/// <summary>
/// Ensures the tree follows red-black properties after an insertion of the given node.
/// </summary>
private void InsertRepair(PointerBackedBinaryTreeNode <RedBlackTreeNode <T> > currentNode)
{
if (currentNode.Parent == null)
{
// If this is the root node, it must be black. It will currently be red, because
// that is the color new nodes are added as.
currentNode.Data.IsBlack = true;
}
else if (currentNode.Parent.Data.IsBlack)
{
// If the parent node is black, the red-black ordering isn't invalidated, so do nothing.
}
else if (currentNode.GetUncle()?.Data.IsBlack == false)
{
// There is a red uncle (and the current node is red), so we can switch then both to black with the
// grandparent becoming red. This may violate #2, so we need to rerun the recursion.
currentNode.Parent.Data.IsBlack = true;
currentNode.GetUncle().Data.IsBlack = true;
// Because we found an uncle, a grandparent is guaranteed to exist.
currentNode.Parent.Parent.Data.IsBlack = false;
InsertRepair(currentNode.Parent.Parent);
}
else // parent is red, uncle is black or missing, grandparent definitely exists.
{
// Rotate left or right and then reverse the rotation later to effectively switch nodes around
// and satisfy red-black height and color properties
if (currentNode == currentNode.Parent.Right && currentNode.Parent == currentNode.Parent.Parent.Left)
{
this.RotateLeft(currentNode.Parent);
currentNode = currentNode.Left;
currentNode.Parent.Data.IsBlack = true;
currentNode.Parent.Parent.Data.IsBlack = false;
this.RotateRight(currentNode.Parent.Parent);
}
else if (currentNode == currentNode.Parent.Left && currentNode.Parent == currentNode.Parent.Parent.Right)
{
this.RotateRight(currentNode.Parent);
currentNode = currentNode.Right;
currentNode.Parent.Data.IsBlack = true;
currentNode.Parent.Parent.Data.IsBlack = false;
this.RotateLeft(currentNode.Parent.Parent);
}
}
}
public void GetUncleFindsNoUncleWhenNoneExists()
{
PointerBackedBinaryTreeNode <string> testNode = new PointerBackedBinaryTreeNode <string>("abc", null);
Assert.IsNull(testNode.GetUncle());
testNode.Left = new PointerBackedBinaryTreeNode <string>("def", testNode);
testNode.Left.Left = new PointerBackedBinaryTreeNode <string>("geh", testNode.Left);
Assert.IsNull(testNode.Left.GetUncle());
Assert.IsNull(testNode.Left.Left.GetUncle());
}