/// <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 <= this.Count)
{
throw new ArgumentException("insufficient space in array to store skip list starting at specified index");
}
SkipListNode <T> current = this.head[0];
int i = 0;
while (current != null)
{
array[i + index] = current.Value;
i++;
}
}
// Clears out the contents of the SkipList and creates a new head, with height 1.
public void Clear()
{
// create a new head
this.head = null;
this.head = new SkipListNode <T>(1);
this.Count = 0;
}
/// <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 = this.head[0];
System.Text.StringBuilder stringBuilder = new System.Text.StringBuilder();
while (current != null)
{
stringBuilder.Append(current.Value.ToString());
stringBuilder.Append(" [ H=").Append(current.Height);
for (int i = current.Height - 1; i >= 0; i--)
{
stringBuilder.Append(" | ");
if (current[i] == null)
{
stringBuilder.Append("NULL");
}
else
{
stringBuilder.Append(current[i].Value.ToString());
}
}
stringBuilder.Append(" ] ; ");
current = current[0];
}
return(stringBuilder.ToString());
}
/// <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 = this.BuildUpdateTable(value);
SkipListNode <T> current = updates[0];
// see if a duplicate is being inserted
if (current[0] != null && this.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, this.ChooseRandomHeight(head.Height + 1));
this.Count++;
// if the node's level is greater than the head's level, increase the head's level
if (n.Height > this.head.Height)
{
this.head.IncrementHeight();
this.head[this.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;
}
}
}
public SkipListNode <T> FindByValue(T value)
{
SkipListNode <T> current = this.head;
// first, determine the nodes that need to be updated at each level
for (int i = this.head.Height - 1; i >= 0; i--)
{
while (current[i] != null)
{
this.ComparisonCount++;
int results = this.comparer.Compare(current[i].Value, value);
if (results == 0)
{
return(current[i]); // 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(null);
}
/// <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 = this.head[0];
while (current != null)
{
yield return(current.Value);
current = current[0];
}
}
public SkipList(int randomSeed, IComparer <T> comparer)
{
this.head = new SkipListNode <T>(1);
this.ComparisonCount = 0;
this.Count = 0;
this.Count++;
if (randomSeed < 0)
{
randomNum = new Random();
}
else
{
randomNum = new Random(randomSeed);
}
if (comparer != null)
{
this.comparer = comparer;
}
}
/// <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 = this.BuildUpdateTable(value);
SkipListNode <T> current = updates[0][0];
if (current != null && this.comparer.Compare(current.Value, value) == 0)
{
this.Count--;
// We found the data to delete
for (int i = 0; i < this.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 (this.head[this.head.Height - 1] == null)
{
// we removed the single, tallest item... reduce the list height
this.head.DecrementHeight();
}
current = null;
return(true); // the item was successfully removed
}
else
{
// the data to delete wasn't found.
return(false);
}
}
/// <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> [this.head.Height];
SkipListNode <T> current = this.head;
// determine the nodes that need to be updated at each level
for (int i = this.head.Height - 1; i >= 0; i--)
{
if (!(current[i] != null && this.comparer.Compare(current[i].Value, value) < 0))
{
this.ComparisonCount++;
}
while (current[i] != null && this.comparer.Compare(current[i].Value, value) < 0)
{
current = current[i];
this.ComparisonCount++;
}
updates[i] = current;
}
return(updates);
}
