Ariel Rosenfeld

 Input: a stream of m integers i1, i2,..., im. (over 1,…,n)  Output: the number of distinct elements in the stream.  Example – count the distinct number of IP addresses you encounter.

 Bit vector of size n (mark 1 when encountered)  Keeping all m integers and naively answer. ◦ Sort and count O(min{n,mlogm})

 a determinitic exact algorithm is impossible using o(n) bits.  A deterministic approximation algorithm for this problem providing a (1 ± 1/1000)- approximation using o(n) bits is impossible.

 Pick random hash function h : [n] → [0, 1]  Calculate z = min i ∈ stream h(i)  Output 1/z − 1

 Same ints gets same hash value.  We will show that the output is a good approximation.

 This is idealized for 2 reasons: 1.We don’t have perfect precision. 2. We need n bits at least to remember the randomness associated with every i. Lets ignore it for now…

 S = {j1,…jt} (unique elements in the stream)  h(j1),..., h(jt) = X1,..., Xt are independent variables from Unif[0, 1]  Z = min{Xi}

P= F(x) 1 1

(HW) We get a bounded variance.

 q increases -> better approximation Chebyshev

 We want a function that doesn't need n bits or more to represent.  So we will use k-wise independent hash functions (H) each can be represented using a small number of bits (log|H|). ◦ In lecture.

 An example - Set q > k a prime power, and define H poly,k to be the set of all degree ≤ (k − 1) polynomials in Fq[x].  H poly,k is a k-wise independent family.  Size: q k  Needs: k log q bits.