Fast Splittable Pseudorandom Number Generators Guy L. Steele Jr. Doug Lea Christine H. Flood Oracle Labs SUNY Oswego Red Hat Inc
[email protected] [email protected] [email protected] Abstract 1. Introduction and Background We describe a new algorithm SPLITMIX for an object- Many programming language environments provide a pseu- oriented and splittable pseudorandom number generator dorandom number generator (PRNG), a deterministic algo- (PRNG) that is quite fast: 9 64-bit arithmetic/logical opera- rithm to generate a sequence of values that is likely to pass tions per 64 bits generated. A conventional linear PRNG ob- certain statistical tests for “randomness.” Such an algorithm ject provides a generate method that returns one pseudoran- is typically described as a finite-state machine defined by dom value and updates the state of the PRNG, but a splittable a transition function τ, an output function µ, and an ini- PRNG object also has a second operation, split, that replaces tial state (or seed) s0; the generated sequence is zj where the original PRNG object with two (seemingly) independent sj = τ(sj−1) and zj = µ(sj) for all j ≥ 1. (An alternate PRNG objects, by creating and returning a new such object formulation is sj = τ(sj−1) and zj = µ(sj−1) for all j ≥ 1, and updating the state of the original object. Splittable PRNG which may be more efficient when implemented on a super- objects make it easy to organize the use of pseudorandom scalar processor because it allows τ(sj−1) and µ(sj−1) to numbers in multithreaded programs structured using fork- be computed in parallel.) Because the state is finite, the se- join parallelism.