Hashing Part Two: Static Perfect Hashing

Advanced Algorithms – COMS31900

Hashing part two

Static Perfect Hashing

Benjamin Sach

Dictionaries and Hashing recap

� A dynamic dictionary stores (key, value)-pairs and supports:

Universe U of u keys.Hash table T of size m > n.

Collisions were fixed by chaining

A hash function maps a key x to position h(x)

- i.e T [h(x)] = (key, value).

n arbitrary operations arrive online, one at a time.

add(key, value), lookup(key) (which returns value) and delete(key)

(building linked lists)

A set H of hash functions is weakly universal if for any

two keys x, y ∈ U (with x 6= y),

Pr(h(x) = h(y)

(h is picked uniformly at random from H)

Pr(h(x) = h(y)

For any n operations, the expected

run-time is O(1) per operation.

Using weakly universal hashing:

Pr(h(x) = h(y)

For any n operations, the expected

run-time is O(1) per operation.

But this doesn’t tell us much about the

worst-case behaviour

Using weakly universal hashing:

Static Dictionaries and Perfect hashing

� A static dictionary stores (key, value)-pairs and supports:

Hash table T of size m > n.

we are given n different (key, value)-pairs and want to pick a good h

lookup(key) (which returns value) - no inserts or deletes are allowed

Universe U of u keys.

Collisions were fixed by chaining(building linked lists)

THEOREM

The FKS hashing scheme:

• Has no collisions

• Every lookup takes O(1) worst-case time,

• Uses O(n) space,

• Can be built in O(n) expected time.

THEOREM

The rest of this lecture is devoted to the

FKS scheme

THEOREM

FKS scheme

The construction is based on weak

universal hashing

THEOREM

FKS scheme

The construction is based on weak

universal hashing

(with an O(1) time hash function)

Perfect hashing - a first attempt

A set H of hash functions is weakly universal if for any two keys x, y ∈ U (x 6= y),

Pr(h(x) = h(y)

mwhere h is picked uniformly at random from H

Pr(h(x) = h(y)

Step 1: Insert everything into a hash table of size m = nusing a weakly universal hash function

Pr(h(x) = h(y)

n(where any h(x) can be computed in O(1) time)

Pr(h(x) = h(y)

Step 2: Check for collisionsn

Pr(h(x) = h(y)

Step 2: Check for collisions

Step 3: Profit!

Pr(h(x) = h(y)

Step 3: Repeat if necessary

Pr(h(x) = h(y)

How many collisions do we get on average?

Pr(h(x) = h(y)

where indicator random variable Ix,y = 1 iff h(x) = h(y).

number of

collisions

E(C) = E( ∑

x,y∈T,x<y

Ix,y)=

∑x,y∈T, x<y

E(Ix,y) 6∑

x,y∈T, x<y

Pr(h(x) = h(y)

number of

collisions

E(C) = E( ∑

x,y∈T,x<y

Ix,y)=

∑x,y∈T, x<y

E(Ix,y) 6∑

x,y∈T, x<y

Pr(h(x) = h(y)

number of

collisions

Linearity of Expectation

Let Y1, Y2, . . . , Yk be k random variables. Then

E( k∑i=1

k∑i=1

E(C) = E( ∑

x,y∈T,x<y

Ix,y)=

∑x,y∈T, x<y

E(Ix,y) 6∑

x,y∈T, x<y

Pr(h(x) = h(y)

number of

collisions

linearity of

expectation

E(C) = E( ∑

x,y∈T,x<y

Ix,y)=

∑x,y∈T, x<y

E(Ix,y) 6∑

x,y∈T, x<y

Pr(h(x) = h(y)

number of

collisions

linearity of

expectation

E(C) = E( ∑

x,y∈T,x<y

Ix,y)=

∑x,y∈T, x<y

E(Ix,y) 6∑

x,y∈T, x<y

Pr(h(x) = h(y)

number of

collisions

linearity of

expectation

definition of

expectation

E(C) = E( ∑

x,y∈T,x<y

Ix,y)=

∑x,y∈T, x<y

E(Ix,y) 6∑

x,y∈T, x<y

Pr(h(x) = h(y)

number of

collisions

linearity of

expectation

E(Ix,y) = 1 ·Pr(Ix,y = 1)+0 ·Pr(Ix,y = 0) 61

E(C) = E( ∑

x,y∈T,x<y

Ix,y)=

∑x,y∈T, x<y

E(Ix,y) 6∑

x,y∈T, x<y

By the definition of expectation. . .

Pr(h(x) = h(y)

number of

collisions

linearity of

expectation

definition of

expectation

E(C) = E( ∑

x,y∈T,x<y

Ix,y)=

∑x,y∈T, x<y

E(Ix,y) 6∑

x,y∈T, x<y

Pr(h(x) = h(y)

number of

collisions

linearity of

expectation

definition of

expectation

E(C) = E( ∑

x,y∈T,x<y

Ix,y)=

∑x,y∈T, x<y

E(Ix,y) 6∑

x,y∈T, x<y

Pr(h(x) = h(y)

number of

collisions

linearity of

expectation

definition of

expectation

n(n− 1)

E(C) = E( ∑

x,y∈T,x<y

Ix,y)=

∑x,y∈T, x<y

E(Ix,y) 6∑

x,y∈T, x<y

Pr(h(x) = h(y)

number of

collisions

linearity of

expectation

definition of

expectation 6 n2/2

E(C) = E( ∑

x,y∈T,x<y

Ix,y)=

∑x,y∈T, x<y

E(Ix,y) 6∑

x,y∈T, x<y

Pr(h(x) = h(y)

number of

collisions

linearity of

expectation

definition of

expectation 6 n2/2

E(C) = E( ∑

x,y∈T,x<y

Ix,y)=

∑x,y∈T, x<y

E(Ix,y) 6∑

x,y∈T, x<y

Pr(h(x) = h(y)

number of

collisions

linearity of

expectation

definition of

expectation 6 n2/2

E(C) = E( ∑

x,y∈T,x<y

Ix,y)=

∑x,y∈T, x<y

E(Ix,y) 6∑

x,y∈T, x<y

Perfect hashing - a second attempt

Pr(h(x) = h(y)

using a weakly universal hash function

Step 1: Insert everything into a hash table of size m = n2

Pr(h(x) = h(y)

number of

collisions

linearity of

expectation

definition of

expectation 6 n2/2

E(C) = E( ∑

x,y∈T,x<y

Ix,y)=

∑x,y∈T, x<y

E(Ix,y) 6∑

x,y∈T, x<y

Pr(h(x) = h(y)

number of

collisions

linearity of

expectation

definition of

expectation 6 n2/2

E(C) = E( ∑

x,y∈T,x<y

Ix,y)=

∑x,y∈T, x<y

E(Ix,y) 6∑

x,y∈T, x<y

Pr(h(x) = h(y)

number of

collisions

linearity of

expectation

definition of

expectation 6 n2/2

E(C) = E( ∑

x,y∈T,x<y

Ix,y)=

∑x,y∈T, x<y

E(Ix,y) 6∑

x,y∈T, x<y

muchbetter!

Pr(h(x) = h(y)

number of

collisions

linearity of

expectation

definition of

expectation 6 n2/2

E(C) = E( ∑

x,y∈T,x<y

Ix,y)=

∑x,y∈T, x<y

E(Ix,y) 6∑

x,y∈T, x<y

(except we cheated)

better!

Expected construction time

Step 3: Repeat if there was a collision

How many times do we repeat on average?

The expected number of collisions: E(C) 6 12

The probability of at least one collision: Pr(C > 1) 6 12

Markov’s inequality

If X is a non-negative r.v., then for all a > 0,

Pr(X > a) 6E(X)

The probability of zero collisions is at least 12

i.e. at least as good as tossing a heads on a fair coin

E(runs) 6 E(coin tosses to get a heads) = 2

E(construction time) = O(m)·E(runs) = O(m) = O(n2)

. . . and then the look-up time is always O(1)

(because any h(x) can be computed in O(1) time)

Step 3: Repeat if there are more than n collisions

Step 1: Insert everything into a hash table of size m = n

This looks rubbish but

it will be useful in a bit!

The expected number of collisions: E(C) 6 n2

The probability of at least n collisions: Pr(C > n) 6 12

(where a = n)

If X is a non-negative r.v., then for all a > 0,

Pr(X > a) 6E(X)

E(construction time) = O(m)·E(runs) = O(m) = O(n)

The probability of at most n collisions is at least 12

E(construction time) = O(m)·E(runs) = O(m) = O(n)

. . . but the look-up time could be rubbish (lots of collisions)

The probability of at most n collisions is at least 12

Perfect hashing - attempt three

Step 1: Insert everything into a hash table, T , of size nusing a weakly universal hash function, h

Let ni be the number of items in T [i]

n1 = 2

n5 = 2

n8 = 3

. . . but don’t use chaining

Step 2: The ni items in T [i] are inserted into

another hash table Ti of size n2i

using another weakly universal hash function

denoted hi (there is one for each i)

denoted hi (there is one for each i)n2i

(Step 3) Immediately repeat a step if eithera) T has more than n collisionsb) some Ti has a collision

i.e. check (and if necessary rebuild)

each table immediately after building it

(Step 3) Immediately repeat a step if eithera) T has more than n collisions

The look-up time is always O(1)

1. Compute i = h(x) (x is the key)2. Compute j = hi(x)

3. The item is in Ti[j]

b) some Ti has a collision

(Step 3) Immediately repeat a step if eithera) T has more than n collisions

What is the expected construction time?

What is the space usage?

Two questions remain:

b) some Ti has a collision

Perfect Hashing - Space usage

Step 1: Insert everything into a hash table, T , of size nusing a weakly universal (w.u.) hash function, h

Step 2: The ni items in T [i] are inserted into another hash table Ti

of size n2i using w.u hash function hi

How much space does this use?

(Step 3) Immediately repeat if eithera) T has more than n collisionsb) some Ti has a collision

The size of T is O(n)

The size of Ti is O(ni2)

Storing hi uses O(1) space

So the total space is. . .

O(n)+∑i

O(n2i ) = O(n)+O

O(n)+∑i

O(n2i ) = O(n)+O

Storing hi uses O(1) spacehow big is this?

O(n)+∑i

O(n2i ) = O(n)+O

O(n)+∑i

O(n2i ) = O(n)+O

How big is∑

i n2i ?

O(n)+∑i

O(n2i ) = O(n)+O

How big is∑

i n2i ?

There are(ni2

)collisions in T [i]

O(n)+∑i

O(n2i ) = O(n)+O

How big is∑

i n2i ?

There are(ni2

)collisions in T [i] so there are

)collisions in T

O(n)+∑i

O(n2i ) = O(n)+O

How big is∑

i n2i ?

There are(ni2

)collisions in T

but we know that there are at most n collisions in T . . .

O(n)+∑i

O(n2i ) = O(n)+O

How big is∑

i n2i ?

There are(ni2

)collisions in T

but we know that there are at most n collisions in T . . .∑i

O(n)+∑i

O(n2i ) = O(n)+O

How big is∑

i n2i ?

There are(ni2

)collisions in T

O(n)+∑i

O(n2i ) = O(n)+O

How big is∑

i n2i ?

There are(ni2

)collisions in T

but we know that there are at most n collisions in T . . .

ni(ni−1)2 >

O(n)+∑i

O(n2i ) = O(n)+O

How big is∑

i n2i ?

There are(ni2

)collisions in T

O(n)+∑i

O(n2i ) = O(n)+O

How big is∑

i n2i ?

There are(ni2

)collisions in T

n2i 6 4nor

How big is∑

i n2i ?

There are(ni2

)collisions in T

n2i 6 4nor

O(n)+∑i

O(n2i ) = O(n)+O

= O(n)

O(n)+∑i

O(n2i ) = O(n)+O

= O(n)

Perfect Hashing - Expected construction time

The expected construction time for T is O(n)

(we considered this on a previous slide)

The expected construction time for each Ti is O(ni2)

- we insert ni items into a table of size m = n2i- then repeat if there was a collision

- we insert ni items into a table of size m = n2i

(we also considered this on a previous slide)- then repeat if there was a collision

- we insert ni items into a table of size m = n2i

(we also considered this on a previous slide)- then repeat if there was a collision

The overall expected constuction time is therefore:

E(construction time) = E

construction time of T +∑i

construction time of Ti

The overall expected construction time is therefore:

(construction time of T )+

E(construction time of Ti)

= O(n)+∑i

O(n2i ) = O(n)+O

= O(n)+∑i

O(n2i ) = O(n)+O

= O(n)

= O(n)+∑i

O(n2i ) = O(n)+O

= O(n)

n2i 6 4n

= O(n)+∑i

O(n2i ) = O(n)+O

= O(n)

Perfect Hashing - Summary

THEOREM

Hashing Part Two: Static Perfect Hashing

Education

Transcript of Hashing Part Two: Static Perfect Hashing

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