# Lecture 017

## Red-Black Tree

datatype 'a dict = Empty
| Red of 'a dict * 'a entry * 'a dict
| Black of 'a dict * 'a entry * 'a dict


functor

functor RBT (K: ORDERED) :> DICT where type Key.t = K.t
= struct
structure Key = K
type 'a entry = Key.t * 'a
val empty = Empty
fun insert...
fun loopup...
end

structure StringDict = RBT(StringLt)
val r1 = StringDIct.insert(StringDict.empty, ("A", 1))

- r1;
val r1 - -: int stringDict.dict


invariant

1. Sorting: tree is sorted by key
2. Red-Black Invariant: Children of red nodes are black
3. Black Height Invariant: The number of black nodes on any path from the root to a leaf is equal (the black height is (black_root + non-nil-black-nodes))

Note: (root) and leaf(Empty) are black

### Implement Rotation

(*restorLeft: 'a dict -> 'a dict
REQUIRES: d is RBT || (or d is black && left child is ARBT && right child RBT)
ENSURES: restoreLeft(d) == RBT
*)
fun restoreLeft (Black(ed(Red(T1, x, T2), y, T3), z, T4))
= Red(Black(T1, x, T2), y, Black(T3, z, T4))
| restoreLeft (Black(Red(T1, x, Red(T2, y, T3)), 2, T4))
= Red(Black(T1, x, T2), y, Black(T3, z, T4))
| restoreLeft d = d


### Implement Insert

(*insert: 'a dict * 'a entry -> 'a dict
REQURIES: d is a RBT
ENSURES: insert(d ,e) is a RBT such that ... inserted

ins: 'a dict -> 'a dict
REQUIRES: d is a RBT
ENSURES: ins(d) is sorted, contain right element, has same black height as d
ins(Black(t)) is a RBT
ins(Red(t)) is a ARBT
*)
fun insert (d, e as (k, v))
= let
fun ins Empty = Red(Empty, e, Empty)
| ins (Black(l, e' as (k', _), r)))
= (case Key.compare(k, k') of
Equal => Black(l, e, r)
| LESS => restoreLeft (Black(ins l, e', r))
| GREATER => restoreRight (Black(l, e', ins r))) (*here ins l might be ARBT before restore*)
| ins (Red(l, e' as (k', _), r))
= (case Key.compare(k, k') of
Equal => Red(l, e, r)
| LESS => Red(ins l, e', r) (*here ins l has to be RBT*)
| GREATER => Red(l, e', ins r))
in
(case ins d of
| Red(t as (Red(_), _, _)) => Black t
| Red(t as (_, _, Red(_)) => Black t
| d' => d'
end


insert: works because an ARBT with the root recolored Black is an RBT

## Red-Black Theorem

In a RBT, depth <= 2 log_2 (|nodes|+1)