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Update SkipList. Fixed Issues #137, #138, #139, #140 #147

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Update SkipList. Fixed Issues #137, #138, #139, #140 #147

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f769be0
Fixed Issue #137 Items are not always added to a SkipList
Gutsonok Aug 5, 2020
9cab153
Fixed Issue #139 A new SkipList<int> contains 0
Gutsonok Aug 5, 2020
a9ce311
Revert "Fixed Issue #139 A new SkipList<int> contains 0"
Gutsonok Aug 5, 2020
377a8d1
Fix #138 and #139 issues. Update skipList logic
Gutsonok Aug 8, 2020
321e838
Fix #140 Program crashes upon search for null in a SkipList<string>
Gutsonok Aug 8, 2020
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141 changes: 82 additions & 59 deletions DataStructures/Lists/SkipList.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -28,22 +28,6 @@ public class SkipList<T> : ICollection<T>, IEnumerable<T> where T : IComparable<
private readonly int MaxLevel = 32;
private readonly double Probability = 0.5;


/// <summary>
/// Private helper. Used in Add method.
/// </summary>
/// <returns></returns>
private int _getNextLevel()
{
int lvl = 0;

while (_randomizer.NextDouble() < Probability && lvl <= _currentMaxLevel && lvl < MaxLevel)
++lvl;

return lvl;
}


/// <summary>
/// CONSTRUCTOR
/// </summary>
Expand All @@ -53,9 +37,6 @@ public SkipList()
_currentMaxLevel = 1;
_randomizer = new Random();
_firstNode = new SkipListNode<T>(default(T), MaxLevel);

for (int i = 0; i < MaxLevel; ++i)
_firstNode.Forwards[i] = _firstNode;
}


Expand Down Expand Up @@ -94,12 +75,15 @@ public int Level
/// <summary>
/// Access elements by index
/// </summary>
public T this[int index]
public T this[T item]
{
get
{
// TODO:
throw new NotImplementedException();
return Find(item, out var result) ? result : throw new KeyNotFoundException();
}
set
{
Add(item);
}
}

Expand All @@ -111,47 +95,51 @@ public void Add(T item)
var current = _firstNode;
var toBeUpdated = new SkipListNode<T>[MaxLevel];

// Get nodes for updated
for (int i = _currentMaxLevel - 1; i >= 0; --i)
{
while (current.Forwards[i] != _firstNode && current.Forwards[i].Value.IsLessThan(item))
while (current.Forwards[i] != null && current.Forwards[i].Value.IsLessThan(item))
{
current = current.Forwards[i];
}

toBeUpdated[i] = current;
}

// Desired position to insert key
current = current.Forwards[0];

// Get the next node level, and update list level if required.
int lvl = _getNextLevel();
if (lvl > _currentMaxLevel)
if(current == null || !current.Value.Equals(item))
{
for (int i = _currentMaxLevel; i < lvl; ++i)
toBeUpdated[i] = _firstNode;

_currentMaxLevel = lvl;
// Get the next node level, and update list level if required.
int lvl = _getNextLevel();
if (lvl > _currentMaxLevel)
{
for (int i = _currentMaxLevel; i < lvl; ++i)
toBeUpdated[i] = _firstNode;

_currentMaxLevel = lvl;
}

var newNode = new SkipListNode<T>(item, lvl);

// Insert the new node into the skip list
for (int i = 0; i < lvl; ++i)
{
newNode.Forwards[i] = toBeUpdated[i].Forwards[i];
toBeUpdated[i].Forwards[i] = newNode;
}

++_count;
}

// New node
var newNode = new SkipListNode<T>(item, lvl);

// Insert the new node into the skip list
for (int i = 0; i < lvl; ++i)
{
newNode.Forwards[i] = toBeUpdated[i].Forwards[i];
toBeUpdated[i].Forwards[i] = newNode;
}

// Increment the count
++_count;
}

/// <summary>
/// Remove element from the list.
/// </summary>
public bool Remove(T item)
{
T deleted;
return Remove(item, out deleted);
return Remove(item, out var _);
}

/// <summary>
Expand All @@ -167,7 +155,7 @@ public bool Remove(T item, out T deleted)
// Mark all nodes as toBeUpdated.
for (int i = _currentMaxLevel - 1; i >= 0; --i)
{
while (current.Forwards[i] != _firstNode && current.Forwards[i].Value.IsLessThan(item))
while (current.Forwards[i] != null && current.Forwards[i].Value.IsLessThan(item))
current = current.Forwards[i];

toBeUpdated[i] = current;
Expand All @@ -176,7 +164,7 @@ public bool Remove(T item, out T deleted)
current = current.Forwards[0];

// Return default value of T if the item was not found
if (current.Value.IsEqualTo(item) == false)
if (current == null || current.Value.IsEqualTo(item) == false)
{
deleted = default(T);
return false;
Expand All @@ -185,15 +173,21 @@ public bool Remove(T item, out T deleted)
// We know that the node is in the list.
// Unlink it from the levels where it exists.
for (int i = 0; i < _currentMaxLevel; ++i)
if (toBeUpdated[i].Forwards[i] == current)
toBeUpdated[i].Forwards[i] = current.Forwards[i];
{
if (toBeUpdated[i].Forwards[i] != current)
{
break;
}

toBeUpdated[i].Forwards[i] = current.Forwards[i];
}

// Decrement the count
--_count;

// Check to see if we've deleted the highest-level node
// Decrement level
while (_currentMaxLevel > 1 && _firstNode.Forwards[_currentMaxLevel - 1] == _firstNode)
while (_currentMaxLevel > 1 && _firstNode.Forwards[_currentMaxLevel - 1] == null)
--_currentMaxLevel;

// Assign the deleted output parameter to the node.Value
Expand All @@ -206,32 +200,49 @@ public bool Remove(T item, out T deleted)
/// </summary>
public bool Contains(T item)
{
T itemOut;
return Find(item, out itemOut);
return Find(item, out var _);
}

/// <summary>
/// Look for an element and return it if found
/// </summary>
public bool Find(T item, out T result)
{
result = default(T);

var current = _firstNode;

// If find null element then check first element after first node
if (item == null)
{
current = current.Forwards[0];
return current != null && current.Value == null;
}

// Skip null element (in first postion) if contain
if (!IsEmpty && current.Forwards[0].Value == null)
{
current = current.Forwards[0];
}

// Walk after all the nodes that have values less than the node we are looking for
for (int i = _currentMaxLevel - 1; i >= 0; --i)
while (current.Forwards[i] != _firstNode && current.Forwards[i].Value.IsLessThan(item))
{
while (current.Forwards[i] != null && current.Forwards[i].Value.IsLessThan(item))
{
current = current.Forwards[i];
}
}

current = current.Forwards[0];

// Return true if we found the element; false otherwise
if (current.Value.IsEqualTo(item))
if (current != null && current.Value.IsEqualTo(item))
{
result = current.Value;
return true;
}

result = default(T);
return false;
}

Expand Down Expand Up @@ -302,7 +313,7 @@ public bool TryPeek(out T result)
public IEnumerator<T> GetEnumerator()
{
var node = _firstNode;
while (node.Forwards[0] != null && node.Forwards[0] != _firstNode)
while (node.Forwards[0] != null && node.Forwards[0] != null)
{
node = node.Forwards[0];
yield return node.Value;
Expand Down Expand Up @@ -361,12 +372,24 @@ public void Clear()
_currentMaxLevel = 1;
_randomizer = new Random();
_firstNode = new SkipListNode<T>(default(T), MaxLevel);

for (int i = 0; i < MaxLevel; ++i)
_firstNode.Forwards[i] = _firstNode;
}
#endregion

/// <summary>
/// Private helper. Used in Add method.
/// </summary>
/// <returns></returns>
private int _getNextLevel()
{
int lvl = 1;

while (_randomizer.NextDouble() < Probability && lvl <= _currentMaxLevel && lvl < MaxLevel)
{
++lvl;
}

return lvl;
}
}

}
20 changes: 3 additions & 17 deletions DataStructures/Lists/SkipListNode.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -4,12 +4,6 @@ namespace DataStructures.Lists
{
public class SkipListNode<T> : IComparable<SkipListNode<T>> where T : IComparable<T>
{
/// <summary>
/// Instance variables
/// </summary>
private T _value;
private SkipListNode<T>[] _forwards;

/// <summary>
/// CONSTRUCTORS
/// </summary>
Expand All @@ -25,20 +19,12 @@ public SkipListNode(T value, int level)
/// <summary>
/// Get and set node's value
/// </summary>
public virtual T Value
{
get { return this._value; }
private set { this._value = value; }
}
public virtual T Value { get; private set; }

/// <summary>
/// Get and set node's forwards links
/// </summary>
public virtual SkipListNode<T>[] Forwards
{
get { return this._forwards; }
private set { this._forwards = value; }
}
public virtual SkipListNode<T>[] Forwards { get; private set; }

/// <summary>
/// Return level of node.
Expand All @@ -55,7 +41,7 @@ public int CompareTo(SkipListNode<T> other)
{
if (other == null)
return -1;

return this.Value.CompareTo(other.Value);
}
}
Expand Down
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