CSE 326: Data Structures Lecture #13 Extendible Hashing and Splay Trees

CSE 326: Data StructuresLecture #13

Extendible Hashing and Splay Trees

Alon Halevy

Spring Quarter 2001

Extendible Hashing

• Hashing technique for huge data sets– optimizes to reduce disk accesses

– each hash bucket fits on one disk block

– better than B-Trees if order is not important

• Table contains– buckets, each fitting in one disk block, with the data

– a directory that fits in one disk block used to hash to the correct bucket

001 010 011 110 111 101

Extendible Hash Table• Directory contains entries labeled by k bits plus a

pointer to the bucket with all keys starting with its bits• Each block contains keys+data matching on the first

j k bits

000 100

(2)00001000110010000110

(2)010010101101100

(3)1000110011

(3)101011011010111

(2)11001110111110011110

directory for k = 3

Inserting (easy case)

001 010 011 110 111 101000 100

(2)00001000110010000110

(2)010010101101100

(3)1000110011

(3)101011011010111

(2)11001110111110011110

insert(11011)001 010 011 110 111 101000 100

(2)00001000110010000110

(2)010010101101100

(3)1000110011

(3)101011011010111

(2)110011110011110

Splitting

001 010 011 110 111 101000 100

(2)00001000110010000110

(2)010010101101100

(3)1000110011

(3)101011011010111

(2)11001110111110011110

insert(11000)

001 010 011 110 111 101000 100

(2)00001000110010000110

(2)010010101101100

(3)1000110011

(3)101011011010111

(3)110001100111011

(3)1110011110

Rehashing

01 10 1100

(2)01101

(2)10000100011001110111

(2)1100111110

insert(10010)

001 010 011 110 111 101000 100

No room to insert and no adoption!

Expand directory

Now, it’s just a normal split.

Rehash of Hashing• Hashing is a great data structure for storing unordered data

that supports insert, delete, and find• Both separate chaining (open) and open addressing

(closed) hashing are useful– separate chaining flexible– closed hashing uses less storage, but performs badly with load

factors near 1– extendible hashing for very large disk-based data

• Hashing pros and cons+ very fast+ simple to implement, supports insert, delete, find- lazy deletion necessary in open addressing, can waste storage- does not support operations dependent on order: min, max, range

Recall: AVL Tree Dictionary Data Structure

4

121062

115

8

14137 9

• Binary search tree properties– binary tree property

– search tree property

• Balance property– balance of every node is:

-1 b 1– result:

• depth is (log n)

15

Splay Trees

“blind” rebalancing – no height info kept• amortized time for all operations is O(log n)• worst case time is O(n)• insert/find always rotates node to the root!

– Good locality – most common keys move high in tree

Idea

17

10

92

5

3

You’re forced to make a really deep access:

Since you’re down there anyway,fix up a lot of deep nodes!

Splay Operations: Find

• Find(x)1. do a normal BST search to find n such that

n->key = x

2. move n to root by series of zig-zag and zig-zig rotations, followed by a final zig if necessary

Zig-Zag*

g

Xp

Y

n

Z

W

*This is just a double rotation

n

Y

g

W

p

ZX

Helped

Unchanged

Hurt

Zig-Zig

n

Z

Y

p

X

g

W

g

W

X

p

Y

n

Z

Zig

p

X

n

Y

Z

n

Z

p

Y

X

root root

Why Splaying Helps• Node n and its children are always helped (raised)• Except for final zig, nodes that are hurt by a zig-

zag or zig-zig are later helped by a rotation higher up the tree!

• Result: – shallow (zig) nodes may increase depth by one or two– helped nodes may decrease depth by a large amount

• If a node n on the access path is at depth d before the splay, it’s at about depth d/2 after the splay– Exceptions are the root, the child of the root, and the

node splayed

Locality

• Assume m n access in a tree of size n– Total amortized time O(m log n)

– O(log n) per access on average

• Gets better when you only access k distinct items in the m accesses.– Exercise.

Splaying Example

2

1

3

4

5

6

Find(6)

2

1

3

6

5

4

zig-zig

Still Splaying 6

zig-zig2

1

3

6

5

4

1

6

3

2 5

4

Almost There, Stay on Target

zig

1

6

3

2 5

4

6

1

3

2 5

4

Splay Again

Find(4)

zig-zag

6

1

3

2 5

4

6

1

4

3 5

2

Example Splayed Out

zig-zag

6

1

4

3 5

2

61

4

3 5

2

Splay Tree Summary

• All operations are in amortized O(log n) time• Splaying can be done top-down; better because:

– only one pass– no recursion or parent pointers necessary

• Invented by Sleator and Tarjan (1985), now widely used in place of AVL trees

• Splay trees are very effective search trees– relatively simple– no extra fields required– excellent locality properties: frequently accessed keys are cheap to

find

Coming Up

• Project 3: implement a “smart” web server.

• Heaps!

• Disjoint Sets

• Graphs and more.