/// <summary>
/// Increases the height of the <see cref="SkipListNode{T}"/> by 1. Used for the head node of the <see cref="SkipList{T}"/>.
/// </summary>
internal void IncrementHeight()
{
var newForwards = new SkipListNode <T> [Height + 1];
for (int i = 0; i < Height; i++)
{
newForwards[i] = forwards[i];
}
forwards = newForwards;
}
/// <summary>
/// Decreases the height of the <see cref="SkipListNode{T}"/> by 1. Used for the head node of the <see cref="SkipList{T}"/>.
/// </summary>
internal void DecrementHeight()
{
if (Height == 1)
{
return;
}
var newForwards = new SkipListNode <T> [Height - 1];
for (int i = 0; i < Height - 1; i++)
{
newForwards[i] = forwards[i];
}
forwards = newForwards;
}
/// <summary>
/// Removes all elements from the <see cref="SkipList{T}"/>.
/// </summary>
public void Clear()
{
var curNode = Head;
var lastNode = Head;
while (curNode != null)
{
curNode = curNode[0];
lastNode.Invalidate();
lastNode = curNode;
}
Head = new SkipListNode <T>(default(T), 1);
Count = 0;
randomizer = new Random();
}
/// <summary>
/// Adds a value in the <see cref="SkipList{T}"/>.
/// </summary>
/// <param name="value">The value to add in the <see cref="SkipList{T}"/>.</param>
public void Add(T value)
{
// Gets the nodes which have to be updated(the nodes before the one we want to insert)
var nodesForUpdate = new SkipListNode <T> [Height];
var curNode = Head;
for (int i = Height - 1; i >= 0; i--)
{
while (curNode[i] != null && value.CompareTo(curNode[i].Value) > 0)
{
curNode = curNode[i];
}
nodesForUpdate[i] = curNode;
}
// Check if inserting a duplicate
var nodeBeforeNodeForInsertion = nodesForUpdate[0];
if (nodeBeforeNodeForInsertion[0] != null && value.CompareTo(nodeBeforeNodeForInsertion[0].Value) == 0)
{
throw new ArgumentException("Tried to insert duplicate value!");
}
// Generates the height of the new node
int height = 1;
while (randomizer.NextDouble() < probability && height < Head.Height + 1)
{
height++;
}
// Creating the new node
var newNode = new SkipListNode <T>(value, height);
if (height > Head.Height)// If the new node is higher that the head node
{
// Icrease the head level
Head.IncrementHeight();
Head[Head.Height - 1] = newNode;
}
// Adding the new node in the list
for (int i = 0; i < newNode.Height; i++)
{
// For every level which have to be updated
if (i < nodesForUpdate.Length)
{
// The new node starts to point where the node before it points.
newNode[i] = nodesForUpdate[i][i];
// The old node starts to point to the new inserted node.
nodesForUpdate[i][i] = newNode;
// Note:
// nodesForUpdate is an array of the nodes before the one we add.
// nodesForUpdate[i] is the node on the level i before our new node.
// nodesForUpdate[i][i] is the reference to the next node on level i.
}
}
Count++;
}
/// <summary>
/// Creates a new instance of the <see cref="DoublyLinkedList{T}"/> class that is empty.
/// </summary>
public SkipList()
{
Head = new SkipListNode <T>(default(T), 1);
Count = 0;
randomizer = new Random();
}
/// <summary>
/// Removes a value from the <see cref="SkipList{T}"/>.
/// </summary>
/// <param name="item">The value to remove from the <see cref="SkipList{T}"/>.</param>
/// <returns>true if the value is removed successfully; otherwise false.</returns>
public bool Remove(T value)
{
if (Count == 0)
{
return(false);
}
// Gets the nodes which have to be updated(the nodes before the one we will delete)
var nodesForUpdate = new SkipListNode <T> [Height];
var curNode = Head;
for (int i = Height - 1; i >= 0; i--)
{
while (curNode[i] != null && value.CompareTo(curNode[i].Value) > 0)
{
curNode = curNode[i];
}
nodesForUpdate[i] = curNode;
}
var nodeToDelete = nodesForUpdate[0][0];
if (nodeToDelete != null && value.CompareTo(nodeToDelete.Value) == 0)
{
// Removing references to the node for deletion and changing them to the node after it
for (int i = 0; i < Head.Height; i++)
{
if (nodesForUpdate[i][i] != nodeToDelete)
{
break;
}
else
{
nodesForUpdate[i][i] = nodeToDelete[i];
}
// the node now points to the node after the node for removal
// Note:
// nodesForUpdate is an array of the nodes before the one we are deleting.
// nodesForUpdate[i] is the node on the level i before the node for removal.
// nodesForUpdate[i][i] is the reference to the next node on level i.
}
// Removing references of deleted node
nodeToDelete.Invalidate();
// Check if we have to decrease the list level
if (Head[Head.Height - 1] == null)
{
Head.DecrementHeight();
}
Count--;
return(true);
}
else
{
return(false);
}
}