/// <summary>
/// Fixes this node by combining parent entry downwards.
/// </summary>
void FixByCombine(int targetParentIndex,
TreeNode <K, V> leftChild, TreeNode <K, V> rightChild, TreeNode <K, V> parent)
{
// Move the entry at targetParentIndex from parent to left child.
leftChild.entries.Add(parent.GetEntry(targetParentIndex));
// Move all elements in the right child to the left child.
leftChild.entries.AddRange(rightChild.entries);
leftChild.childrenIds.AddRange(rightChild.childrenIds);
// Update parent of the children moved from right child.
for (int i = 0; i < rightChild.childrenIds.Count; i++)
{
var childNode = nodeManager.Find(rightChild.childrenIds[i]);
childNode.parentId = leftChild.id;
nodeManager.MarkAsChanged(childNode);
}
// Remove the entry at targetParentIndex and rightChild node from parent.
parent.entries.RemoveAt(targetParentIndex);
parent.childrenIds.RemoveAt(targetParentIndex + 1);
// Node no longer used.
nodeManager.Delete(rightChild);
// If the parent node is a root and no longer has an entry
if (parent.parentId == 0 && parent.EntryCount == 0)
{
// Make the left child root
leftChild.parentId = 0;
nodeManager.MarkAsChanged(leftChild);
nodeManager.MakeRoot(leftChild);
nodeManager.Delete(parent);
}
// If the parent node is not a root and doesn't have the minimum entry count
else if (parent.parentId != 0 && parent.EntryCount < nodeManager.MinEntriesPerNode)
{
nodeManager.MarkAsChanged(leftChild);
nodeManager.MarkAsChanged(parent);
// Fix the parent node.
parent.FixNode();
}
// Else, just finalize.
else
{
nodeManager.MarkAsChanged(leftChild);
nodeManager.MarkAsChanged(parent);
}
}
//
// Private Methods
//
/// <summary>
/// Rebalance this node after an element has been removed causing it to underflow
/// </summary>
void Rebalance()
{
// If the deficient node's right sibling exists and has more
// than the minimum number of elements, then rotate left
var indexInParent = IndexInParent();
var parent = nodeManager.Find(parentId);
var rightSibling = ((indexInParent + 1) < parent.ChildrenNodeCount) ? parent.GetChildNode(indexInParent + 1) : null;
if ((rightSibling != null) && (rightSibling.EntriesCount > nodeManager.MinEntriesPerNode))
{
// Copy the separator from the parent to the end of the deficient node
// (the separator moves down; the deficient node now has the minimum number of elements)
entries.Add(parent.GetEntry(indexInParent));
// Replace the separator in the parent with the first element of the right sibling
// (right sibling loses one node but still has at least the minimum number of elements)
parent.entries[indexInParent] = rightSibling.entries[0];
rightSibling.entries.RemoveAt(0);
// Move the first child reference from right sibling to me.
if (false == rightSibling.IsLeaf)
{
// First, update parentId of the child that will be moved
var n = nodeManager.Find(rightSibling.childrenIds[0]);
n.parentId = this.id;
nodeManager.MarkAsChanged(n);
// Then move it
childrenIds.Add(rightSibling.childrenIds[0]);
rightSibling.childrenIds.RemoveAt(0);
}
// The tree is now balanced
nodeManager.MarkAsChanged(this);
nodeManager.MarkAsChanged(parent);
nodeManager.MarkAsChanged(rightSibling);
return;
}
// Otherwise, if the deficient node's left sibling exists and has more
// than the minimum number of elements, then rotate right
var leftSibling = ((indexInParent - 1) >= 0) ? parent.GetChildNode(indexInParent - 1) : null;
if ((leftSibling != null) && (leftSibling.EntriesCount > nodeManager.MinEntriesPerNode))
{
// Copy the separator from the parent to the start of the deficient node
// (the separator moves down; deficient node now has the minimum number of elements)
entries.Insert(0, parent.GetEntry(indexInParent - 1));
// Replace the separator in the parent with the last element
// of the left sibling (left sibling loses one node but still has
// at least the minimum number of elements)
parent.entries[indexInParent - 1] = leftSibling.entries[leftSibling.entries.Count - 1];
leftSibling.entries.RemoveAt(leftSibling.entries.Count - 1);
// Move the last child reference from the left sibing to me.
// First, update parentId of the child that will be moved.
if (false == IsLeaf)
{
var n = nodeManager.Find(leftSibling.childrenIds[leftSibling.childrenIds.Count - 1]);
n.parentId = this.id;
nodeManager.MarkAsChanged(n);
// Then move it
childrenIds.Insert(0, leftSibling.childrenIds[leftSibling.childrenIds.Count - 1]);
leftSibling.childrenIds.RemoveAt(leftSibling.childrenIds.Count - 1);
}
// The tree is now balanced;
nodeManager.MarkAsChanged(this);
nodeManager.MarkAsChanged(parent);
nodeManager.MarkAsChanged(leftSibling);
return;
}
// Otherwise, if both immediate siblings have only the minimum number of elements,
// then merge with a sibling sandwiching their separator taken off from their parent
var leftChild = rightSibling != null ? this : leftSibling;
var rightChild = rightSibling != null ? rightSibling : this;
var seperatorParentIndex = rightSibling != null ? indexInParent : (indexInParent - 1);
// Step 1:
// Copy the separator to the end of the left node (the left node may be the
// deficient node or it may be the sibling with the minimum number of elements)
leftChild.entries.Add(parent.GetEntry(seperatorParentIndex));
// Move all elements from the right node to the left
// node (the left node now has the maximum number of elements, and the right node – empty)
leftChild.entries.AddRange(rightChild.entries);
leftChild.childrenIds.AddRange(rightChild.childrenIds);
// Update parent id of the children that has been moved from rightChild to leftChild
foreach (var id in rightChild.childrenIds)
{
var n = nodeManager.Find(id);
n.parentId = leftChild.id;
nodeManager.MarkAsChanged(n);;
}
// Remove the separator from the parent along with its
// empty right child (the parent loses an element)
parent.entries.RemoveAt(seperatorParentIndex);
parent.childrenIds.RemoveAt(seperatorParentIndex + 1);
nodeManager.Delete(rightChild);
// If the parent is the root and now has no elements,
// then free it and make the merged node the new root (tree becomes shallower)
if (parent.parentId == 0 && parent.EntriesCount == 0)
{
leftChild.parentId = 0;
nodeManager.MarkAsChanged(leftChild); // Changed left one
nodeManager.MakeRoot(leftChild);
nodeManager.Delete(parent); // Deleted parent
}
// Otherwise, if the parent has fewer than
// the required number of elements, then rebalance the parent
else if ((parent.parentId != 0) && (parent.EntriesCount < nodeManager.MinEntriesPerNode))
{
nodeManager.MarkAsChanged(leftChild); // Changed left one
nodeManager.MarkAsChanged(parent); // Changed parent
parent.Rebalance();
}
else
{
nodeManager.MarkAsChanged(leftChild); // Changed left one
nodeManager.MarkAsChanged(parent); // Changed parent
}
}