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The Power of Tabulation Hashing

Speaker: Mikkel Thorup University of Copenhagen
Time: 2011-12-15 14:00-2011-12-15 15:00
Venue: FIT 1-222
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Randomized algorithms are often enjoyed for their simplicity, but the hash functions used to yield the desired theoretical guarantees are often neither simple nor practical. Here we show that the simplest possible tabulation hashing provides unexpectedly strong guarantees. The scheme itself dates back to Carter and Wegman (STOC'77). Keys are viewed as consisting of c characters. We initialize c tables T1,,Tc mapping characters to random hash codes. A key x=(x1,,xc) is hashed to T1[x1]Tc[xc], where denotes xor. While this scheme is not even 4-independent, we show that it provides many of the guarantees that are normally obtained via higher independence, e.g., Chernoff-type concentration, min-wise hashing for estimating set intersection, and cuckoo hashing. We shall also discuss a twist to simple tabulation that leads to extremely robust performance for linear probing with small buffers.

Short Bio:

Mikkel Thorup has a D.Phil. from Oxford University from 1993. From 1993 to 1998 he was at the University of Copenhagen. Since then he has been at AT&T Labs-Research. He is a Fellow of the ACM and of AT&T, Member of the Royal Danish Academy of Sciences and Letters, and co-winner of the 2011 MAA Robbins Award. His main work is in algorithms and data structures and he is the editor of this area for the Journal of the ACM. One of his best-known results is a linear-time algorithm for the single-source shortest paths problem in undirected graphs. Mikkel prefers to seek his mathematical inspiration in nature, combining the quest with his hobbies of bird watching and mushroom picking.