File: Base\MS\Internal\PriorityQueue.cs
Project: wpf\src\WindowsBase.csproj (WindowsBase)
using System;
using System.Collections.Generic;
using System.Diagnostics;
 
namespace System.Windows.Threading
{
    internal class PriorityQueue<T>
    {
        public PriorityQueue()
        {
            // Build the collection of priority chains.
            _priorityChains = new SortedList<int, PriorityChain<T>>(); // NOTE: should be Priority
            _cacheReusableChains = new Stack<PriorityChain<T>>(10);
                
            _head = _tail = null;
            _count = 0;
        }
 
        // NOTE: not used
        // public int Count {get{return _count;}}
        
        public DispatcherPriority MaxPriority // NOTE: should be Priority
        {
            get
            {
                int count = _priorityChains.Count;
                
                if(count > 0)
                {
                    return (DispatcherPriority) _priorityChains.Keys[count - 1];
                }
                else
                {
                    return DispatcherPriority.Invalid; // NOTE: should be Priority.Invalid;
                }
            }
        }
 
        public PriorityItem<T> Enqueue(DispatcherPriority 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;
        }
 
        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();
                
            }
        }
 
        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 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, DispatcherPriority 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);
        }
 
        private PriorityChain<T> GetChain(DispatcherPriority priority) // NOTE: should be Priority
        {
            PriorityChain<T> chain = null;
 
            int count = _priorityChains.Count;
            if(count > 0)
            {
                if(priority == (DispatcherPriority) _priorityChains.Keys[0])
                {
                    chain = _priorityChains.Values[0];
                }
                else if(priority == (DispatcherPriority) _priorityChains.Keys[count - 1])
                {
                    chain = _priorityChains.Values[count - 1];
                }
                else if((priority > (DispatcherPriority) _priorityChains.Keys[0]) &&
                        (priority < (DispatcherPriority) _priorityChains.Keys[count - 1]))
                {
                    _priorityChains.TryGetValue((int)priority, out chain);
                }
            }
 
            if(chain == null)            
            {
                if(_cacheReusableChains.Count > 0)
                {
                    chain = _cacheReusableChains.Pop();
                    chain.Priority = priority;
                }
                else
                {
                    chain = new PriorityChain<T>(priority);
                }
                
                _priorityChains.Add((int)priority, chain);
            }
 
            return chain;
        }
        
        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);
            }
        }
 
        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++;
        }
 
        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 == (DispatcherPriority) _priorityChains.Keys[_priorityChains.Count - 1])
                {
                    _priorityChains.RemoveAt(_priorityChains.Count - 1);
                }
                else
                {
                    _priorityChains.Remove((int) item.Chain.Priority);
                }
 
                if(_cacheReusableChains.Count < 10)
                {
                    item.Chain.Priority = DispatcherPriority.Invalid; // NOTE: should be Priority.Invalid
                    _cacheReusableChains.Push(item.Chain);
                }
            }
            
            item.Chain = null;
        }
 
        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++;
        }
 
        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--;
        }
 
        // Priority chains...
        private SortedList<int, PriorityChain<T>> _priorityChains; // NOTE: should be Priority
        private Stack<PriorityChain<T>> _cacheReusableChains;
        
        // Sequential chain...
        private PriorityItem<T> _head;
        private PriorityItem<T> _tail;
        private int _count;
    }
}