Cache-oblivious peeling of random hypergraphs

Belazzougui D, Boldi P, Ottaviano G, Venturini R, Vigna S

Perfect hash functions  Hypergraph algorithms  Cache-oblivious algorithms 

The computation of a peeling order in a randomly generated hypergraph is the most time- consuming step in a number of constructions, such as perfect hashing schemes, random r-SAT solvers, error-correcting codes, and approximate set encodings. While there exists a straightforward linear time algorithm, its poor I/O performance makes it impractical for hypergraphs whose size exceeds the available internal memory. We show how to reduce the computation of a peeling order to a small number of sequential scans and sorts, and analyze its I/O complexity in the cache-oblivious model. The resulting algorithm requires O(sort(n)) I/Os and O(n log n) time to peel a random hypergraph with n edges. We experimentally evaluate the performance of our implementation of this algorithm in a real- world scenario by using the construction of minimal perfect hash functions (MPHF) as our test case: our algorithm builds a MPHF of 7.6 billion keys in less than 21 hours on a single machine. The resulting data structure is both more space-efficient and faster than that obtained with the current state-of-the-art MPHF construction for large-scale key sets.