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A Fast Priority Queue Implementation of the Dijkstra Shortest Path Algorithm

, 4 Aug 2013
Anyone needs a fast, efficient algorithm to compute the shortest path in C#? This article provides one.
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using System;
using System.Collections;

namespace DataStructTest
{
	public interface IPriorityQueue : ICollection, ICloneable, IList
	{
		int Push(object O);
		object Pop();
		object Peek();
		void Update(int i);
	}
	public class BinaryPriorityQueue : IPriorityQueue, ICollection, ICloneable, IList
	{
		protected ArrayList InnerList = new ArrayList();
		protected IComparer Comparer;

		#region contructors
		public BinaryPriorityQueue() : this(System.Collections.Comparer.Default)
		{}
		public BinaryPriorityQueue(IComparer c)
		{
			Comparer = c;
		}
		public BinaryPriorityQueue(int C) : this(System.Collections.Comparer.Default,C)
		{}
		public BinaryPriorityQueue(IComparer c, int Capacity)
		{
			Comparer = c;
			InnerList.Capacity = Capacity;
		}

		protected BinaryPriorityQueue(ArrayList Core, IComparer Comp, bool Copy)
		{
			if(Copy)
				InnerList = Core.Clone() as ArrayList;
			else
				InnerList = Core;
			Comparer = Comp;
		}

		#endregion
		protected void SwitchElements(int i, int j)
		{
			object h = InnerList[i];
			InnerList[i] = InnerList[j];
			InnerList[j] = h;
		}

		protected virtual int OnCompare(int i, int j)
		{
			return Comparer.Compare(InnerList[i],InnerList[j]);
		}

		#region public methods
		/// <summary>
		/// Push an object onto the PQ
		/// </summary>
		/// <param name="O">The new object</param>
		/// <returns>The index in the list where the object is _now_. This will change when objects are taken from or put onto the PQ.</returns>
		public int Push(object O)
		{
			int p = InnerList.Count,p2;
			InnerList.Add(O); // E[p] = O
			do
			{
				if(p==0)
					break;
				p2 = (p-1)/2;
				if(OnCompare(p,p2)<0)
				{
					SwitchElements(p,p2);
					p = p2;
				}
				else
					break;
			}while(true);
			return p;
		}

		/// <summary>
		/// Get the smallest object and remove it.
		/// </summary>
		/// <returns>The smallest object</returns>
		public object Pop()
		{
			object result = InnerList[0];
			int p = 0,p1,p2,pn;
			InnerList[0] = InnerList[InnerList.Count-1];
			InnerList.RemoveAt(InnerList.Count-1);
			do
			{
				pn = p;
				p1 = 2*p+1;
				p2 = 2*p+2;
				if(InnerList.Count>p1 && OnCompare(p,p1)>0) // links kleiner
					p = p1;
				if(InnerList.Count>p2 && OnCompare(p,p2)>0) // rechts noch kleiner
					p = p2;
				
				if(p==pn)
					break;
				SwitchElements(p,pn);
			}while(true);
			return result;
		}

		/// <summary>
		/// Notify the PQ that the object at position i has changed
		/// and the PQ needs to restore order.
		/// Since you dont have access to any indexes (except by using the
		/// explicit IList.this) you should not call this function without knowing exactly
		/// what you do.
		/// </summary>
		/// <param name="i">The index of the changed object.</param>
		public void Update(int i)
		{
			int p = i,pn;
			int p1,p2;
			do	// aufsteigen
			{
				if(p==0)
					break;
				p2 = (p-1)/2;
				if(OnCompare(p,p2)<0)
				{
					SwitchElements(p,p2);
					p = p2;
				}
				else
					break;
			}while(true);
			if(p<i)
				return;
			do	   // absteigen
			{
				pn = p;
				p1 = 2*p+1;
				p2 = 2*p+2;
				if(InnerList.Count>p1 && OnCompare(p,p1)>0) // links kleiner
					p = p1;
				if(InnerList.Count>p2 && OnCompare(p,p2)>0) // rechts noch kleiner
					p = p2;
				
				if(p==pn)
					break;
				SwitchElements(p,pn);
			}while(true);
		}

		/// <summary>
		/// Get the smallest object without removing it.
		/// </summary>
		/// <returns>The smallest object</returns>
		public object Peek()
		{
			if(InnerList.Count>0)
				return InnerList[0];
			return null;
		}

		public bool Contains(object value)
		{
			return InnerList.Contains(value);
		}

		public void Clear()
		{
			InnerList.Clear();
		}

		public int Count
		{
			get
			{
				return InnerList.Count;
			}
		}
		IEnumerator IEnumerable.GetEnumerator()
		{
			return InnerList.GetEnumerator();
		}

		public void CopyTo(Array array, int index)
		{
			InnerList.CopyTo(array,index);
		}

		public object Clone()
		{
			return new BinaryPriorityQueue(InnerList,Comparer,true);	
		}

		public bool IsSynchronized
		{
			get
			{
				return InnerList.IsSynchronized;
			}
		}

		public object SyncRoot
		{
			get
			{
				return this;
			}
		}
		#endregion
		#region explicit implementation
		bool IList.IsReadOnly
		{
			get
			{
				return false;
			}
		}

		object IList.this[int index]
		{
			get
			{
				return InnerList[index];
			}
			set
			{
				InnerList[index] = value;
				Update(index);
			}
		}

		int IList.Add(object o)
		{
			return Push(o);
		}

		void IList.RemoveAt(int index)
		{
			throw new NotSupportedException();
		}

		void IList.Insert(int index, object value)
		{
			throw new NotSupportedException();
		}

		void IList.Remove(object value)
		{
			throw new NotSupportedException();
		}

		int IList.IndexOf(object value)
		{
			throw new NotSupportedException();
		}

		bool IList.IsFixedSize
		{
			get
			{
				return false;
			}
		}

		public static BinaryPriorityQueue Syncronized(BinaryPriorityQueue P)
		{
			return new BinaryPriorityQueue(ArrayList.Synchronized(P.InnerList),P.Comparer,false);
		}
		public static BinaryPriorityQueue ReadOnly(BinaryPriorityQueue P)
		{
			return new BinaryPriorityQueue(ArrayList.ReadOnly(P.InnerList),P.Comparer,false);
		}
		#endregion
	}
}

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This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

Tolga Birdal
CEO Gravi Information Technologies and Consultancy Ltd
Turkey Turkey
Currently, also an MSc. student in Technical University of Munich, I develop practical application in computer vision for more than 5 years. I design real-time solutions to industrial and practical vision problems, both 3D and 2D. Very interested in developing algorithms in C relating math and vision.
 
Please visit Gravi's web page (http://www.gravi.com.tr) and my page (http://www.tbirdal.me) to learn more about what we develop.
 
I admire performance in algorithms.
 
"Great minds never die, they just tend to infinity..."
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