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Persistent Data Structures

, 24 Feb 2005 MIT
An article describing the basic principles of persistent data structures.
persistentdatastructures_src.zip
ImmutableCollections
ImmutableCollections
AVL Tree Classes
ImmutableCollections.csproj.user
RAL Helper Classes
/*
 * Created by: Leslie Sanford 
 * 
 * Last modified: 02/23/2005
 * 
 * Contact: jabberdabber@hotmail.com
 */

using System;
using System.Collections;
using System.Diagnostics;

namespace ImmutableCollections
{
    /// <summary>
    /// Provides functionality for iterating over an AVL tree.
    /// </summary> 
    internal class AvlEnumerator : IEnumerator
    {
        #region AvlEnumerator Members

        #region Instance Fields

        // The root of the AVL tree.
        private IAvlNode root;

        // The number of nodes in the tree.
        private readonly int count;

        // The object at the current position.
        private object current = null;

        // The current index.
        private int index;

        // Used for traversing the tree inorder.
        private System.Collections.Stack nodeStack = new System.Collections.Stack();

        #endregion

        #region Construction

        /// <summary>
        /// Initializes a new instance of the AvlEnumerator class.
        /// </summary>
        /// <param name="root">
        /// The root of the AVL tree to iterate over.
        /// </param>
        public AvlEnumerator(IAvlNode root)
        {
            this.root = root;
            this.count = root.Count;

            Reset();
        }

        /// <summary>
        /// Initializes a new instance of the AvlEnumerator class.
        /// </summary>
        /// <param name="root">
        /// The root of the AVL tree to iterate over.
        /// </param>
        /// <param name="count">
        /// The number of nodes in the tree.
        /// </param>
        public AvlEnumerator(IAvlNode root, int count)
        {
            Debug.Assert(count <= root.Count);

            this.root = root;
            this.count = count;

            Reset();
        }

        #endregion

        #endregion

        #region IEnumerator Members

        /// <summary>
        /// Sets the enumerator to its initial position, which is before 
        /// the first element in the AVL tree.
        /// </summary>
        public void Reset()
        {
            index = 0;

            nodeStack.Clear();

            IAvlNode currentNode = root;

            // Push nodes on to the stack to get to the first item.
            while(currentNode != AvlNode.NullNode)
            {
                nodeStack.Push(currentNode);
                currentNode = currentNode.LeftChild;
            }
        }

        /// <summary>
        /// Gets the current element in the AVL tree.
        /// </summary>
        /// <exception cref="InvalidOperationException">
        /// The enumerator is positioned before the first element in the AVL
        /// tree or after the last element.
        /// </exception>
        public object Current
        {
            get
            {
                if(index == 0)
                {
                    throw new InvalidOperationException(
                        "The enumerator is positioned before the first " +
                        "element of the collection or after the last " +
                        "element.");
                }

                return current;
            }
        }

        /// <summary>
        /// Advances the enumerator to the next element of the AVL tree.
        /// </summary>
        /// <returns>
        /// <b>true</b> if the enumerator was successfully advanced to the 
        /// next element; <b>false</b> if the enumerator has passed the end 
        /// of the collection.
        /// </returns>
        public bool MoveNext()
        {
            bool result;

            // If the end of the AVL tree has not yet been reached.
            if(index < count)
            {
                // Get the next node.
                IAvlNode currentNode = (IAvlNode)nodeStack.Pop();

                current = currentNode.Data;

                currentNode = currentNode.RightChild;

                while(currentNode != AvlNode.NullNode)
                {
                    nodeStack.Push(currentNode);
                    currentNode = currentNode.LeftChild;
                }

                index++;

                result = true;
            }
            else
            {
                result = false;
            }

            return result;
        }

        #endregion
    }
}

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This article, along with any associated source code and files, is licensed under The MIT License

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About the Author

Leslie Sanford

United States United States
Aside from dabbling in BASIC on his old Atari 1040ST years ago, Leslie's programming experience didn't really begin until he discovered the Internet in the late 90s. There he found a treasure trove of information about two of his favorite interests: MIDI and sound synthesis.
 
After spending a good deal of time calculating formulas he found on the Internet for creating new sounds by hand, he decided that an easier way would be to program the computer to do the work for him. This led him to learn C. He discovered that beyond using programming as a tool for synthesizing sound, he loved programming in and of itself.
 
Eventually he taught himself C++ and C#, and along the way he immersed himself in the ideas of object oriented programming. Like many of us, he gotten bitten by the design patterns bug and a copy of GOF is never far from his hands.
 
Now his primary interest is in creating a complete MIDI toolkit using the C# language. He hopes to create something that will become an indispensable tool for those wanting to write MIDI applications for the .NET framework.
 
Besides programming, his other interests are photography and playing his Les Paul guitars.

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