Struct rand::rngs::SmallRng

pub struct SmallRng(_);

A small-state, fast non-crypto PRNG

SmallRng may be a good choice when a PRNG with small state, cheap initialization, good statistical quality and good performance are required. It is not a good choice when security against prediction or reproducibility are important.

This PRNG is feature-gated: to use, you must enable the crate feature small_rng.

The algorithm is deterministic but should not be considered reproducible due to dependence on platform and possible replacement in future library versions. For a reproducible generator, use a named PRNG from an external crate, e.g. rand_pcg or rand_chacha. Refer also to The Book.

The PRNG algorithm in SmallRng is chosen to be efficient on the current platform, without consideration for cryptography or security. The size of its state is much smaller than StdRng. The current algorithm is Pcg64Mcg on 64-bit platforms and Pcg32 on 32-bit platforms. Both are implemented by the rand_pcg crate.

Examples

Initializing SmallRng with a random seed can be done using SeedableRng::from_entropy:

use rand::{Rng, SeedableRng};
use rand::rngs::SmallRng;

// Create small, cheap to initialize and fast RNG with a random seed.
// The randomness is supplied by the operating system.
let mut small_rng = SmallRng::from_entropy();

When initializing a lot of SmallRng's, using thread_rng can be more efficient:

use rand::{SeedableRng, thread_rng};
use rand::rngs::SmallRng;

// Create a big, expensive to initialize and slower, but unpredictable RNG.
// This is cached and done only once per thread.
let mut thread_rng = thread_rng();
// Create small, cheap to initialize and fast RNGs with random seeds.
// One can generally assume this won't fail.
let rngs: Vec<SmallRng> = (0..10)
    .map(|_| SmallRng::from_rng(&mut thread_rng).unwrap())
    .collect();

Trait Implementations

impl Clone for SmallRng

fn clone(&self) -> SmallRng

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for SmallRng

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl RngCore for SmallRng

fn next_u32(&mut self) -> u32

Return the next random u32.

fn next_u64(&mut self) -> u64

Return the next random u64.

fn fill_bytes(&mut self, dest: &mut [u8])

Fill dest with random data.

fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error>

Fill dest entirely with random data.

impl SeedableRng for SmallRng

type Seed = <Pcg64Mcg as SeedableRng>::Seed

Seed type, which is restricted to types mutably-dereferencable as u8 arrays (we recommend [u8; N] for some N).

fn from_seed(seed: Self::Seed) -> Self

Create a new PRNG using the given seed.

fn from_rng<R: RngCore>(rng: R) -> Result<Self, Error>

Create a new PRNG seeded from another Rng.

fn seed_from_u64(state: u64) -> Self

Create a new PRNG using a u64 seed.

fn from_entropy() -> Self

Creates a new instance of the RNG seeded via getrandom.