display the character using a glyph from a
// less-than-best physical font, when the best candidate fonts don't match.
// (We have no reports of this actually happening, possibly because it might escape
// notice by all but the most font-obsessed users.)
namespace MS.Internal
{
/// <summary>
/// PriorityQueue provides a stack-like interface, except that objects
/// "pushed" in arbitrary order are "popped" in order of priority, i.e.,
/// from least to greatest as defined by the specified comparer.
/// </summary>
/// <remarks>
/// Push and Pop are each O(log N). Pushing N objects and them popping
/// them all is equivalent to performing a heap sort and is O(N log N).
/// </remarks>
internal class LegacyPriorityQueue<T>
{
//
// 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 LegacyPriorityQueue(int capacity, IComparer<T> comparer)
{
_heap = new T[capacity > 0 ? capacity : DefaultCapacity];
_count = 0;
_comparer = comparer;
}
#endregion
#region internal members
/// <summary>
/// Gets the number of items in the priority queue.
/// </summary>
internal int Count
{
get { return _count; }
}
/// <summary>
/// Gets the first or topmost object in the priority queue, which is the
/// object with the minimum value.
/// </summary>
internal T Top
{
get
{
Debug.Assert(_count > 0);
return _heap[0];
}
}
/// <summary>
/// Adds an object to the priority queue.
/// </summary>
internal void Push(T value)
{
// Increase the size of the array if necessary.
if (_count == _heap.Length)
{
T[] temp = new T[_count * 2];
for (int i = 0; i < _count; ++i)
{
temp[i] = _heap[i];
}
_heap = temp;
}
// Loop invariant:
//
// 1. index is a gap where we might insert the new node; initially
// it's the end of the array (bottom-right of the logical tree).
//
int index = _count;
while (index > 0)
{
int parentIndex = HeapParent(index);
if (_comparer.Compare(value, _heap[parentIndex]) < 0)
{
// value is a better match than the parent node so exchange
// places to preserve the "heap" property.
_heap[index] = _heap[parentIndex];
index = parentIndex;
}
else
{
// we can insert here.
break;
}
}
_heap[index] = value;
_count++;
}
/// <summary>
/// Removes the first node (i.e., the logical root) from the heap.
/// </summary>
internal void Pop()
{
Debug.Assert(_count != 0);
if (_count > 1)
{
// Loop invariants:
//
// 1. parent is the index of a gap in the logical tree
// 2. leftChild is
// (a) the index of parent's left child if it has one, or
// (b) a value >= _count if parent is a leaf node
//
int parent = 0;
int leftChild = HeapLeftChild(parent);
while (leftChild < _count)
{
int rightChild = HeapRightFromLeft(leftChild);
int bestChild =
(rightChild < _count && _comparer.Compare(_heap[rightChild], _heap[leftChild]) < 0) ?
rightChild : leftChild;
// Promote bestChild to fill the gap left by parent.
_heap[parent] = _heap[bestChild];
// Restore invariants, i.e., let parent point to the gap.
parent = bestChild;
leftChild = HeapLeftChild(parent);
}
// Fill the last gap by moving the last (i.e., bottom-rightmost) node.
_heap[parent] = _heap[_count - 1];
}
_count--;
}
#endregion
#region private members
/// <summary>
/// Calculate the parent node index given a child node's index, taking advantage
/// of the "shape" property.
/// </summary>
private static int HeapParent(int i)
{
return (i - 1) / 2;
}
/// <summary>
/// Calculate the left child's index given the parent's index, taking advantage of
/// the "shape" property. If there is no left child, the return value is >= _count.
/// </summary>
private static int HeapLeftChild(int i)
{
return (i * 2) + 1;
}
/// <summary>
/// Calculate the right child's index from the left child's index, taking advantage
/// of the "shape" property (i.e., sibling nodes are always adjacent). If there is
/// no right child, the return value >= _count.
/// </summary>
private static int HeapRightFromLeft(int i)
{
return i + 1;
}
private T[] _heap;
private int _count;
private IComparer<T> _comparer;
private const int DefaultCapacity = 6;
#endregion
}
}
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