SkipListNodeList
/// <summary>
/// Represents a collection of SkipListNodes. This class differs from the base class - NodeList -
/// in that it contains an internal method to increment or decrement the height of the SkipListNodeList.
/// Incrementing the height adds a new neighbor to the list, decrementing the height removes the
/// top-most neighbor.
/// </summary>
/// <typeparam name="T">The type of data stored in the SkipListNode instances that are contained
/// within this SkipListNodeList.</typeparam>
public class SkipListNodeList<T> : NodeList<T>
{
#region Constructors
public SkipListNodeList(int height)
: base(height)
{
}
#endregion
#region Internal Methods
/// <summary>
/// Increases the size of the SkipListNodeList by one, adding a default SkipListNode.
/// </summary>
internal void IncrementHeight()
{
// add a dummy entry
base.Items.Add(default(Node<T>));
}
/// <summary>
/// Decreases the size of the SkipListNodeList by one, removing the "top-most" SkipListNode.
/// </summary>
internal void DecrementHeight()
{
// delete the last entry
base.Items.RemoveAt(base.Items.Count - 1);
}
#endregion
}
/// <summary>
/// Represnts a SkipList. A SkipList is a combination of a BST and a sorted link list, providing
/// sub-linear access, insert, and deletion running times. It is a randomized data structure, randomly
/// choosing the heights of the nodes in the SkipList.
/// </summary>
/// <typeparam name="T">Type type of elements contained within the SkipList.</typeparam>
public class SkipList<T> : ICollection<T>, IEnumerable<T>
{
#region Fields
protected readonly double _prob = 0.5; // the probability used in determining the heights of the SkipListNodes
private SkipListNode<T> _head; // a reference to the head of the SkipList
private long _comparisons; // an internal counter used for performance testing of the SkipList
private int _count;
private Random _rndNum;
private IComparer<T> comparer;
#endregion
#region Properties
/// <summary>
/// Returns the height of the tallest SkipListNode in the SkipList.
/// </summary>
public int Height
{
get { return _head.Height; }
}
/// <summary>
/// Returns the number of total comparisons made - used for perf. testing.
/// </summary>
/// <value></value>
public long Comparisons
{
get { return _comparisons; }
}
#region ICollection Properties
/// <summary>
/// Returns the number of elements in the SkipList
/// </summary>
public int Count
{
get { return _count; }
}
bool ICollection<T>.IsReadOnly
{
get { return false; }
}
#endregion
#endregion
#region Constructors
public SkipList()
: this(-1, null)
{
}
public SkipList(int randomSeed)
: this(randomSeed, null)
{
}
public SkipList(IComparer<T> comparer)
: this(-1, comparer)
{
}
public SkipList(int randomSeed, IComparer<T> comparer)
{
_head = new SkipListNode<T>(1);
_comparisons = 0;
_count = 0;
_count++;
if (randomSeed < 0)
{
_rndNum = new Random();
}
else
{
_rndNum = new Random(randomSeed);
}
this.comparer = comparer ?? Comparer<T>.Default;
}
#endregion
#region Methods
/// <summary>
/// Selects a height for a new SkipListNode using the "loaded dice" technique.
/// The value selected is between 1 and maxLevel.
/// </summary>
/// <param name="maxLevel">The maximum value ChooseRandomHeight can return.</param>
/// <returns>A randomly chosen integer value between 1 and maxLevel.</returns>
protected virtual int ChooseRandomHeight(int maxLevel)
{
int level = 1;
while (_rndNum.NextDouble() < _prob && level < maxLevel)
{
level++;
}
return level;
}
#region ICollection Methods and Related Helpers
#region Add
/// <summary>
/// Adds a new element to the SkipList.
/// </summary>
/// <param name="value">The value to add.</param>
/// <remarks>This SkipList implementation does not allow for duplicates. Attempting to add a
/// duplicate value will not raise an exception, it will simply exit the method without
/// changing the SkipList.</remarks>
public void Add(T value)
{
SkipListNode<T>[] updates = BuildUpdateTable(value);
SkipListNode<T> current = updates[0];
// see if a duplicate is being inserted
if (current[0] != null && comparer.Compare(current[0].Value, value) == 0)
{
// cannot enter a duplicate, handle this case by either just returning or by throwing an exception
return;
}
// create a new node
SkipListNode<T> n = new SkipListNode<T>(value, ChooseRandomHeight(_head.Height + 1));
_count++;
// if the node's level is greater than the head's level, increase the head's level
if (n.Height > _head.Height)
{
_head.IncrementHeight();
_head[_head.Height - 1] = n;
}
// splice the new node into the list
for (int i = 0; i < n.Height; i++)
{
if (i < updates.Length)
{
n[i] = updates[i][i];
updates[i][i] = n;
}
}
}
#endregion
#region Clear
/// <summary>
/// Clears out the contents of the SkipList and creates a new head, with height 1.
/// </summary>
public void Clear()
{
_head = null;
_head = new SkipListNode<T>(1);
_count = 0;
}
#endregion
#region Contains
/// <summary>
/// Determines if a particular element is contained within the SkipList.
/// </summary>
/// <param name="value">The value to search for.</param>
/// <returns>True if value is found in the SkipList; false otherwise.</returns>
public bool Contains(T value)
{
SkipListNode<T> current = _head;
// first, determine the nodes that need to be updated at each level
for (int i = _head.Height - 1; i >= 0; i--)
{
while (current[i] != null)
{
_comparisons++;
int results = comparer.Compare(current[i].Value, value);
if (results == 0)
{
return true; // we found the item
}
else if (results < 0)
{
current = current[i]; // move on to the next neighbor
}
else
{
break; // exit while loop, we need to move down the height of the current node
}
}
}
// if we reach here, we searched to the end of the list without finding the element
return false;
}
#endregion
#region Remove
/// <summary>
/// Attempts to remove a value from the SkipList.
/// </summary>
/// <param name="value">The value to remove from the SkipList.</param>
/// <returns>True if the value is found and removed; false if the value is not found
/// in the SkipList.</returns>
public bool Remove(T value)
{
SkipListNode<T>[] updates = BuildUpdateTable(value);
SkipListNode<T> current = updates[0][0];
if (current != null && comparer.Compare(current.Value, value) == 0)
{
_count--;
// We found the data to delete
for (int i = 0; i < _head.Height; i++)
{
if (updates[i][i] != current)
{
break;
}
else
{
updates[i][i] = current[i];
}
}
// finally, see if we need to trim the height of the list
if (_head[_head.Height - 1] == null)
{
// we removed the single, tallest item... reduce the list height
_head.DecrementHeight();
}
current = null;
return true; // the item was successfully removed
}
else
{
// the data to delete wasn't found.
return false;
}
}
#endregion
#region BuildUpdateTable
/// <summary>
/// Creates a table of the SkipListNode instances that will need to be updated when an item is
/// added or removed from the SkipList.
/// </summary>
/// <param name="value">The value to be added or removed.</param>
/// <returns>An array of SkipListNode instances, as many as the height of the head node.
/// A SkipListNode instance in array index k represents the SkipListNode at height k that must
/// be updated following the addition/deletion.</returns>
protected SkipListNode<T>[] BuildUpdateTable(T value)
{
SkipListNode<T>[] updates = new SkipListNode<T>[_head.Height];
SkipListNode<T> current = _head;
// determine the nodes that need to be updated at each level
for (int i = _head.Height - 1; i >= 0; i--)
{
if (!(current[i] != null && comparer.Compare(current[i].Value, value) < 0))
{
_comparisons++;
}
while (current[i] != null && comparer.Compare(current[i].Value, value) < 0)
{
current = current[i];
_comparisons++;
}
updates[i] = current;
}
return updates;
}
#endregion
#region CopyTo
/// <summary>
/// Copies the contents of the SkipList to the passed-in array.
/// </summary>
public void CopyTo(T[] array)
{
CopyTo(array, 0);
}
/// <summary>
/// Copies the contents of the SkipList to the passed-in array.
/// </summary>
public void CopyTo(T[] array, int index)
{
// copy the values from the skip list to array
if (array == null)
{
throw new ArgumentNullException("array is null");
}
if (index < 0)
{
throw new ArgumentOutOfRangeException("index is less than 0");
}
if (index >= array.Length)
{
throw new ArithmeticException("index is greater than the length of array");
}
if (array.Length - index <= _count)
{
throw new ArgumentException("insufficient space in array to store skip list starting at specified index");
}
SkipListNode<T> current = _head[0];
int i = 0;
while (current != null)
{
array[i + index] = current.Value;
i++;
}
}
#endregion
#endregion
#region GetEnumerator
/// <summary>
/// Returns an enumerator to access the contents of the SkipList.
/// </summary>
public IEnumerator<T> GetEnumerator()
{
// enumerate through the skip list one element at a time
SkipListNode<T> current = _head[0];
while (current != null)
{
yield return current.Value;
current = current[0];
}
}
IEnumerator IEnumerable.GetEnumerator()
{
return this.GetEnumerator();
}
#endregion
#region ToString
/// <summary>
/// This overridden form of ToString() is simply for displaying detailed information
/// about the contents of the SkipList, used by SkipListTester - feel free to remove it.
/// </summary>
public override string ToString()
{
SkipListNode<T> current = _head[0];
StringBuilder sb = new StringBuilder();
while (current != null)
{
sb.Append(current.Value.ToString());
sb.Append(" [ H=").Append(current.Height);
for (int i = current.Height - 1; i >= 0; i--)
{
sb.Append(" | ").Append(current[i] == null ? "NULL" : current[i].Value.ToString());
}
sb.Append(" ] ; ");
current = current[0];
}
return sb.ToString();
}
#endregion
#region ResetComparisons
/// <summary>
/// Resets the internal comparison counter back to zero. Used for performance testing (can be removed).
/// </summary>
public void ResetComparisons()
{
_comparisons = 0;
}
#endregion
#endregion
}
