// Delete element already found at path.
private void Delete(TreePath <TKey, TValue> path)
{
int leafIndex = path.TopNodeIndex;
Leaf <TKey, TValue> leaf = (Leaf <TKey, TValue>)path.TopNode;
leaf.Remove(leafIndex);
--Count;
if (leafIndex == 0)
{
if (leaf.KeyCount != 0)
{
path.SetPivot(path.TopNode.GetKey(0));
}
else
{
Debug.Assert(leaf.RightLeaf == null, "only the rightmost leaf should ever be emptied");
// Leaf is empty. Prune it unless it is the only leaf in the tree.
Leaf <TKey, TValue> leftLeaf = (Leaf <TKey, TValue>)path.GetLeftNode();
if (leftLeaf != null)
{
leftLeaf.RightLeaf = leaf.RightLeaf;
Demote(path);
}
return;
}
}
// Leaf underflow?
if (leaf.KeyCount < leaf.KeyCapacity / 2)
{
Leaf <TKey, TValue> rightLeaf = leaf.RightLeaf;
if (rightLeaf != null)
{
if (leaf.KeyCount + rightLeaf.KeyCount > leaf.KeyCapacity)
{
// Balance leaves by shifting pairs from right leaf.
int shifts = leaf.KeyCapacity - (leaf.KeyCount + rightLeaf.KeyCount - 1) / 2;
leaf.Add(rightLeaf, 0, shifts);
rightLeaf.Remove(0, shifts);
path.TraverseRight();
path.SetPivot(path.TopNode.GetKey(0));
}
else
{
// Coalesce right leaf to current leaf and prune right leaf.
leaf.Add(rightLeaf, 0, rightLeaf.KeyCount);
leaf.RightLeaf = rightLeaf.RightLeaf;
path.TraverseRight();
Demote(path);
}
}
}
}
// Leaf has been emptied, now non-lazy delete its pivot.
private void Demote(TreePath <TKey, TValue> path)
{
for (;;)
{
Debug.Assert(path.Height > 0);
path.Pop();
Branch <TKey> branch = (Branch <TKey>)path.TopNode;
if (path.TopNodeIndex == 0)
{
if (branch.KeyCount == 0)
{
// Cascade when rightmost branch is empty.
continue;
}
// Rotate pivot for first child.
TKey pivot0 = branch.GetKey(0);
branch.RemoveKey(0);
branch.RemoveChild(0);
path.SetPivot(pivot0);
}
else
{
// Typical branch pivot delete.
branch.RemoveKey(path.TopNodeIndex - 1);
branch.RemoveChild(path.TopNodeIndex);
}
Branch <TKey> right = (Branch <TKey>)path.TraverseRight();
if (right == null)
{
if (branch == root && branch.KeyCount == 0)
{
// Prune the empty root.
Branch <TKey> newRoot = branch.FirstChild as Branch <TKey>;
if (newRoot != null)
{
root = (Branch <TKey>)branch.FirstChild;
--height;
}
}
return;
}
if (branch.KeyCount + right.KeyCount < branch.KeyCapacity)
{
// Coalesce left: move pivot and right sibling nodes.
branch.AddKey(path.GetPivot());
for (int i = 0;; ++i)
{
branch.Add(right.GetChild(i));
if (i >= right.KeyCount)
{
break;
}
branch.AddKey(right.GetKey(i));
}
// Cascade demotion.
continue;
}
// Branch underflow?
if (branch.KeyCount < branch.KeyCapacity / 2)
{
int shifts = (right.KeyCount - branch.KeyCount) / 2 - 1;
// Balance branches to keep ratio. Rotate thru the pivot.
branch.AddKey(path.GetPivot());
// Shift pairs from right sibling.
for (int rightIndex = 0;; ++rightIndex)
{
branch.Add(right.GetChild(rightIndex));
if (rightIndex >= shifts)
{
break;
}
branch.AddKey(right.GetKey(rightIndex));
}
path.SetPivot(right.GetKey(shifts));
right.Remove(0, shifts + 1);
}
return;
}
}
