## ⓘ AA tree

An AA tree in computer science is a form of balanced tree used for storing and retrieving ordered data efficiently. AA trees are named for Arne Andersson, their inventor.

AA trees are a variation of the red-black tree, a form of binary search tree which supports efficient addition and deletion of entries. Unlike red-black trees, red nodes on an AA tree can only be added as a right subchild. In other words, no red node can be a left sub-child. This results in the simulation of a 2-3 tree instead of a 2-3-4 tree, which greatly simplifies the maintenance operations. The maintenance algorithms for a red-black tree need to consider seven different shapes to properly balance the tree:

An AA tree on the other hand only needs to consider two shapes due to the strict requirement that only right links can be red:

## 1. Balancing rotations

Whereas red-black trees require one bit of balancing metadata per node the color, AA trees require OloglogN) bits of metadata per node, in the form of an integer "level". The following invariants hold for AA trees:

- The level of every right child is equal to or one less than that of its parent.
- The level of every left child is exactly one less than that of its parent.
- Every node of level greater than one has two children.
- The level of every right grandchild is strictly less than that of its grandparent.
- The level of every leaf node is one.

A link where the childs level is equal to that of its parent is called a horizontal link, and is analogous to a red link in the red-black tree. Individual right horizontal links are allowed, but consecutive ones are forbidden; all left horizontal links are forbidden. These are more restrictive constraints than the analogous ones on red-black trees, with the result that re-balancing an AA tree is procedurally much simpler than re-balancing a red-black tree.

Insertions and deletions may transiently cause an AA tree to become unbalanced that is, to violate the AA tree invariants. Only two distinct operations are needed for restoring balance: "skew" and "split". Skew is a right rotation to replace a subtree containing a left horizontal link with one containing a right horizontal link instead. Split is a left rotation and level increase to replace a subtree containing two or more consecutive right horizontal links with one containing two fewer consecutive right horizontal links. Implementation of balance-preserving insertion and deletion is simplified by relying on the skew and split operations to modify the tree only if needed, instead of making their callers decide whether to skew or split.

function skew is input: T, a node representing an AA tree that needs to be rebalanced. output: Another node representing the rebalanced AA tree. if nilT then return Nil else if nilleftT) then return T else if levelleftT) == levelT then Swap the pointers of horizontal left links. L = leftT leftT:= rightL rightL:= T return L else return T end if end functionSkew:

function split is input: T, a node representing an AA tree that needs to be rebalanced. output: Another node representing the rebalanced AA tree. if nilT then return Nil else if nilrightT) or nilrightrightT) then return T else if levelT == levelrightrightT) then We have two horizontal right links. Take the middle node, elevate it, and return it. R = rightT rightT:= leftR leftR:= T levelR:= levelR + 1 return R else return T end if end functionSplit:

## 2. Insertion

Insertion begins with the normal binary tree search and insertion procedure. Then, as the call stack unwinds assuming a recursive implementation of the search, its easy to check the validity of the tree and perform any rotations as necessary. If a horizontal left link arises, a skew will be performed, and if two horizontal right links arise, a split will be performed, possibly incrementing the level of the new root node of the current subtree. Note, in the code as given above, the increment of levelT. This makes it necessary to continue checking the validity of the tree as the modifications bubble up from the leaves.

function insert is input: X, the value to be inserted, and T, the root of the tree to insert it into. output: A balanced version T including X. Do the normal binary tree insertion procedure. Set the result of the recursive call to the correct child in case a new node was created or the root of the subtree changes. if nilT then Create a new leaf node with X. return node else if X < valueT then leftT:= insertX, leftT) else if X > valueT then rightT:= insertX, rightT) end if Note that the case of X == valueT is unspecified. As given, an insert will have no effect. The implementor may desire different behavior. Perform skew and then split. The conditionals that determine whether or not a rotation will occur or not are inside of the procedures, as given above. T:= skewT T:= splitT return T end function## 3. Deletion

As in most balanced binary trees, the deletion of an internal node can be turned into the deletion of a leaf node by swapping the internal node with either its closest predecessor or successor, depending on which are in the tree or on the implementors whims. Retrieving a predecessor is simply a matter of following one left link and then all of the remaining right links. Similarly, the successor can be found by going right once and left until a null pointer is found. Because of the AA property of all nodes of level greater than one having two children, the successor or predecessor node will be in level 1, making their removal trivial.

To re-balance a tree, there are a few approaches. The one described by Andersson in his original paper is the simplest, and it is described here, although actual implementations may opt for a more optimized approach. After a removal, the first step to maintaining tree validity is to lower the level of any nodes whose children are two levels below them, or who are missing children. Then, the entire level must be skewed and split. This approach was favored, because when laid down conceptually, it has three easily understood separate steps:

- Skew the level.
- Decrease the level, if appropriate.
- Split the level.

However, we have to skew and split the entire level this time instead of just a node, complicating our code.

function delete is input: X, the value to delete, and T, the root of the tree from which it should be deleted. output: T, balanced, without the value X. if nilT then return T else if X > valueT then rightT:= deleteX, rightT) else if X < valueT then leftT:= deleteX, leftT) else If were a leaf, easy, otherwise reduce to leaf case. if leafT then return rightT else if nilleftT) then L:= successorT rightT:= deletevalueL, rightT) valueT:= valueL else L:= predecessorT leftT:= deletevalueL, leftT) valueT:= valueL end if end if Rebalance the tree. Decrease the level of all nodes in this level if necessary, and then skew and split all nodes in the new level. T:= decrease_levelT T:= skewT rightT:= skewrightT) if not nilrightT) rightrightT):= skewrightrightT) end if T:= splitT rightT:= splitrightT) return T end function decrease_level is input: T, a tree for which we want to remove links that skip levels. output: T with its level decreased. should_be = minlevelleftT), levelrightT) + 1 if should_be < levelT then levelT:= should_be if should_be < levelrightT) then levelrightT):= should_be end if end if return T end functionA good example of deletion by this algorithm is present in the Andersson paper.

## 4. Performance

The performance of an AA tree is equivalent to the performance of a red-black tree. While an AA tree makes more rotations than a red-black tree, the simpler algorithms tend to be faster, and all of this balances out to result in similar performance. A red-black tree is more consistent in its performance than an AA tree, but an AA tree tends to be flatter, which results in slightly faster search times.

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