Fast generation of random permutations via networks simulation

We consider the problem of generating random permutations with uniform distribution. That is, we require that for an arbitrary permutation pi of n elements, with probability 1/n! the machine halts with the ith output cell containing pi(i), for 1 less than or equal to i less than or equal to n, We study this problem on two models of parallel computations: the CREW PRAM and the EREW PRAM. The main result of the paper is an algorithm for generating random permutations that runs in O(log log n) time and uses O(n (1+o(1))) processors on the CREW PRAM. This is the first o(log n)-time CREW PRAM algorithm for this problem. On the EREW PRAM we present a simple algorithm that generates a random permutation in time O (log n) using n processors and O(n) space. This algorithm outperforms each of the previously known algorithms for the exclusive write PRAMs. The common and novel feature of both our algorithms is first to design a suitable random switching network generating a permutation and then to simulate this network on the PRAM model in a fast way.